AFFINITIES.—Until recently the Nemertines were regarded as a sub-order of

the Turbellaria. They were afterwards separated from the Turbellaria and

placed as a distinct class of the phylum Platyhelminthes.

Some zoologists have considered them to be so different in many respects

from the other classes of the Platyhelminthes as to justify their being

altogether separated from that phylum, and treated as a distinct group.

{120}If, however, the recent work of Bürger on the excretory system is to

be relied upon, the existence of flame cells would be a strong reason for

classing them among the Platyhelminthes.

Hubrecht[153] has instituted an interesting comparison between Nemertines

and Vertebrates. He compares the median dorsal nerve of Nemertines to the

spinal cord of Vertebrates; the lateral nerve-cords to the nerve of the

Vertebrate lateral line; and the lateral swellings which constitute the

brain in Nemertines to the lateral ganglia of the cephalic region in

Vertebrates. This view is strengthened by the existence of transverse

nerves connecting the lateral and dorsal nerves of Nemertines, since these

may be compared with the spinal nerves of Vertebrates. He suggests that

both Nemertines and Vertebrates may have arisen from a vermiform animal

possessing a nervous layer in the form of a plexus of nerve-fibres, the

nerve tissue having become concentrated along three lines to form a median

dorsal and two lateral nerve trunks; the former being specially developed

in the Vertebrata and the latter in the Nemertines. Hubrecht further

suggests that the notochord of Vertebrates may be a survival of the

proboscis sheath of Nemertines, while the proboscis of the latter may be

represented by the invagination to form the pituitary body in Vertebrates.

Certain authors[154] have suggested that indications exist of a

relationship between Nemertines and _Balanoglossus_.

The features which are supposed to indicate this are the elongated

vermiform shape showing no external signs of segmentation; the ciliated

smooth skin and the possession of unicellular mucous glands; and the

protrusible proboscis, which may be comparable to the non-retractile

proboscis of _Balanoglossus_, a comparison which is strengthened by the

fact that in some Nemertines a sheath of nerve-fibres exists in the wall of

the proboscis corresponding to the nerve plexus in the proboscis of

_Balanoglossus_. In both cases an ectodermic nerve plexus exists with local

thickenings along definite lines, although these lines are not the same in

the two cases. Both possess a straight alimentary canal, ending in a

terminal anus and thrown out into paired lateral caeca, between which are

the paired metamerically-arranged generative sacs.

NEMATHELMINTHES & CHAETOGNATHA

BY

ARTHUR E. SHIPLEY, M.A.

Fellow and Tutor of Christ's College, Cambridge.

{123}CHAPTER VI

NEMATHELMINTHES

INTRODUCTION—NEMATODA—ANATOMY—EMBRYOLOGY—CLASSIFICATION—ASCARIDAE—

STRONGYLIDAE—TRICHOTRACHELIDAE—FILARIIDAE—MERMITHIDAE—ANGUILLULIDAE—

ENOPLIDAE—PARASITISM—NEMATOMORPHA—ANATOMY—CLASSIFICATION—

LIFE-HISTORY—ACANTHOCEPHALA—ANATOMY—EMBRYOLOGY—CLASSIFICATION.

The Nemathelminthes include three sub-Orders of very different size and

importance. These are—

i. The Nematoda.

ii. The Nematomorpha (Gordiidae).

iii. The Acanthocephala.

Although the members of these groups differ considerably from one another,

on the whole there is a closer resemblance between them than between any

one of them and any other group of animals, and there is a certain

convenience in arranging them under one head.

The following characteristics are common to all three groups of the

Nemathelminthes: they are worm-like in form, and with few exceptions are

parasitic in the bodies of other animals, either Vertebrate or

Invertebrate. Some of them spend their whole existence within the bodies of

their hosts, but more commonly they are only parasitic during a certain

period of their life; a few, however, lead a free life in water or in damp

earth. None of the Nemathelminthes are segmented—that is, their bodies are

not divided into a number of parts which serially repeat each other, and

which resemble more or less closely the preceding and {124}succeeding

parts. They are not provided with any appendages or limbs, but sometimes

bear a few bristles or hooks, and in rarer cases suckers. The body, which

is elongated and, as a rule, thread-like and tapering at each end, is

enclosed in a thick cuticle or hardened secretion of the underlying cells.

In no Nemathelminth is there any closed vascular system, nor are special

respiratory organs developed.

In many respects the most remarkable peculiarity of these animals is that,

with the possible exception of the excretory organs of the Acanthocephala,

there is a complete absence of cilia throughout the whole group. In this

respect they resemble the Arthropoda. The universal presence of these small

flickering processes of cells from man down to the simplest unicellular

organisms makes the absence of these structures most remarkable. In many

animals they are the sole organs of locomotion, and in almost all they

perform very important functions, both in bringing food and oxygen to the

body, and in removing waste matter from it. At present there seems to be no

adequate explanation for their absence in the two large groups mentioned

above.

Nemathelminthes are, with hardly an exception, dioecious—that is to say,

their male and female reproductive organs are in different individuals.

Their young do not differ markedly from the adults, except in the absence

of sexual organs, but the immature stages are usually termed larvae, and

not infrequently either inhabit a different host from the adult, or are

free when the adults are parasitic, or _vice versâ_.

SUB-ORDER I. NEMATODA.

ANATOMY.—The Nematode worms, or thread-worms, form by far the largest and

most important division of the group Nemathelminthes. The number of species

is great, and although the conditions under which they live are of the most

varied kind, there is, as a rule, little corresponding difference in

structure, and hence the determination of the species is attended with no

small difficulty.

With few exceptions the shape of the body is filiform (Figs. 66 and 71),

the two ends being more or less pointed, and the posterior end of the male,

which is generally a smaller animal than the female, is usually slightly

recurved. The worms are, as a rule, {125}white, or of the colour of

polished ivory; they may be opaque or semi-transparent, but pigment spots

are rarely developed.

Minute Nematodes abound in moist soil, around the roots of plants, etc.,

and may easily be detected with the aid of a lens wriggling about amongst

the particles of sand and earth. Of the animal parasites perhaps the most

familiar is the "round worm" (_Ascaris lumbricoides_, Figs. 66 and 67),

which inhabits the alimentary canal of man; others are common in

domesticated animals, as _A. mystax_ in the cat and dog, and _A.

megalocephala_ in the horse and ox. They are also found living

parasitically in plants (Fig. 77), causing the formation of galls and other

pathological growths; _Anguillula_ (_Tylenchus_) _tritici_ causes in this

way considerable damage to corn, and others attack root-crops, cabbages,

etc. The "vinegar eel" (_Anguillula aceti_), which occurs so often in weak

vinegar, is another familiar example of this group.

THE SKIN.—The body of the worm is encased in a relatively thick,

transparent, smooth cuticle, which is turned in at the various apertures,

and lines the tubes connected with them for a greater or less distance. The

cuticle is in some cases raised to form spikes or hooks, and in certain

species, _e.g._ _Ascaris mystax_ and _A. transfuga_, it is produced into

two lateral fins, which are supported by a thickened triradiate rod of

specialised cuticle (Fig. 62); these fins, however, do not run far down the

body. As a rule the cuticle is quite smooth, but it may be ringed, as in

_Filaria laticaudata_ and in _F. denticulata_; and the rings may bear

backwardly-projecting teeth.

The skin of Nematodes consists of three layers—(i.) the above-mentioned

_cuticle_, which is presumably secreted by (ii.) the _sub-cuticle_ or

_epidermis_ which underlies it; the latter surrounds in its turn (iii.) the

_muscular layer_.

The nature of the sub-cuticle is one of the debateable points in the

morphology of the Nematoda. No cell outlines have been detected in it,

although nuclei are scattered through it; it is in fact a _syncytium_, or

protoplasmic mass in which cell limits cannot be distinguished. Many of the

cells forming it have broken down into fibrils, and these form a close

meshwork, which is occasionally specialised, as, for instance, round the

nerve-cords. Along the median dorsal and ventral lines, and along the

lateral lines, this tissue is heaped up in such a way as to divide the

{126}enclosed muscle-cells into four quadrants. These thickenings surround

dorsally and ventrally a specialised nerve-cord, and laterally the

excretory canals.

According to Jammes[155] this lack of differentiation in the sub-cuticular

layer is caused by the early appearance of the cuticle, which he thinks is

necessitated, at any rate in many of the parasitic forms, by the action

which the digestive juices of the host would have on the otherwise

unprotected body-wall.

[Illustration: FIG. 62.—A transverse section through the body of _Ascaris

transfuga_ Rud., in the region of the oesophagus: _a_, the muscular

oesophagus with its triradiate lumen; _b_, the cuticle; _c_, the

sub-cuticle; _d_, the muscular layer; _e_, the lateral nerves running in

the lateral line; _f_, the excretory canal; _g_, the dorsal, and _h_, the

ventral nerve; _i_, the triradiate rod in the fin.]

The nervous system, according to the same writer, is of the same nature as

this sub-cuticular tissue, only it is more differentiated, or perhaps we

should say it has retained more of the primitive cellular character of the

embryonic tissue. The fibres of the sub-cuticular tissue are closely

connected with the fibrils which compose the spongioplasm (Fig. 64, _d_) of

the muscles,[156] and form also the sheaths of the various nerves; in fact

the passage of these fibrils into the nerves is so gradual that it is

impossible to make any separation between them.

{127}THE NERVOUS SYSTEM.—The central organ of the nervous system is the

circumoesophageal ring which surrounds the pharynx, close to the anterior

end of the body, in _A. megalocephala_ 1½ to 2 mm. behind the mouth.[157]

Ganglion cells are found in the ring, but they are not numerous, and are

chiefly aggregated round the points of origin of the nerves.

Six short nerves, three on each side of the median line, run forward from

the ring, a pair of these ending in each of the three papillae which

surround the mouth.

Behind, the nerve-ring gives off six main nerve trunks, of which the dorsal

and ventral nerves are usually the largest. These run in the median dorsal

and ventral thickenings of the sub-cuticular tissue, and are connected one

with another by numerous fine lateral branches running through the

sub-cuticle.

[Illustration: FIG. 63.—Diagram of the nervous system at the two ends of

the body in _Ascaris megalocephala_ Cloq., ♂. (After Hesse.) _a_,

Circumoesophageal nerve-ring; _b_, opening of excretory ducts; _c_, dorsal

nerve; _d_, dorso-lateral nerve; _e_, ventro-lateral nerve becoming the

bursal nerve posteriorly; _f_, the ventral nerve; _g_, cloacal opening;

_h_, sub-cuticular nerves running from _c_ to _f_; _k_, spicules.]

The lateral nerves, which consist of two or four bundles, one or two lying

dorsal and one or two ventral to each excretory canal, have a double

origin. The dorsal branches arise directly from the nerve-ring, and at

their point of origin there is a considerable accumulation of ganglion

cells, from which two commissures on each side run into the ventral nerve

(Fig. 63, _f_). The ventral branches arise from the ventral nerve-cord

immediately in front of the excretory pore. At the posterior end the

lateral nerves pass into the two branches into which the ventral nerve

divides. Just before the point where the ventral {128}nerve splits it

swells out into an anal ganglion situated just in front of the anus. In the

male[158] this anal ganglion gives off two lateral nerves which pass round

the cloaca and form a ring, and in this sex the ventro-lateral nerve, which

is much strengthened by fibres from the ventral nerve, and has received,

owing to the mistaken impression that it was a special _nervus recurrens_,

the name of the "bursal nerve," gives off numerous branches to the sense

papillae which are found in this region of the body and on the tail. The

arrangement of these parts is shown in Fig. 63.

Sense organs are but poorly developed in the Nematoda, as is usual in

animals which are, as a rule, either parasitic or live underground. Eyes,

consisting of masses of dark pigment with or without a lens, occur in the

neighbourhood of the circumoesophageal nerve-ring in some free-living

forms. Leuckart described as possible auditory organs certain giant-cells

lying near the orifice of the excretory ducts. Later research has shown

these cells to have some phagocytic action on the contents of the

body-cavity. The chief sense organs are the papillae, of which in _A.

megalocephala_ there are two kinds, the lip papillae being distinguished

from the genital papillae by the fact that the nerve supplying them ends in

a fine point and pierces the cuticle in the former case, whilst in the

latter it swells out into an "end-organ," which is always covered by a

layer of cuticle, though sometimes by a very thin one.

MUSCULAR SYSTEM.—The muscular system is one of the most characteristic

features of the Nematoda, both as regards the histology of the muscle-cells

and the way in which the cells are arranged.

Each muscle-cell is of considerable size, and is of the shape of a somewhat

flattened spindle produced into a process near the middle. Each end of the

spindle cell is said to be continuous with the fibrils of the sub-cuticular

layer.[159] The muscle-cell consists of two portions, a contractile part

which lies next the sub-cuticle, and which usually, to some extent, wraps

round the second or medullary half. The latter consists of a fibrillar

spongioplasm, in the meshes of which lies a clear structureless hyaloplasm.

The nucleus always lies in the medullary half.

{129}The contractile portion consists of a number of columns, very

regularly arranged in two rows and close together, but allowing sufficient

space between adjacent columns for fibrils of the spongioplasm to

penetrate; and these become continuous with the fibrils of the sub-cuticle,

which is thus intimately connected with both nervous and muscular systems.

The medullary portion of the cell varies greatly in size; it may stretch

far into the body-cavity, which may be thereby almost occluded, or it may

be flattened out, leaving a large space around the alimentary canal. At one

point, usually about its middle, it is produced into a process, which bends

inwards towards the dorsal or ventral nerve-cord, and by means of this

process the muscle receives its nerve supply.

In most Nematodes there are numerous muscle-cells to be seen in any

transverse section, forming a layer within the sub-cuticle, and broken up

into four quadrants (Fig. 62) by the projection of the dorsal, ventral, and

lateral thickenings of the sub-cuticular tissue. In some genera, however,

such as _Oxyuris_, _Strongylus_, _Pelodera_, _Leptodera_, etc., there are

but eight muscle-cells in a row, two in each quadrant. Such genera are

classed together by Schneider,[160] and termed Meromyarii (_vide_ p. 137).

[Illustration: FIG. 64.—A, transverse section through the centre of a

muscle-cell; B, the same through a nerve fibre showing the sub-cuticular

fibres running into the sheath. (After Rohde.) _a_, Cuticle; _b_,

sub-cuticular fibres continuous with _d_; _c_, contractile columns; _d_,

network of spongioplasm; _e_, nucleus.]

In addition to the characteristic muscles of the body-wall there are

others, such as those which move the spicules in the male, which cross the

body-cavity obliquely near the anus, and such as sphincter muscles near the

latter orifice, which have not {130}the characteristic arrangement of

contractile and medullary parts described above.

THE BODY-CAVITY.—The skin of a Nematode, as described above, contains most

of the important organs of the body within its thickness. The chief

muscular system, the nervous system with its sense organs, and the

excretory organs are all embedded in or form part of the skin, which in its

turn encloses a cavity—the body-cavity—in which the other two systems of

organs which are found in Nematodes lie. These are the digestive system and

the reproductive system.

The body-cavity is continuous from one end of the animal to the other, and

is in no case divided up into compartments by the presence of septa or

mesenteries. It contains a coagulable fluid with numerous corpuscles; this

is, as a rule, colourless, but in _Syngamus trachealis_ Sieb. (Fig. 70),

which lives on blood, the haemoglobin of its host tinges it red, though the

colour is said to disappear if the parasite be isolated and starved.

The morphological nature of this body-cavity affords an interesting

problem. It is not a true _coelom_, such as exists in the earthworm, since

it is not surrounded by mesoderm, nor do the excretory organs, with the

possible exception of one or two genera, open into it, nor do the

generative cells arise from its walls. Essentially it is a space between

the mesodermic muscle-cells which line the skin and the endodermic cells of

the alimentary canal, and although in many of its functions it resembles

the coelom of other animals, its morphological character is quite

different.

There are no respiratory or circulatory organs in the Nematoda; possibly

the fluid in the body-cavity acts, to some extent, as a carrier of oxygen,

but from the inert and almost vegetative life of these animals it seems

probable that their respiratory processes are slow, and in fact Bunge[161]

has shown that _Ascaris mystax_, found in the intestine of the cat, will

live for four or five days in media quite free from oxygen, and that _A.

acus_ from the pike will live and exhibit movements in the same media for

from four to six days.

THE DIGESTIVE SYSTEM.—The mouth of the Nematoda is usually anterior and

terminal, and is surrounded by from two to six projecting lips, the most

common number being three. These {131}lips are well provided with sense

papillae. The mouth leads into an alimentary canal, which with hardly an

exception runs straight through the body to the anus without twists or

loops. The anus is usually placed ventrally and is not terminal, but in

_Trichina_ and _Trichocephalus_ it is at the end of the body, and in

_Mermis_, where the several parts of the alimentary canal are said not to

communicate, it is absent altogether. _Ichthyonema_, _Dracunculus_,

_Allantonema_, _Atractonema_, and other Filariae are also aproctous.

The alimentary canal is divisible into three parts—(i.) the oesophagus,

(ii.) the intestine, and (iii.) the rectum. The suctorial oesophagus is a

very muscular, thick-walled tube, lined with cuticle continuous with that

which covers the body, and like it cast from time to time. Its lumen is

usually much reduced, and is almost invariably triangular or triradiate in

section (Fig. 62). In many genera the hinder end of the oesophagus is

swollen into a muscular bulb, which is armed with teeth in _Heterakis_,

_Oxyuris_, _Pelodera_, _Leptodera_, etc. Other species, such as

_Tylenchus_, _Aphelenchus_, _Dorylaimus_, are armed with a spear, which in

_Onyx_,[162] a genus recently described and allied to the last named, is

borne on a special bulb. The use of the spear is to pierce the tissue upon

the juices of which the animal lives. A gland lies embedded in the thick

walls of the oesophagus, and opens into its lumen by a fine tube. This was

first described by Schneider[163] in _A. megalocephala_, and more recently

it has been found by Hamann[164] in a number of Ascaridae and Strongylidae

from the Adriatic, and also in _Lecanocephalus_.

With a few exceptions, such as _Mermis_, where it is blind, the oesophagus

opens posteriorly into the intestine. This is a somewhat flattened tube,

whose shape and position are often altered by the development of the

generative organs. Its wall consists of a single layer of columnar cells,

with large nuclei coated internally and externally by a layer of cuticle.

The inner layer of cuticle is usually perforated by very numerous minute

pores. In some species the intestine is degenerate, in _Mermis_ it is a

closed tube opening neither into the oesophagus nor into the rectum; in

_Trichina spiralis_ and in the larva of _Tylenchus tritici_ it consists of

a single row of cells perforated by a duct, but in the adult of the last

named there are many cells in a transverse section.

{132}[Illustration: FIG. 65.—A longitudinal section through the body of

_Strongylus filaria_ Rud. (From O. Augstein.[165]) A portion of the body,

on each side of the excretory pore, is seen in optical section. _a_, Mouth;

_b_, oesophagus; _c_, intestine; _d_, excretory canal; _e_, excretory pore,

and the opening of the poison glands, _i_; _f_, circumoesophageal

nerve-ring; _g_, ventral nerve; _h_, dorsal nerve; _i_, unicellular poison

glands; _k_, ovary, with the ova separate; _l_, oviduct; _m_, uterus, the

first egg in the uterus is surrounded by spermatozoa; _n_, opening of

uterus; _o_, inner end of ovary with the ova undifferentiated.]

{133}In some genera, _Leptodera_ and _Pelodera_, the lumen of the intestine

at any one level is bounded by two horseshoe-shaped cells, but by far the

commonest arrangement is a tube formed of fairly numerous columnar cells

crowded with granules and with large nuclei.

The rectum is usually short; its cuticular lining, like that of the

oesophagus, is cast at intervals. At its anterior end there is usually a

sphincter muscle, and its walls are divaricated by muscular strands which

run from it to the body-wall. The anus is a transverse slit, which in the

male Strongylidae is surrounded by a funnel-shaped membrane.

The food of Nematodes seems to be almost entirely fluid, and consists, at

any rate in the parasitic forms, of the elaborated juices of their hosts.

Little is known about the nutriment of the free-living forms.

THE EXCRETORY SYSTEM.—The excretory organs are peculiar, and, like many

other Nematode structures, do not fall readily into line with what is known

of similar organs in other animals. They consist of two canals embedded in

the lateral thickenings of the sub-cuticular tissue. The canals end blindly

behind, but near the anterior end of the body they bend inwards, and after

uniting, open by a common pore situated in the middle ventral line, a

little way behind the mouth. The lateral canals are in some cases continued

in front of the transverse branch, and they then end blindly in the head.

The walls of these canals consist of an internal, structureless, refractive

layer surrounded by a granular layer with nuclei. They contain a fluid, but

nothing is known of its composition.

An interesting divergence from the usual form of excretory organ has been

described by Hamann[166] in the genus _Lecanocephalus_. Here there is only

one canal, the right; anteriorly this bends towards the ventral surface and

opens by a small median pore close behind the nerve-ring. Posteriorly the

canal does not extend much beyond the middle of the body, where it forms a

coiled mass, and diminishing in size, opens into the body-cavity. The same

author also states that both canals in _Dochmius_ have a similar internal

opening; these observations, if confirmed,[167] show a conformity to the

ordinary structure of {134}excretory organs which was not supposed to exist

in the lateral canals of the Nematoda.

THE REPRODUCTIVE ORGANS.—With the exception of the genera _Angiostomum_,

_Pelodytes_, and of _Rhabdonema nigrovenosum_, which are physiologically

hermaphrodite and self-impregnating, the Nematodes have separate sexes. The

males are, as a rule, smaller than the females, and may usually be

distinguished by the posterior end of the body being curved towards the

ventral surface; a genital bursa, and one or more spicules are often found

in this sex. Further, the position of the genital opening differs; in the

male the vas deferens opens on the ventral surface of the rectum close to

the anus, but the oviduct in the female opens in the ventral middle line,

usually near the middle of the body, but sometimes close behind the

excretory pore, or in some Strongylidae just in front of the anus. The tail

of the male bears very numerous papillae, which are of considerable

systematic importance.

[Illustration: FIG. 66.—_Ascaris lumbricoides_ Cloq. ♂, natural size, cut

open along the dorsal middle line. _a_, Oesophagus; _b_, intestine; _c_,

testis; _d_, vas deferens; _h_, lateral excretory canals.]

With rare exceptions, _e.g._ _Filaria attenuata_, where it is double, the

male reproductive organ consists of a single tube divisible into a testis

proper, a vas deferens, a vesicula seminalis, where the spermatozoa are

stored up, and a ductus ejaculatorius. The tube stretches through the body

in a straight line in the small free-living forms, but is thrown into loops

and coils in the larger parasitic Nematodes. Within the testis the

mother-cells of the spermatozoa are attached to a rhachis or axial cord;

the mother-cells divide, and their products ultimately form spermatozoa.

The latter have a very peculiar shape; in accordance with the universal

absence of cilia in the Nematoda the spermatozoon has no flagellum, and at

first consists of a spherical nucleated cell, on one side of which a cap or

covering of some refractive substance appears. The cap elongates and

{135}becomes conical, whilst the protoplasmic portion of the spermatozoon

throws out pseudopodia and becomes amoeboid, but ultimately rounds itself

off again. The spermatozoa do not attain maturity until they reach the

uterus of the female.

The internal female reproductive organs are, with few exceptions

(_Trichina_, etc.), double, but the vagina, which is lined with cuticle

continuous with that covering the body, is always single. They are usually

much coiled, and may be divided into ovary, oviduct, and uterus. The ova

arise from a polynucleated mass of protoplasm or syncytium (Fig. 65, _o_)

at the upper end, and acquire distinctness as they approach the oviduct.

Fertilisation takes place in the uterus, but the segmentation may not begin

until some time after the eggs are laid; in _Dochmius_, however, it is well

advanced at this period, and in many genera, _e.g._ _Pseudalius_,

_Trichina_, _Dracunculus_, etc., the whole development of the larva takes

place in the body of the mother.

[Illustration: FIG. 67.—_Ascaris lumbricoides_ Cloq. ♀, natural size, cut

open along the median dorsal line to show the internal organs. _a_, The

muscular oesophagus; _b_, the intestine; _c_, the ovary; _d_, the uterus;

_e_, the vagina; _f_, the external opening; _h_, the excretory canals; _i_,

their opening.]

EMBRYOLOGY.—The eggs of many of the parasitic forms require a considerable

degree of warmth to develop. Those of _Ascaris lumbricoides_ require a

temperature of 20° C., those of _Trichocephalus_ 22.5° C., and those of

_Oxyuris vermicularis_, 40° C. The latter develop in a few hours, the eggs

of _Dochmius_ in a few days, whilst those of _A. {136}lumbricoides_ take

weeks or even months, and the young of _Trichocephalus_ seldom develop

within a year.[168] The ova only develop in a damp atmosphere, and they can

be arrested at almost any stage, and for considerable periods, by

desiccation.

Our knowledge of the processes by which the fertilised egg-cell develops

into the larva is very imperfect. As a rule the segmentation is complete

and equal; it results in the formation of a blastula, which may take the

form of a hollow sphere of cells—_A. megalocephala_—or the cavity may be

reduced, and the blastula may consist of a double-layered plate, as in

_Cucullanus_.[169] The distinction into cells which will form the three

embryonic layers, the ectoderm, mesoderm, and endoderm, is very early

evident,—in the eight-cell stage. By the growth of one side of the blastula

and the tucking in of the other the blastula becomes converted into a

gastrula, which is a two-layered stage with a cavity opening to the

exterior by a pore termed the blastopore. In Nematodes the blastopore is

elongated and slit-like; it either forms the mouth (_Cucullanus_) or closes

from behind forwards, the mouth ultimately arising at the point where the

blastopore finally closed (_Rhabdonema nigrovenosum_). The mesodermal cells

lie between the ectoderm and the endoderm; they ultimately develop into the

muscles of the body-wall, the lateral excretory canals, and the

reproductive organs; the last-named two systems arise each[170] from a

single cell. The nervous system arises from the ectoderm, which also forms

the sub-cuticle, and is turned in slightly at the mouth and anus; the

remainder of the alimentary canal develops from the endoderm.

The post-embryonic development, which is very variable, and in many cases

very extraordinary, will be dealt with under the several families.

CLASSIFICATION.—The classification of the Nematodes is a matter of very

considerable difficulty; their structure is unusually monotonous, and,

owing perhaps to their largely parasitic mode of life, they show

practically none of those external features which are so useful to the

systematist in other groups. Schneider in his Monograph divides the group

into three subdivisions—(i.) {137}the POLYMYARII, in which numerous muscle

cells are seen in a transverse section; (ii.) the MEROMYARII, in which only

eight are seen, two in each quadrant; and (iii.) the HOLOMYARII, in which

the muscles are either not divided, or only divided by longitudinal lines.

This grouping has, however, to some extent broken down, since Bütschli[171]

and others have shown that the third subdivision is founded on insufficient

observation, whilst the first two include, in different subdivisions,

Nematodes which are closely allied in all respects except as regards their

muscle cells.

The details of the life-history have been used by other writers as a basis

of classification. Linstow[172] enumerates fourteen distinct modifications

of the post-embryonic development (_vide_ p. 159), and Örley[173] has

grouped these under three headings. The animals which fall under each group

to some extent resemble one another in structure. Örley's groups are:—

(i.) NEMATOZOA.—Thread-worms with free larval life, the mature forms being

parasitic in animals. Enormous numbers of eggs are produced, and the

development is indirect. The genital organs are complicated by many

convolutions.

(ii.) RHABDITIFORMAE.—Small, as a rule microscopic, thread-worms, usually

living free, but rarely parasitic. They become sexually mature only in

decomposing organic substances, or in earth saturated with such substances.

They live gregariously and do not produce immense numbers of ova. The

metamorphosis is slight, or is complicated by sexual metamorphosis. The

oesophagus has two dilatations. The genital tubes are simple and not

coiled.

(iii.) ANGUILLULIDAE.—Small microscopic thread-worms, with a free existence

in mould or water, throughout all stages. They produce large eggs. They are

provided with a caudal sucker and bristles, sometimes with eyes and other

structures characteristic of a free life. Genital tube simple and not

coiled.

The disadvantage of such a system is, that to accurately place a specimen

in its proper class we must be acquainted with its life-history, and this

is known in but few cases.

The determination of the species to which a Nematode belongs is a matter of

considerable difficulty. Amongst the more important features for purposes

of classification are the arrangement of the {138}muscles, the character of

the tail in the male, especially when papillae are present, the number and

the size of the spicules, and the arrangement of the lips and mouth-parts

generally.

Cobb[174] has recently devised an ingenious formula in which measurements

of different parts of the body appear as percentages of the whole length of

the body. The nature of this will be understood by reference to Fig. 68.

Such a formula should, however, be used with caution, since it rests on the

assumption that the proportions of the various parts of the body are

constant in different individuals, and it is by no means certain that this

is the case.

[Illustration: FIG. 68.—Diagram to explain the descriptive formula used for

Nematodes. (From Cobb.) 6, 7, 8, 10, 6 are the transverse measurements,

while 7, 14, 28, 50, 88 are the corresponding longitudinal measurements.

The formula in this case is

7 14 28 50 88

-------------

6 7 8 10 6

The unit of measurement is the one-hundredth part of the length of the

worm. The measurements are therefore percentages of the length.

The measurements are taken with the animal viewed in profile; the first is

taken at the base of the oesophagus, the second at the nerve-ring, the

third at the cardiac constriction, the fourth at the vulva in females and

at the middle in males, the fifth at the anus.]

Taking everything into consideration, it has seemed advisable in the

following systematic account of the Nematoda to abandon the larger groups,

and to deal directly with the families. Claus distinguishes seven of these,

and the diagnoses given at the head of each are mainly taken from his

_Grundzüge der Zoologie_.[175]

I. FAMILY ASCARIDAE.

Body rather stout. A dorsal and two ventro-lateral lips, bearing papillae.

Buccal cavity distinct, seldom provided with chitinous armature. The

oesophagus often has two dilatations. {139}The tail of the male is

ventrally curved, and usually there are two horny spicules. The Ascaridae

are found in the intestines of their respective hosts.

Genera: _Ascaris_, _Heterakis_, _Oxyuris_, _Nematoxys_, _Oxysoma_, and many

others.

Von Linstow[176] enumerates over 250 species of _Ascaris_, of which it will

only be possible to mention here one or two. They are all parasitic in

Vertebrata.

_A. lumbricoides_ Linn. is one of the largest known Nematodes ♂ = 4-6 in.,

♀ = 10-14 in.; Figs. 66 and 67). It is a common parasite in man, and has

been found in the ox. It is now generally recognised as the same parasite

which inhabits the pig, and which Dujardin regarded as specifically

distinct, and named _A. suillae_. In the latter host, however, it never

attains the dimensions it does in man. It inhabits the upper and middle

parts of the small intestine, and has been known to escape into the

body-cavity and set up abscesses there, or to make its way into the

stomach, and to be voided through the mouth. It is practically cosmopolitan

in distribution, and is very common in Japan—Baely found it in twenty-one

out of twenty-three _post-mortems_—and in Tonquin and tropical Africa.

Heller[177] states that no one is free from these worms in Finland, and

they are common wherever there is a plentiful water supply, as in the

marshy districts of Holland and Sweden. In Iceland alone they seem absent.

When examined alive they give off an irritating vapour which seriously

affects some observers, causing catarrhal symptoms, which in Bastian's case

lasted six weeks. The usual number found in one host is small, one to six

or eight, but cases are on record where many hundreds occurred in one

person.

The details of the life-history of this form are not yet completely worked

out. The eggs leave the body of the host with the excreta, and formerly it

was thought they re-entered the alimentary canal in drinking-water, etc.,

and there developed into the adult without change of host. This view has

been combated by Leuckart, who failed to rear the Nematodes by direct

feeding, and it has been noticed that the youngest parasites found in the

{140}intestine are already 2 to 3 mm. long. Von Linstow has recently

suggested that the larval stages may be hatched out in the body of the

millipede _Julus guttulatus_, whose habits might easily lead it to eat the

eggs of the parasite in manured gardens, etc., and which is itself

sometimes unconsciously eaten when hidden in fruit or vegetables. This

would account for the frequent presence of the parasite in pigs, and also

for the fact that in man it is commonest in children who are apt to eat

windfalls, and in maniacs and people with perverted tastes.

_A. megalocephala_, which is found in the horse, ass, zebra, ox, etc.,

attains even greater dimensions than the foregoing. The male rarely exceeds

7 inches in length, but the female sometimes reaches 17 inches. They are

found in the small intestine of their hosts. Cobbold[178] succeeded in

rearing larvae which attained a high degree of organisation when the eggs

were placed amongst moist horse-dung, and it seems probable that the larvae

pass into the body of their hosts in drinking water; at any rate no

intermediate host has yet been found, and Davaine, who fed cows, and

Leuckart, who fed horses with the unhatched eggs, both failed to infect the

animals they experimented on. _A. mystax_, which lives in cats, dogs, and

other Carnivora, has also been found in man. It is provided with fin-like

extensions on the side of its head (cf. Fig. 62), and varies much in size

in different hosts. When first found in man it received the name of _A.

alata_. It becomes sexually mature in about three weeks.

One of the most remarkable cycles of development amongst the many curious

life-histories met with amongst Nematodes, is that presented by

_Rhabdonema_ (_Ascaris_) _nigrovenosum_. The free form of this, formerly

known as a distinct species, _Rhabditis nigrovenosa_, lives in the

excrement of frogs, and attains sexual maturity in a very short time. The

sexes pair, and the fertilised ova give rise to embryos which hatch out

within the body of the mother, and then begin to devour her internal

organs. After the destruction of the mother, the embryos escape and live in

water or slime, and sometimes burrow into water snails, but they undergo no

change until swallowed by a frog. Then they make their way into its lungs

and grow enormously, attaining a length of almost an inch. This form,

parasitic in the frog, is a protandrous hermaphrodite, which first produces

spermatozoa and afterwards {141}ova; the latter are fertilised by the

spermatozoa, and give rise to rhabditiform embryos, which escape by the

alimentary canal and form the free-living sexual stage mentioned above.

Thus in the life-history of this form we find an alternation of generation,

a sexual free-living form alternating with a hermaphrodite parasitic form.

Of the enormous number of other species of the genus, only a very few can

be mentioned. _A. transfuga_ Rud. inhabits bears; _A. leptoptera_ Rud.,

lions; _A. ferox_ H. and Ehrbg., Hyracoidea; _A. depressa_ Rud., vultures;

_A. rubicunda_ Schn., pythons; _A. sulcata_ Rud., turtles; _A. mucronata_

Schn., the cod and pike; _A. incurva_ Rud., the sword-fish.

[Illustration: FIG. 69.—A male and female _Oxyuris diesingi_ Ham. _in

copula_, × 60. _a_, Anus; _b_, oesophagus; _c_, bulb; _d_, testis; _e_,

intestine; _f_, ovary. (From Galeb.[179])]

_Oxyuris_ is Meromyarian (see p. 137), and is characterised by the long

capillary tail of the female. It includes another human parasite, _O.

vermicularis_, and it is one which it is difficult to get rid of. The

female has the characteristic tail and is about 10 mm. long. The male is

smaller. They are found in the caecum and rectum of man, and cause great

irritation and sometimes serious functional disturbance. The eggs are laid

in immense numbers but perish in water. If whilst still in the egg-shell

the larvae are swallowed on fruit or raw vegetables, etc., they are set

free in the stomach and small intestine by the action of the digestive

secretions. The distribution of this parasite is universal. Besides

numerous species that inhabit the alimentary canal of Vertebrates, such as

_O. ambigua_ Rud., found in hares and rabbits; _O. curvula_ Rud., in

{142}the caecum of horses; _O. megatyphlon_ Rud., in iguanas; several

species inhabit the rectum of insects, such as _O. blattae_, _O. diesingi_,

_O. blatticola_, found in the cockroach; _O. spirotheca_, and _O.

hydrophili_ in the water beetle _Hydrophilus_.[180]

The genus _Nematoxys_ has the most complex arrangement of muscles of any

Meromyarian, and forms a transition to the Polymyarian type. The whole body

of both sexes is covered with numerous irregularly scattered papillae. The

members of this genus have hitherto been found in snakes, Amphibia, and

eels; there are but few species.

_Oxysoma_ is another small genus with but three species, found in the

intestines of opossums, frogs, and turtles respectively.

II. FAMILY STRONGYLIDAE.

Mouth surrounded by papillae; an armature of teeth or spines often present.

The chitinous lining of the intestine projects into the interior as ridges.

No oesophageal bulb. The male orifice at the posterior end of the body is

surrounded by a bell-shaped bursa.

Genera: _Eustrongylus_, _Strongylus_, _Dochmius_, _Sclerostomum_,

_Cucullanus_, _Syngamus_, _Pseudalius_, _Ollulanus_, and others.

The genus _Eustrongylus_ includes two species, _E. gigas_ Rud. and _E.

tubifex_ Nitsch. The former attains in the female the gigantic length of

860 mm., with a breadth of 7 mm. and a weight of over 40 grs.[181] The male

is a quarter to a third as long as the female. This parasite inhabits the

kidney capsules of carnivorous animals, especially of those that eat fish,

such as dogs, seals, etc., and has occasionally been found in man, the

horse, and the deer. It frequently destroys the substance of the kidney.

The worms are red in colour. The eggs die when exposed to desiccation for a

few days, but have been kept alive for fifteen months in water; it is

believed by Schneider and Leuckart that they are eaten by fish, and that

the larvae form the _Filaria cystica_ found in the peritoneal membrane of

the fishes _Galaxias scriba_ and _Symbranchus laticaudatus_, and that they

pass into their final host, where they become sexually mature, by the

latter eating raw fish. _E. tubifex_ is found in aquatic birds, _e.g._

ducks, grebes, and divers, etc.

{143}The genus _Strongylus_ is easily recognised by its conspicuous genital

bursa, strengthened by variously arranged ridges which are of specific

value. There are numerous species, found in man and many other mammals, and

also in birds and reptiles. Some species inhabit the intestine, others form

aneurisms in the large blood-vessels, and cause considerable mortality

amongst horses; others live in the tracheae and lungs of cattle and sheep,

their presence often causing great loss to the farmer. No intermediate host

has been satisfactorily demonstrated; the larvae live in damp earth, and it

seems almost certain that they pass directly into their host with its food.

_Dochmius_ (_Ancylostomum_) _duodenalis_, called by Neumann[182] _Uncinaria

duodenalis_, is one of the most dangerous parasites that attack man. It

lives in the duodenum and jejunum, and the fertilised eggs leave the body

of its host with the excreta, and in damp earth develop into larvae in the

course of a few days. These at first eat voraciously, but after undergoing

several moults they cease to take food and pass into the resting stage. If

now they are swallowed with drinking water, they come to rest in the small

intestine of their host, and in a few weeks become sexually mature. They

cause great harm by burrowing in the intestinal walls and destroying the

capillaries. They are found by hundreds, and even thousands, in the same

host, and produce profound anaemia, which is frequently fatal to miners,

and was the cause of a great mortality amongst the workers in the St.

Gothard Tunnel some fifteen years ago. This species is very widely spread

over the face of the globe. _Dochmius trigonocephala_ Rud. and _D.

stenocephala_ produce similar diseases in dogs and cats, and _D. cernua_

Crep. is found in sheep and goats.

The genus _Cucullanus_ exists in the adult form in the intestines of

fishes, and more rarely of reptiles. _C. elegans_ Zed., which live in

fresh-water fish, _e.g._ the perch, is viviparous; after birth the young

pass into the water and make their way into the alimentary canal of the

small crustacean _Cyclops_, and thence into its body-cavity. Here they

undergo two moults, accompanied by certain changes in structure. If this

second host be swallowed by a fish the parasites are set free, and develop

generative organs. {144}_Ollulanus tricuspis_ Leuck., which in the adult

state is found in the cat, chiefly in the intestine but also in the bronchi

and other parts, gives rise to larvae which are of enormous size compared

with the parent; these leave the body, and if eaten by a mouse encyst in

its muscles, and if the mouse be devoured by a cat, they complete their

life-cycle by becoming sexually mature.

The genus _Syngamus_ infests the trachea and bronchi of birds, more rarely

of mammals. The _red-_ or _forked-worm_, _Syngamus trachealis_ Sieb., is

common in poultry and game birds, and causes the disease known as gapes,

which is especially common in young birds, and often gives rise to

extensive loss. The peculiarity of this genus is that the male is

permanently attached to the female, its genital bursa being so closely

adherent to the opening of the oviduct that two specimens cannot be

separated without tearing the tissues. The ova are not laid, but escape

from the body with fully-formed embryos in them, by the decay or rupture of

their parent's body. They hatch in damp earth or water in from one to six

weeks according to the temperature. When swallowed by a fowl they develop

into adults, which reproduce eggs in less than three weeks. No second host

is needed, but the embryos remain alive in the alimentary canal of

earthworms, and these doubtless to some extent serve to spread the disease.

[Illustration: FIG. 70.—_Syngamus trachealis_ Sieb., natural size and

magnified four diameters. The small ♂ is permanently attached to the

female. (From Warburton.[183])]

III. FAMILY TRICHOTRACHELIDAE.

This family is characterised by the anterior end of the body being produced

into a long whip-like neck. The mouth is small and devoid of papillae. The

oesophagus is very long, and it traverses a peculiar strand of cells.

Genera: _Trichocephalus_, _Trichina_, _Trichosoma_, and others.

{145}_Trichocephalus dispar_ Rud. (_hominis_ Gmel.) is common in man, and

also occurs in some species of monkey. It does not live freely within the

intestine, but buries its long whip-like anterior end in the mucous lining

of the caecum or colon. The eggs pass out of the body of the host. The

development of the embryo is slow, lasting many months; whilst still in the

egg-shell the embryos are swallowed, and give rise to the sexually-mature

parasite without the intervention of an intermediate host. They are by no

means uncommon. Davaine calculated that about 50 per cent of the

inhabitants of Paris were infested with them, but they give rise to little

disturbance, and only very occasionally cause serious harm. _T. affinis_

Rud. infests sheep; _T. crenatus_ Rud. the pig; _T. depressiusculus_ Rud.

the dog; and _T. unguiculatus_ Rud. the hare and rabbit.

[Illustration: FIG. 71.—_Trichocephalus dispar_ Rud., attached to part of

the human colon. × 2.]

The genus _Trichosoma_, with many species, is as a rule found in birds, but

it occurs also in mammals, as _T. plica_ Rud. in the bladder of the fox and

wolf, _T. felis cati_ in the bladder of the cat, _T. aerophilum_ Duj. in

the trachea of the fox and marten. The chief interest of this genus is

that, at any rate in _T. crassicauda_ Bel., which infests the rat, the

dwarf males live two, three, or four at a time within the uterus of the

female, a condition of things which recalls the similar arrangement found

in the Gephyrean _Bonellia_.

_Trichina spiralis_ is the cause of the well-known disease trichinosis,

which appears in two forms, intestinal and muscular, according to the

habitat of the parasite. The mature forms of {146}both sexes are found in

the intestine of man and many other mammals. They have been experimentally

developed in birds, though in the latter the larval forms have never been

observed. By keeping such cold-blooded animals as the salamander at a

constant temperature, Goujon and Legros succeeded in infecting them, but

the larvae perished as soon as the artificial heat was withdrawn. Muscular

trichinosis is unknown in fishes, but the sexual form develops in their

intestine.

The adult parasites of the intestine are scarcely visible to the naked eye;

the females are 3 to 4 mm. long and more numerous than the males, which

measure 1.4 to 1.6 mm. The eggs are very numerous, a single female

containing at one time 1200, and probably producing ten times as many

during her life. The embryos are hatched out within the uterus, and the

larvae leave the body of the mother through the generative pore. The minute

larvae bore through the intestinal walls of their host, and then, either

burrowing in the tissues or swept along in the stream of blood or lymph,

make their way all over the body, and come to rest most usually in the

muscles, but occasionally in other parts. When the larva reaches its

resting-place, it either pierces the sarcolemma and establishes itself

within the substance of the muscle-fibre, or it comes to rest between and

not in the fibres. Here its presence sets up the formation of a

spindle-shaped cyst which usually contains but one larva, though any number

up to seven have been found in one cyst. Within this the larva may remain

dormant for years, the walls of the cyst gradually undergoing a fatty or

calcareous degeneration. Almost any muscle may be affected; those most

usually infested being the muscles of the diaphragm, of the shoulder-blade,

and of the lumbar region; the larvae have also been found in the heart. The

ends of the muscles near their points of attachment are always the most

thoroughly infested.

[Illustration: FIG. 72.—_Trichina spiralis_ Owen, encysted in muscle. _a_,

Calcareous deposit. Highly magnified. (From Leuckart.)]

The number of the encapsuled larvae in one host is enormous. Leuckart

counted between 12,000 and 15,000 in a gramme of muscle, which would give a

total of thirty to forty million parasites in one host; other estimates

place the total even higher.

{147}When trichinised meat is eaten, unless it has been thoroughly cooked,

the cysts are dissolved and the larvae are set free. Within three or four

days they become sexually mature and their ova begin to segment. The males

after a time leave the body with the excreta and perish, whilst the larvae

of the new brood make their way into the tissues of the host.

Man usually acquires trichinosis by eating uncooked or improperly-cooked

pork, and the disease is so widely spread and of such a serious nature that

most civilised countries have adopted rigorous methods for the detection of

trichinised meat. The pigs either acquire the disease by eating uncooked

swine's flesh, which is frequently given them in the form of offal, or by

devouring rats, which are very susceptible to the disease.

IV. FAMILY FILARIIDAE.

Mouth with two lips, or without lips. Six oral papillae often present, and

sometimes a horny oral capsule. Four pre-anal pairs of papillae, and

sometimes an unpaired one as well. Two unequal spicula or a single one.

Genera: _Filaria_, _Ichthyonema_, _Hystrichis_, _Spiroptera_,

_Dispharagus_, and others.

The genus _Filaria_ is a very large one. Like _Ascaris_, it is confined to

Vertebrates, but usually lives in the tissues of the body and not in the

intestines. _F._ (_Dracunculus_) _medinensis_ Gmel., the guinea-worm, is

well known as a human parasite in hot countries; it also occurs in the

horse and dog. The female has an average length of 50 to 80 cm., but

gigantic forms with a length of 4 metres have been described. The

alimentary canal is degenerate. In adult females the body is completely

occupied by a uterus crowded with eggs and embryos, which can only escape

by the rupture of the mother's body, as the genital ducts have disappeared.

Its original home is tropical Asia and Africa, but it has been introduced

into South America with the negroes.

The female lives coiled up in the subcutaneous tissues, usually in those of

the legs. Its presence gives rise to painful tumours. When these break the

female protrudes, and may be withdrawn from the body by very carefully

rolling it round a stick or pencil. This must be done very slowly, a few

inches a day, as the rupture of the body sets free the contained embryos,

and may result in {148}the death of the host. The embryos normally bore

their way into the body of the fresh-water _Cyclops_, and are re-introduced

into their Vertebrate hosts with the drinking-water. It is usually stated

that the female alone is known, and that it is uncertain whether it is

hermaphrodite or whether both sexes are present in the _Cyclops_. Recently

Dr. Charles[184] has described a specimen found in the mesentery of a human

subject, from an orifice in the middle of whose body he was able to draw a

much smaller specimen, and he thinks this may be the long-sought-for male.

[Illustration: FIG. 73.—A, View of the heart of a dog infested with

_Filaria immitis_[185] Leidy; the right ventricle and base of the pulmonary

artery have been opened. _a_, Aorta; _b_, pulmonary artery; _c_, vena cava;

_d_, right ventricle; _e_, appendix of left auricle; _f_, appendix of right

auricle. B, A female _F. immitis_ removed from the heart to show its

length. Natural size.]

_Filaria immitis_ Leidy, the cruel worm, is common in dogs in China and the

East generally. It is not unknown in America and Europe. It occurs in such

large clusters in the right ventricle that it is difficult to see how the

circulation can proceed. The intermediate host is unknown, but from the

prevalence of the {149}disease in marshy country it is probably some

aquatic animal. The larvae are said by Manson to disappear from the

peripheral circulation of the dog during the day, but not to such a marked

extent as do the _F. sanguinis hominis_ Lew., var. _nocturna_ Man. They

were found by Galeb and Pourquier in the foetus of an infested bitch, a

fact which establishes the transmission of such parasites through the

placenta.

_Filaria sanguinis hominis nocturna._—The female of this parasite has been

described as living in the lymphatic glands of man. The embryos escape from

it into the lymph, and thus reach the blood. According to Manson the

intermediate host is the mosquito, in whose stomach the embryos undergo

their larval changes. When the mosquito dies the larvae escape into the

water, and then make their way into the alimentary canal of man, where they

are believed to pair, and whence the female makes its way to the

lymphatics. The presence of this _Filaria_ causes great functional

disturbance. One of the most remarkable features of it is that the larvae,

which are very numerous in the blood during the night, disappear during the

day, and are not to be found. Recently Manson[186] has described two new

varieties: _F. san. hom. diurna_, in which the conditions of things are

reversed, the larvae being found by day and not by night; and _F. san. hom.

perstans_, in which the larvae occur both by day and by night. The larvae

are long-lived, and were found by Manson in the blood of a negro who had

not been in Africa, where it is endemic, for six years. The same observer

is inclined to associate the presence of _F. san. hom. perstans_ with the

fatal disease known as "sleeping sickness." He also suggests that the

mature form of the variety _diurna_ is the _F. loa_, which is not uncommon

in the eyes of negroes, and that its intermediate host may be one of the

blood-sucking flies so common on the west coast of Africa.

The genus _Ichthyonema_ is confined to fishes. The male is very minute and

the female partly degenerate. It has no anus and no external opening to its

generative organs. The uterus fills up almost the whole of the body-cavity.

_I. sanguineum_ Rud. is found encapsuled in the peritoneum of many fish.

_Hystrichis_ and _Dispharagus_ are confined to birds, where they occur in

the oesophagus and stomach. _Spiroptera reticulata_ {150}Crep. occurs in

horses, twisted in a spiral round tendons and muscles, forming tumours

which require to be opened.

V. FAMILY MERMITHIDAE.

Nematodes without anus and with six mouth papillae. Two spicules in the

males and three rows of numerous papillae.

Genera: _Mermis_, _Bradynema_, _Atractonema_, _Allantonema_,

_Sphaerularia_, and others.

As a rule the Nematoda show but little trace of their parasitic mode of

life, but in this family there is considerable degeneration, and in extreme

cases the body of the female is reduced to a simple sac crowded with eggs.

They are exclusively parasitic in insects. In some respects their structure

shows a transition towards _Nectonema_ and the Gordiidae; especially is

this the case in the structure of their ventral nerve-cord.

The sexual form of _Mermis nigrescens_ Duj.[187] lives in damp earth, and

after storms and in the early morning is sometimes found in such numbers

crawling up the stalks of plants, as to give rise to the popular idea that

there has been a shower of worms. The male is unknown; the female lays her

eggs in the ground, and there they hatch out. It is not known exactly how

the larvae make their way into the grasshoppers in whose body-cavity they

live, but in an allied species, _M. albicans_ v. Sieb., the larvae have

been observed boring their way into small caterpillars through their skin,

and it seems probable that the larvae of _M. nigrescens_ burrow in a

similar way into young Orthoptera.

_Bradynema rigidum_ Leuck.[188] is found in the adult stage living freely

in the body-cavity of a small beetle _Aphodius fimetarius_, one of the

Scarabeidae, from two to three to as many as thirty being found in one

host, which does not seem much injured by their presence. The parasite is

without mouth, anus, or excretory pore. The eggs hatch out in the uterus of

the mother, and the larvae are male and female; they make their way into

the body-cavity of the host, and here they pass an unusually long time,

five months, soaking in osmotically the nutriment contained in the blood of

the insect. Eventually they burrow through the walls of {151}the intestine,

and leaving the body of their host through the anus, find their way to the

earth. Here, according to zur Strassen, the females die without playing any

part in the perpetuation of the species. The males, on the other hand,

having developed spermatozoa whilst in the larval stage (paedogenesis),

afterwards form ova, and are in fact protandrous hermaphrodites, and become

the mature parasites of the beetle, though how they enter the body of the

host is unknown.

[Illustration: FIG. 74.—_Allantonema mirabile_ Leuck. (From Leuckart.) A,

Male Rhabditis stage, sexually mature, × 100; B, the mature female

parasitic form, × 17, showing at the upper end part of the capsule richly

supplied with the tracheae of the host, a beetle; C, female Rhabditis

stage, sexually mature, × 100; D, the larva developed from the Rhabditis

form, × 102.]

The phenomenon presented by the hermaphroditism of _Bradynema_ is, as far

as we know, at present unique, as, though some other Nematodes are

hermaphrodite, in their case the hermaphrodite form alternates with a

bisexual generation. It is further interesting as showing a means by which

hermaphroditism may arise, by the suppression of the females and the

assumption of their functions by the male. In the case of _Rhabdonema

nigrovenosum_, no females appear in the alternate generation.

{152}[Illustration: FIG. 75.—_Atractonema gibbosum_ Leuck. (From Leuckart.)

1, Female with commencing prolapsus of the uterus and neighbouring parts, ×

130; 2, a further stage, the female being now sexually mature, × 15; 3, a

still older stage, with commencing degeneration of the body of the female,

× 15.]

A similar protandry exists in the parasitic forms of _Allantonema_,[189] of

which there are several species—_A. mirabile_ in _Hylobius pini_, _A.

sylvaticum_ in _Geotrupes sylvatica_, _A. diplogaster_ in _Tomicus

typographicus_; but in their case the male and female forms which leave

their host pair in the damp earth and give rise to larvae which make their

way into the body of the beetle-grubs. Here they undergo very extensive

retrogressive change. The body of the female, which becomes the shape of a

thick sausage, is encapsuled and surrounded by a curious hypertrophied

network of tracheae (Fig. 74). As is usually the case with the degenerate

parasitic forms, there are practically no organs but the ovary, and this is

{153}embedded in a fatty parenchyma which fills all the space within the

skin.

[Illustration: FIG. 76.—Four stages in the life-history of _Sphaerularia

bombi_ Dufour, ♀. (From Leuckart.) A, Beginning of the protrusion of the

uterus (_b_), × 66; B, later stage, × 66; C, later stage, × 12; D, the

protrusion is complete, × 6. In each case _a_ represents the Nematode, and

_b_ its protruded uterus.]

_Atractonema gibbosum_, which lives in the body-cavity of the larva of

_Cecidomyia pini_, has a similar life-history, but the parasitic form has a

structural peculiarity which merits attention (Fig. 75). At the time of

sexual maturity a swelling, which is caused by the prolapsus of the uterus

and vagina, appears at the posterior end of the body; this swelling

increases until it equals the rest of the body of the Nematode in size.

Even this is far surpassed by a similar protuberance in _Sphaerularia

bombi_, where the evaginated sac grows with such extreme rapidity that in a

few weeks its length increases from .25 mm. to 15 mm. and its volume

60,000-fold, the increase being due, according to Leuckart, to the increase

in size of the individual cells and not to their multiplication. The

Nematode which has produced this enormous growth gets relatively smaller

and smaller, and ultimately drops off (Fig. 76). The sexual larvae which

arise from the eggs in this sac leave the body of the bee in which this

species is parasitic by the anus, {154}and may live in damp earth, moss,

etc., for months without taking nourishment, until the autumn, when they

become sexually mature and, according to Leuckart, pair. The fertilised

female is believed to bore her way into the humble-bee whilst the latter is

seeking her underground winter quarters; this accounts for the fact that

only queen bees are infected. The parasite is widely distributed both in

Europe and North America; it is found in many species of _Bombus_, but most

frequently in _B. lapidarius_ and _B. terrestris_. The presence of the

_Sphaerularia_ affects the reproductive organs of the host, and reduces

their fertility, so that an infected queen bee never succeeds in forming a

colony.

VI. FAMILY ANGUILLULIDAE.

For the most part free living and of small size. The oesophagus has usually

a double swelling or two oesophageal bulbs. The male has two equal spicula.

Genera: _Diplogaster_, _Mononchus_, _Rhabditis_, _Tylenchus_, _Anguillula_,

and many others.

Many species of this family live in humus or decaying matter; others live

on, or are parasitic in, plants; some, such as _Anguillula aceti_, which is

found in vinegar and in paste, live in organic fluids.

The part played by the presence of these Nematodes in the soil is not

thoroughly understood; sometimes they occur in great numbers, and even when

not directly parasitic in plants, probably do them much damage. Cobb[190]

has recently described from Australia and Fiji over eighty species,

one-half of them new, which occur mostly in the earth, and many of them

among plant roots. They frequently crawl up on to plants, especially on to

seedlings. An instance of this is given as follows: "The edible part of

three bunches of nice-looking celery bought of a Chinaman in Sydney was cut

off as far up as it was tender, nearly to the first leaflets. It was washed

by hand in a tin dish in tank water, free from Nematodes. The washings gave

about 200 to 300 Nematodes, belonging to five different genera."

It is very probable that many of the free-living forms which have received

distinct specific names may ultimately turn out to be but stages in the

life-history of some of the parasitic species. {155}Von Linstow[191] has

pointed out that the free form of _A. diplogaster_, if found alone, would

be placed in the genus _Diplogaster_; similarly the bisexual form of

_Ascaris nigrovenosa_ is known as _Rhabditis nigrovenosa_.

Those Nematodes which live parasitically in plants, _e.g._ many of the

genera _Tylenchus_ and _Aphelenchus_ and _Heterodera_, as well as those

which only pierce the epidermis of the roots (the remaining species of the

above-named genera), are provided with a spine which works to and fro

through the mouth and assists the animal to bore into the tissues of the

plant. _Tylenchus devastatrix_ lives and reproduces in leaves and stems

(never in the roots, except in the case of hops[192]) of many cultivated

plants, such as rye, oats, onions, etc. "Clover sickness" is probably

caused by this Nematode. The plants become infected by the thread-worms in

the soil during the spring; their presence causes swellings and often kills

the plant, in which case the worms return to the soil or remain in the

straw.

_Tylenchus tritici_ Need. is the cause of "ear-cockles" in corn. These take

the form of brown or purple galls, which replace the grains of corn, and

which contain hundreds of minute Nematodes. In these galls they are

motionless, and are capable of surviving in dryness for at least twenty

years; but when moistened,—for instance, by the gall falling on damp

earth,—they resume their vitality and make their way to the young wheat

plants, and then, wriggling up the leaves and stems, find their way to the

ear. Here they pair, and producing a gall-like growth in the flower, lay

numerous eggs, from which arise the Nematodes of the ear-cockle.

[Illustration: FIG. 77.—A, _a_, Female _Heterodera schachtii_ Schmidt,

breaking through the epidermis of a root; the head is still embedded in the

parenchyma of the root: B, _a_, larvae boring their way into a root; _b_,

larva of the immobile kind surrounded by the old skin, living as an

ectoparasite on the outside of the root. (From Strubell.)]

_Heterodera schachtii_[193] Schmidt, is the cause of the "beet

{156}sickness," and forms galls or swellings on the roots of many plants,

in England especially on the roots of tomatoes and cucumbers. The free

larvae live in the earth and make their way into the smaller rootlets; here

the female larvae shed their skin, lose their characteristic Nematode form,

and become citron-shaped (Fig. 78, D). The male larvae undergo a change,

and after a period of rest cast their skin and, leaving the rootlet, seek

out the females. The female does not undergo this second ecdysis, but its

generative organs grow and mature in what is practically a larval stage.

The embryos develop within the body of the mother, and, escaping through

the uterus, ultimately cause her death. They then make their way into the

earth. The cycle of the development takes but four or five weeks, so that,

as in the case of _Tylenchus devastatrix_, there are several broods in a

year; _T. tritici_, on the other hand, has but one.

[Illustration: FIG. 78.—A, Male _Heterodera schachtii_ strongly magnified;

_a_, head lappets; _b_, mouth cavity; _c_, spine; _d_, muscle of spine;

_e_, gland; _f_, oesophagus; _g_, bulb; _h_, nerve-ring; _i_, excretory

pore; _j_, intestine; _k_, testis; _l_, intestine; _m_, muscles moving

spicule; _n_, spicule: B, first motile larva: C, second immovable parasitic

larva casting its skin: D, a female with one half of the body-wall taken

away to show the coiling generative organs; _a_, boring apparatus; _b_,

oesophageal bulb; _c_, excretory pore; _d_, alimentary canal; _e_, anus;

_f_, ovary: E, a male shortly before casting its larval skin.]

Vuillemin and Legrain[194] point out that while _Heterodera_ is injurious

to cultivated plants growing in damp soil, its presence is advantageous to

those that grow in deserts. It is very common in the Sahara, and attacks

many plants which are {157}immune from it elsewhere. It causes the rootlets

to swell out, and the bladder-like extensions thus formed act as reservoirs

for water.

Many other species attack plants; _Tylenchus millefolii_ Löw forms galls on

_Achillea_, _T. dipsaci_ Kühn. on the teazle. They all seem to have great

powers of resisting desiccation. The former species, when dried and placed

in a herbarium in May, gave rise to active worms when moistened the

following October; and the corn eel-worm is said to survive twenty-seven

years in a state of suspended animation. On the other hand, although these

Nematodes like moisture, they cannot withstand submersion in water for any

time. They can resist a considerable degree of cold, and a species,

_Aphelenchus nivalis_ Auriv.,[195] has been described from Spitzbergen,

where it lives in the snow amongst a small red alga, _Sphaerella nivalis_.

VII. FAMILY ENOPLIDAE.

Small, as a rule free-living, usually marine Nematodes, without a second

oesophageal bulb. Eyes and mouth-armature often present. Fine hairs and

bristles sometimes surround the mouth.

Genera: _Enoplus_, _Dorylaimus_, _Enchelidium_, and others.

The genus _Enoplus_ is exclusively marine, living amongst Algae and

Hydroids in shallow water and moving actively about, but never coiling into

spirals. De Man[196] describes _Enoplus brevis_ Bast. as being attacked by

a plant parasite, probably a Bacterium, of a greenish colour, which

infested the muscles and gave them a peculiar colour.

Numerous other species have been described by De Man from the coast of

Holland. It is probable that some of them are the free stages of parasitic

forms; a brackish water species found in the East Indies (_Dorylaimus

palustris_) is regarded by Carter as the larva of _Filaria medinensis_.

_Oncholaimus echini_ Leyd. is parasitic in the intestine of the sea-urchin

_Echinus esculentus_. _Tricoma cincta_[197] has a strongly striated

cuticle, which gives it almost the appearance of segmentation. _Fimbria

tenuis_ has numerous hairs on the tail, and the mouth is surrounded by

bristle-bearing papillae.

{158}Here must be mentioned two families closely allied to the true

Nematodes.

(i.) CHAETOSOMATIDAE.—This family includes three genera: _Chaetosoma_,

_Rhabdogaster_, and _Tristicochaeta_. According to Metschnikoff,[198]

although they are not true Nematodes, they have a great likeness to the

group. He distinguishes them from the swimming members of the group as

"creeping Nematoda." _Chaetosoma_, of which two species are known, _C.

ophicephalum_ and _C. claparedii_, has a head distinct from the body (Fig.

79). The mouth is at the anterior end, surrounded by a double semicircle of

movable spicules; the whole body is covered by fine hairs, and on the

ventral surface, just in front of the anus, is a double row of about

fifteen cylindrical projections, by whose agency the animal creeps. The

female _C. claparedii_ is 1.5 mm. long, the male 1.14 mm. They were found

creeping about on sea-weeds in the neighbourhood of Salerno.

[Illustration: FIG. 79.—Mature female of _Chaetosoma claparedii_ Metschni.,

× 57. (From Metschnikoff.) _a_, Oesophagus; _b_, intestine; _c_, anus; _d_,

ovary; _e_, generative pore; _f_, ventral bristles.]

The genus _Tristicochaeta_[199] differs from the foregoing in having three

rows of locomotor projections instead of two.

[Illustration: FIG. 80.—_Tristicochaeta inarimense_ Panceri, in one of its

most usual positions, showing the triple row of ventral bristles, × 100.

(From Panceri.)]

_Rhabdogaster_ has no head distinct from the body, though the anterior part

of the body is swollen. A second swelling occurs, as is also the case with

_Chaetosoma_, in the region of the opening of the genital ducts. The female

in _Rh. cygnoides_ attains a length of 0.36 mm. In this genus the hairs are

confined to {159}the dorsal middle line. The locomotor projections are

hooked, and are much finer than those of _Chaetosoma_, and they are

situated farther forward than in the last-named genus. _Rhabdogaster_

occurs in the same surroundings as _Chaetosoma_. _Ch. ophicephalum_ is

recorded from the English Channel.

(ii.) DESMOSCOLECIDAE.—The members of this family are minute, and are

characterised by the presence of well-marked ridges which surround the body

and give it an appearance of segmentation. The head, which is somewhat

swollen, bears four bristles, and single pairs are borne by a certain

number of the ridges, some on the dorsal and some on the ventral surface.

These hairs can be moved independently of one another. Two red eye-spots

are described between the fourth and fifth rings. The sexes are distinct,

and the internal organs generally have a marked resemblance to those of the

true Nematoda. The _Desmoscolecidae_ move by looping their bodies after the

manner of the Geometrid caterpillars, as well as by creeping with their

bristles. The genus contains numerous species[200]: _D. minutus_ Clap.

(English Channel), _D. nematoides_ Greef, _D. adelphus_ Greef, _D.

chaetogaster_ Greef, _D. elongatus_ Panceri, and _D. lanuginosa_ Panceri.

They are exclusively marine.

[Illustration: FIG. 81.—Female _Desmoscolex elongatus_ Panceri, ventral

view, × 260. _a_, Ovary. (From Panceri.)]

_Trichoderma oxycaudatum_ Greef[201] is a minute animal, 0.3 mm. long,

which has no head or ventral spines, but whose body is ringed and covered

with long hair-like bristles. The male has two spicules, and the internal

organisation recalls that of other Nematodes; still its ringed body has

induced some authorities to place it near to _Desmoscolex_.

THE LIFE-HISTORY OF NEMATODES.

Although, considering the enormous number of species of Nematodes and the

remarkable diversity of the conditions under {160}which they live, their

bodily structure shows a very striking uniformity, the same is by no means

the case with their life-history, which exhibits an astounding variety. Von

Linstow[202] has arranged the various modifications, which occur under

fourteen heads. He includes in his list the Gordian worms, which we have

placed under a different heading. The following account has been taken from

his paper, with a few alterations:—

1. The embryos develop, with a larval stage and without any change of

medium, directly into the mature sexual forms. They live in fresh,

brackish, or salt water, in plants, in the earth or in decaying organic

matter: examples, _Dorylaimus_, _Enoplus_, _Plectus_, _Monhystera_.

2. The larvae live in the earth, the sexual forms in plants: examples,

_Tylenchus tritici_ and _T. devastatrix_, _Heterodera schachtii_ (Figs. 77

and 78).

3. The larvae live in animals, after whose death and decay they are set

free and develop into the sexual animals in the earth: example, _Rhabditis

pellio_.

4. The bisexual forms live in the earth, and the fertilised females bore

into animals (insects), and here produce embryos: example, _Sphaerularia

bombi_ (Fig. 76).

5. The bisexual forms live in the earth; the females do not develop, but

the males make their way into Insects (Beetles), and becoming

hermaphrodite, develop ova which give rise to the bisexual form: example,

_Bradynema rigidum_.

6. The larvae live in the earth, the sexual form in Vertebrates: examples,

_Dochmius_, _Strongylus_.

7. The Nematode lives as a hermaphrodite in animals, the offspring of this,

by an alternation of generations, become sexual in the earth: example,

_Rhabdonema_ in Frog.

8. A bisexual free form gives origin to a bisexual parasitic form living in

an animal: example, _Leptodera appendiculata_ in Snails.

9. The eggs develop in the earth, and give rise to embryos which are

transferred whilst still in the egg-cell to the body of an animal. The

embryos hatch out and form bisexual parasites: examples, _Oxyuris_,

_Trichocephalus_.

10. The larvae live in insects, the sexual worms in water or in the earth:

example, _Mermis_.

11. The larva lives encapsuled and is passively transferred to {161}a

second animal: examples, _Ollulanus_, from Mouse to Cat; _Cucullanus

elegans_, from _Cyclops_ to Perch; _Spiroptera obtusa_, from Meal-worm to

Mouse.

12. The sexual form lives for a short time in the intestine of a

Vertebrate, and produces larvae which bore through the intestinal wall and

become encapsuled in the tissues: example, _Trichina spiralis_.

13. The sexual animal lives in the trachea of birds; the ova containing

embryos are coughed up and are taken into other birds with food. They quit

the egg-shell and wander into the air-sacs, and finally into the trachea:

example, _Syngamus_.

14. There are two larval forms; the first lives in water, the second in the

lungs of Amphibia, whence they wander into the intestine and become

sexually mature: example, _Nematoxys longicauda_ in _Triton alpestris_.

PARASITISM.

1. EFFECT OF PARASITISM ON THE PARASITE.—The usual effect of parasitism on

the parasitic organism is that the various organs necessary for a free life

tend to degenerate, whilst there is a multiplication and development of

organs of adhesion, by means of which the parasite maintains its hold on

its host. There is further an immense increase in the powers of

reproduction, which may take the form of an increase in the number of

fertilised eggs produced, or the parasite may at some time of its life

reproduce asexually, by budding, or fission, or parthenogetically.

Of the various classes of animals which are more or less parasitic, the

Nematodes show less difference between the free-living and parasitic

members of the group than obtains in any other class. With few exceptions,

such as _Sphaerularia_, _Allantonema_, and one or two others, the parasitic

forms have undergone but little degeneration. It is true that they have no

eyes such as the free forms often possess, but in other respects, such as

in the nervous, muscular, and digestive systems, they do not show any

marked retrogression; further, the mouth-armature is developed in many free

forms, and is not confined to the parasites.

The group has developed no methods of asexual reproduction by budding or

fission, such as are found in Platyhelminthes; and the cases of an

alternation of generations in which a sexual form alternates with a

parthenogenetic form, are rare, _e.g._ {162}_Rhabdonema nigrovenosum_; and

it seems possible that even when parthenogenesis has been described,

further observation may show that the parthenogenetic stage is really a

protandrous hermaphrodite, in which case the alternation of generations in

Nematodes, _i.e._ the hermaphrodite alternating with the dioecious form, is

a case of _heterogamy_ or the alternation of two sexual generations.

On the other hand, parasitic Nematodes produce enormous numbers of eggs.

Van Beneden states that 60,000,000 have been computed in a single Nematode,

and this multiplication of ova is absolutely necessary, for the chance of

the embryo reaching the right host, in which alone it can develop, is

always a small one.

It is a common thing to find that parasites are either hermaphrodite or

that the male is degenerate, as is the case with many of the parasitic

Crustacea, but with one or two exceptions the Nematoda are bisexual, and

although, as a rule, the males are smaller than the females, they show no

other trace of degeneracy.

In spite of the fact that the class as a whole shows but few special

modifications consequent on a parasitic mode of life, it is clear that the

Nematoda are peculiarly adapted for such a mode of life. Their elongated

thread-like bodies afford little resistance to the passage of the food,

which, as it passes through the intestine of the host, might tend to carry

the parasites out of the body. At the same time their shape enables them to

pierce and wriggle through the various tissues without making any very

serious lesions such as might prove fatal to their host. Their

extraordinary power of resisting desiccation both in the egg and in the

adult state vastly increases their chances of ultimately hitting on the

right host. They are capable of living in a state of suspended animation

for months, and even years when dried (_vide_ p. 136), and of resuming

their activity on being moistened.

The great faculty this group shows for living parasitically is evinced by

the extraordinary variety of life-history presented by the different

species. There is scarcely a stage which may not be parasitic; the eggs,

the larvae, the adults are all in some cases free, in others parasitic, and

in many cases first the one and then the other.

2. OCCURRENCE AND EFFECT OF THE PARASITE ON THE HOST.—Von Linstow states

that the only law that can be derived inductively from the study of the

life-history of Nematodes is that those that live in animals never pass

through all their stages of development in the same organ; consequently, in

considering the distribution of {163}the parasites within the body of their

host we have a double habitat to consider. Many forms, such as _Trichina

spiralis_, wander from the intestine to the muscles; others, such as

_Filaria medinensis_, from the alimentary canal to the lymphatics or blood

vessels or subcutaneous tissues. Others pass from the body-cavity to the

intestine, as the Mermithidae, which infest Insects, or from the stem and

leaves of a plant to its flower, as in the case of _Tylenchus tritici_.

With regard to their occurrence in the different classes of the animal

kingdom, they have been most frequently observed in Vertebrates and in

Insects. They are comparatively rare in the other large divisions. Many

genera are confined to certain hosts: thus _Ascaris_, _Filaria_,

_Trichosoma_ occur only in Vertebrates; _Spiroptera_ (with one exception)

in Mammals and Birds; _Cucullanus_ in Fishes and Amphibia; _Strongylus_ and

_Physaloptera_ in Mammals, Birds, and Reptiles; _Dochmius_, _Pseudalius_,

_Trichocephalus_ in Mammals; _Dispharagus_, _Hystrichis_, _Syngamus_ in

Birds; _Nematoxys_, _Hedruris_ in Amphibia and Reptiles; _Ichthyonema_ in

Fishes; and _Isacis_ and _Mermis_ in Insects.

Twenty-two species have been described as parasitic in man, of which

perhaps the most dangerous are _Filaria medinensis_, the three varieties of

_F. sanguinis hominis_; _Dochmius_ (_Ancylostomum_) _duodenalis_, and

_Trichina spiralis_. The Ascaridae, as _Ascaris lumbricoides_ and _Oxyuris

vermicularis_, though painful, seldom cause death.

The enormous number of parasites harboured by one host is shown by the fact

mentioned in Leuckart's _Parasites of Man_, that Nathusius[203] took from a

single black stork 24 specimens of _Filaria labiata_ from the lungs, 16

_Syngamus trachealis_ from the trachea, more than 100 _Spiroptera alata_

from the coats of the stomach, besides several hundred Trematodes belonging

to several different species (see p. 63). Even this has been surpassed in

the case of a young horse, in whose body Krause found 500 _Ascaris

megalocephala_, 190 _Oxyuris curvula_, several millions of _Strongylus

tetracanthus_, 214 _Sclerostomum armatum_, 287 _Filaria papillosa_, 69

_Taenia perfoliata_, and 6 _Cysticercus_ forms.

It is impossible here to enter into a full description of the destruction

caused to domesticated animals and crops by the presence of these

parasites; full details will be found in books dealing {164}especially with

this question, such as Neumann's _Parasites and Parasitic Diseases of

Domesticated Animals_. A couple of cases will show how important this

matter is to the farmer. Crisp estimates that _Syngamus trachealis_ causes

the death of half a million pullets in England every year, and Mégnin

states that in a single pheasantry 1200 victims died daily; again, the loss

of one-third the crop of beetroot is by no means uncommon when it is

infested with _Heterodera schachtii_. These show the practical importance

of what at first sight seem quite insignificant animals, and the necessity

for the minutest observation, for only when we are fully acquainted with

all the details of the life-history of a parasite are we in a position to

successfully combat it.

SUB-ORDER II. NEMATOMORPHA.

Until the last few years it has been customary to regard the Gordiidae as a

family of Nematodes. Although in external appearance and life-history they

closely resemble the members of this group, yet recent research has shown

so many important morphological differences between them and the Nematoda,

that most zoologists are now agreed in placing them in a different

sub-Order, the Nematomorpha, a name first suggested by Vejdovsky.[204]

[Illustration: FIG. 82.—A water plant around which a female _Gordius_ is

twining and laying eggs. _a_, _a_, Clump and string of eggs. (From von

Linstow.[205])]

The Gordiidae comprise but two genera, _Gordius_ and _Nectonema_. The

latter has but one species, _N. agile_ Verr., and is marine; the former, on

the other hand, is exclusively fresh-water, and contains a very large

number of species. Gordian worms are frequently to be found in ditches,

ponds, or large puddles, moving with an undulating motion through the

water, or twining and writhing round water-plants; they are scarcer in

running water. In shape they are like a piece of thin whip-cord, slightly

tapering {165}at each end; the male, however, is easily distinguished from

the female by its forked tail (Fig. 89). Not unfrequently a considerable

number are found inextricably tangled together into a knot, and the name of

the genus refers to this fact. Where numbers have suddenly appeared in

water hitherto free from them, legends have sprung up which attribute their

presence to a rain of worms; in reality they have come out of the bodies of

Insects in which they are parasitic for the greater part of their life.

The genus _Gordius_ passes through three distinct stages, of which the

first two are larval and parasitic; the third is sexually mature and lives

in water. The second larval stage closely resembles the adult, but the

reproductive organs are not developed. The following account of the

structure of this larval form and of the adult is in the main taken from

von Linstow.[206]

The whole body is covered with a well-developed two-layered cuticle, which

in the adult is marked out into areas, and bears numerous minute sensory

bristles, which are especially developed in the neighbourhood of the cloaca

of the male. Beneath this is a hypodermis which differs markedly from the

sub-cuticle of Nematodes, inasmuch as it consists of a single layer of

polygonal nucleated cells. Within this lies a single layer of longitudinal

muscle-cells, which differ from the corresponding layer of Nematodes in

having that part of their medulla which is not surrounded by the

contractile portion directed outwards towards the hypodermis, and not

inwards towards the body-cavity.

[Illustration: FIG. 83.—Transverse section through a young male _Gordius

tolosanus_ Duj. (From von Linstow.) Highly magnified. _a_, Cuticle; _b_,

hypodermis; _c_, muscular layer; _d_, parenchyma; _e_, alimentary canal;

_f_, nervous system; _g_, cells of the testis.]

The body is in the younger stages practically solid, the interior being

filled with clearly defined polygonal cells which are arranged in definite

rows; in later life certain splits arise in this tissue which {166}subserve

various functions; between these splits strands of tissue are left which

form mesenteries, and some of the cells remain lining the muscular layer

(Fig. 86). These cells have been described by Vejdovsky as a definite

somatic, peritoneal epithelium, but this was not found by von Linstow.

Besides forming the mesenteries, and acting as packing between the various

organs of the body, these cells also form the ova and the spermatozoa.

The splits which have appeared when the animal has reached the second

larval stage, are two dorsal and a ventral; the latter contains the

alimentary canal, and may be termed the body-cavity, the former will

develop the generative organs. The mouth is occluded in the older larvae,

and in the adults there is a distinct but solid oesophagus which passes

into a tubular intestine. The intestine consists of a single layer of cells

surrounding a lumen; it runs straight to the hinder end of the body, where

it opens in both sexes with the ducts of the reproductive organs.

The nervous system consists of a well-defined circumoesophageal ring with

two dorsal swellings, and, arising from this, a median ventral cord which

runs the whole length of the body. The cord consists of three longitudinal

strands with ganglionic cells below them; the latter, though they lie

within the muscle layer, maintain a connexion with the hypodermis. Behind,

the nerve-cord splits in the male, one half passing into each caudal fork.

In the adult a pair of black eyes can be detected on the head; the only

other sense organs are the tactile bristles mentioned above. Excretory

organs are unknown.

[Illustration: FIG. 84.—Section through a young female _Gordius tolosanus_.

(From von Linstow.) _a_, Cuticle; _b_, hypodermis; _c_, muscular layer;

_d_, parenchyma; _e_, alimentary canal; _f_, nervous system; _g_, egg-sac;

_h_, ovary.]

The generative organs only attain maturity in the adult, which is, in fact,

exclusively devoted to reproduction. No trace of testes {167}is found in

the larva, though the two dorsal splits from the walls of which the

spermatozoa will arise are present. They are lined by a definite epithelium

(Fig. 83), and this serves at once to distinguish them from the

body-cavity. Posteriorly the splits narrow and become the two vasa

deferentia which open one on each side into the cloaca. The cells lining

the lumen give rise to secondary cells, and these become spermatozoa, the

process extending from behind forwards. The external organs—bursa,

etc.—described by Vejdovsky were not found by von Linstow.

[Illustration: FIG. 85.—Section through a mature female _Gordius

tolosanus_. (From von Linstow.) Lettering as in Fig. 84; _g_, egg-sac; _h_,

ovary.]

[Illustration: FIG. 86.—Section through a female _Gordius tolosanus_ when

the deposition of ova is almost complete. _a_, _b_, _c_, _d_, _e_, and _f_,

as in Fig. 84; _g_, egg-sac; _h_, ovary almost empty; _i_, dorsal canal

containing eggs; _j_, receptaculum seminis.]

In the female larva two similar splits are present; these form the

egg-sacs. Posteriorly they end in two short oviducts which open into a

uterus, in which fertilisation takes place, and in which the secretion

arises which cements the eggs together. In the adult the ovaries and a

receptaculum seminis are found, in addition to the organs present in the

larva. The ovaries are formed from modifications of the packing tissue;

they begin close behind the head, and soon attain such dimensions as to

compress the egg-sacs and body-cavity to small slits. After a time the wall

between the ovary and the egg-sacs becomes absorbed, and the eggs grow into

the latter. In the old females, where the egg sacs are empty, there is a

considerable space round the exhausted ovary, into which eggs continue to

fall off; there {168}is also a median dorsal canal which contains a few

eggs. By this time the wall between the ovary and the egg-sac has again

appeared.

One of the most interesting points about the female is that, according to

Vejdovsky, the ovary is segmented, the cells which form the ova being

heaped up in segmentally-arranged masses. This observation, if correct, is

almost the only instance of segmentation recorded in the group

Nemathelminthes.

[Illustration: FIG. 87.—_Nectonema agile_ Verrill. A, The adult. Magnified.

(After Fewkes.) B, Longitudinal section through the head. × about 20. (From

Bürger.) _a_, Mouth; _b_, circumoesophageal commissure (dorsal); _c_, cell

of salivary gland; _d_, septum cutting off head from rest of body; _e_,

testis; _f_, ventral cord; _g_, oesophageal cells; _h_, lumen of

oesophagus; _i_, cerebral ganglion (ventral).]

The only other genus which is associated with _Gordius_ in the group

Nematomorpha is _Nectonema_, of which there is as yet but one species

known, _Nectonema agile_ Verr.[207] Our knowledge of the anatomy of this

worm is due mainly to Bürger[208] and Ward.[209] _Nectonema_ is a marine

worm found swimming near the surface of the sea with rapid undulatory

motion. The males are from 50 to 200 mm. long, the females from 30 to 60

mm. The body is faintly ringed, and bears two rows of fine bristles on each

side. Owing to a curious torsion of the body through a right angle, the

lateral bristles of the anterior third seem to be placed in the ventral and

dorsal middle line. They are very easily broken off. The body is divided

into a small anterior and a large posterior {169}chamber by a transverse

septum placed a little way behind the head. The anterior chamber contains

the brain and is lined by a definite epithelium, the posterior is not. The

layers of the skin correspond with those of Nematodes or of _Gordius_, but

the hypodermal cells show no cell outlines; still they are not so modified

as in the former group. The hypodermis is thickened in the median dorsal

and ventral line, and the single nerve-cord lies in the latter.

The alimentary canal is degenerate, as in _Gordius_. A mouth exists, but it

is minute, and opens into a very fine tube lined with chitin, which pierces

through the substance of a single elongated cell. This minute oesophagus,

with its coextensive cell, reaches back to the transverse partition, but

behind this a few other cells become associated with it, and ultimately the

lumen of the alimentary canal is surrounded by four cells; but the number

diminishes behind, and soon only two cells surround the tube at any one

level, and the intestine dwindles away some little distance in front of the

tail. There is no sign of an anus. A circumoesophageal nerve-ring exists,

of which the ventral part is by far the larger (Fig. 87); it gives off a

ventral nerve-cord, which swells posteriorly in the male into a large anal

ganglion, far bigger than the brain, and larger in the male than in the

female.

The testes consist of a dorsally placed sac, continuous behind with a vas

deferens; this opens at the posterior end, which is pointed and slightly

curved ventrally. The ovary is unknown; but females have been found with

their body-cavity crammed with ova; these escape, like the spermatozoa,

from a genital pore at the posterior end of the body.

CLASSIFICATION.—The separation of the Nematomorpha from the Nematoda

depends mainly on the character of the nervous system, the absence of the

lateral lines and of the dorsal line, the character of the contents of the

body-cavity, and the character of the reproductive organs. In Gordiidae the

latter are always placed dorsal to the intestine, and ovaries and testes

open alike at the hinder end of the body. The importance of the differences

in the organs just enumerated has been considered sufficient to justify the

removal of the Gordiidae from the Nematoda, and the establishment of the

special sub-Order Nematomorpha for their reception; and although

_Nectonema_ has a dorsal line, and is in some other respects intermediate

between the two groups, there can be little doubt that it is more closely

allied to _Gordius_ than to any member {170}of the Nematoda, and it must

therefore be placed with it in the Nematomorpha.

On the other hand, it ought to be mentioned that Camerano[210] found that

the chief details of the fertilisation and development of the egg in

_Gordius_ closely conform with what is known of the same processes in

Nematodes, and he is of opinion that these resemblances are sufficiently

important to justify the retention of the group among the Nematoda.

LIFE-HISTORY.—The life-history of _Gordius_ comprises four stages—the early

development of the egg, the first larval form, the second larval form, and

the sexually mature form. Both larval forms are parasitic, and during their

life they are actively engaged in feeding; the free form, on the other

hand, takes in no nourishment, and is exclusively engaged in reproduction.

[Illustration: FIG. 88.—Abdomen of _Pterostichus niger_ with the terga

removed to expose the _Gordius_ larva within. Slightly magnified. (From von

Linstow.)]

Von Linstow[211] gives the following account of the life-history of _G.

tolosanus_, a form which has been more fully worked out than any other. In

the month of April numerous specimens of the beetle _Pterostichus niger_

were found floating on the surface of the ditches and small ponds in the

fields surrounding Göttingen. Some were found dead or dying; others

appeared quite healthy, and these were swimming actively, endeavouring to

reach land. Within the abdomen of these beetles, in about 20 per cent of

those collected, the second larval form of the _G. tolosanus_ was found.

The longest larvae were 122 mm. in length, and very soft, partly snow-white

and partly brown in colour; traces of the boring apparatus of the first

larval form were still to be seen, but in other respects the larva only

differed from the free form in the immaturity of its sexual organs. Besides

the parasite hardly anything was to be found in the abdomen of the beetle,

the larva having eaten up all trace of the fat body and the generative

organs of its host. The larvae bored their way out of the body of the

beetle and became adult animals.

It is rather difficult to say what brings these essentially

{171}terrestrial beetles to the water, but von Linstow suggests that, as

they live partly on snails, and at this time of year there are not many

land-snails about, they may be in search of water-snails such as _Limnaea_.

They may also be sometimes blown into the water by wind storms, but,

whatever the cause is, their presence in water is essential for the

continuance of the life of their parasites.

Once free in the water the _Gordius_ is soon sexually mature; the

fertilisation takes place in April, and then the female may be seen

twisting and writhing round the stems of water-plants and laying the long

bead-like strands of eggs (Fig. 82). The first deposition observed by von

Linstow took place on 14th April, the last on 2nd August, and the period of

egg-laying for each female extended over four weeks. At first the eggs are

snow-white, but within twenty-four hours they turn brown in colour.

The development of the first larva within the egg takes about a month. When

it emerges from the egg-shell it is minute, .065 mm. long, ringed

anteriorly, and provided with a protrusible and retractile boring apparatus

consisting of three chitinous rods; round the base of this piercing

proboscis is a double crown of papillae, each bearing a spine (Fig. 90).

[Illustration: FIG. 89.—The tail ends of a female _Gordius_ (_a_) and a

male (_b_) _in copula_. × 1.5. (From G. Meissner.[212])]

This first larval form breaks through the egg-shell and sinks to the bottom

of the water, where it moves about sluggishly and awaits the arrival of the

right host in which to take up its abode. This host is the larva of the

Alder-fly, _Sialis lutaria_ Lin. (_vide_ vol. v. p. 444), and into this it

bores and comes to rest in the muscles or the fat body. It does not form

distinct capsules. It remains in this larva during the following winter,

and in the spring passes over into the imago _Sialis_. The complete insect

frequents the small plants growing along the water's edge, and falls an

easy prey to the predaceous beetle _Pt. niger_. The larva is eaten, and

undergoing a change becomes the second larval form mentioned above. It

remains in the body of the beetle during the second winter, and finally

returns to the water {172}as the adult some eighteen or twenty months after

it has been hatched from the egg.

[Illustration: FIG. 90.—Embryo or first larval form of _Gordius tolosanus_

taken from the egg. Highly magnified. _a_ and _b_, The bristle-bearing

papillae on the head; _c_, the boring apparatus. (From von Linstow.)]

From the above account of the life-history of _Gordius_ it will be seen

that the chances of an egg reaching maturity are comparatively small, and

to compensate for this a very large number of eggs are laid. In addition to

the risk of the larvae not finding the right host at the right time, and of

the first host not being eaten by the second, and the second not being

drowned, there is the danger that the ditches and ponds in which the adults

live may dry up, and, in fact, great numbers of worms perish by this taking

place.

The sex of the adults may be told from their colour, the males being of a

blackish brown, the females of a light clay brown; the former average 120

mm. in length, the latter 170 mm. The males are also more numerous, the

proportion being seven to three. Camerano[213] has drawn attention to the

fact that there is a certain polymorphism in size, form, and colour which

is especially common amongst the males; dwarf forms with mature

reproductive organs exist, and he is of opinion that these differences

depend both on the size of the second host and on the duration of the

parasitic life.

In addition to the larva of _Sialis lutaria_, the first larval stage has

also been found in the larva of _Ephemera_, _Tanypus_, _Corethra_, and

_Chironomus_; the second in _Carabus hortensis_ Fabr., _Procerus_

(_Carabus_) _coriaceus_ Linn., _Calathus fuscipes_ Goeze, _Molops elatus_

Fabr., several species of _Pterostichus_, and a number of other beetles. It

is probable that its normal hosts are _S. lutaria_ and _Pt. niger_, but it

is clear that it often comes to rest in other insects. The view that the

Gordiidae have no special hosts, but may either pass the whole of their

life-history within one and the same animal, or, on the other hand, may

inhabit animals belonging to very different groups, is held by Villot, who

has paid great attention to the subject. He finds the first larval form

encysted in the walls of the alimentary canal in fishes, such {173}as

_Leuciscus phoxinus_, the minnow, _Cobitis barbatula_, the loach, and

_Petromyzon planeri_, the lamprey; in the larvae of Diptera, _Ephemera_,

and beetles, in _Planorbis_ (a water snail), in _Enchytraeus_ (an

Oligochaet); the second larval form in all kinds of insects, spiders,

Crustacea, fish, frogs, birds (_Otis_), and in man, and these various

habitats lead him to the conclusion that "Les Gordiens n'ont pas d'hôtes

spéciaux." On the other hand, as von Linstow points out, it is contrary to

our knowledge of parasites that a single species should develop equally

well in the body of warm and cold-blooded Vertebrates and of Insects, and

the explanation of the presence of the larvae in these various forms may

either be that they belong to different species of _Gordius_ or, more

probably, that they are accidentally present, having passed into their

hosts with drinking water.

[Illustration: FIG. 91.—Tarsal joint of an Ephemerid larva into which two

_Gordius_ larvae (_a_, _a_) have penetrated. Magnified. (From G.

Meissner.)]

The number of species of _Gordius_ is large; over 100 are enumerated in the

_Compendium der Helminthologie_,[214] the great majority of which inhabit

insects.

The life-history of _Nectonema_ is practically unknown; the adults have

been found swimming near the surface of the sea at two places only:

Newport, R.I., and Wood's Holl, Mass., on the south coast of New England.

It has been fished close to the shore, from the end of June to the

beginning of October, when the tide is going out at evening and there is no

moon. This seems to indicate that it avoids the light. When first caught

the worms move actively about, coiling themselves into figures of eight and

then uncoiling; at the same time there is a rhythmical movement caused by

waves of muscular contraction passing down each side of the body

alternately; by this kind of motion they make rapid and definite progress

through the water.

It seems probable that the adult _Nectonema_ is preceded by one or more

larval stages, and what appears to be a young form has {174}been obtained

from the thoracic cavity of a prawn, _Palaemonetes_,[215] which has thus

some claim to be regarded as the host of this species, but nothing is known

about its early life-history.

SUB-ORDER III. ACANTHOCEPHALA.

The Acanthocephala, which form the third class of the Nemathelminthes,

consists of but few genera; there are, however, numerous species of very

different size, varying from 10 to 65 cm. long in the female

_Gigantorhynchus_ (_Echinorhynchus_) _gigas_, to quite minute forms a few

millimetres in length. The adult stage occurs in the alimentary canal of

Vertebrates, as a rule in those which live in, or frequent water; the

larvae are found in the bodies of certain Invertebrates, very frequently

small Crustacea.

[Illustration: FIG. 92.—Two specimens of _Echinorhynchus proteus_

Westrumb., with their anterior ends embedded in the wall of the intestine

of a Pike. Magnified with a lens. (From Hamann.)]

ANATOMY.—The body of the mature forms can usually be divided into three

sections—the proboscis, the neck, and the trunk, but the middle region is

not always discernible. The proboscis is armed with rings of hooks (Fig.

93) arranged in longitudinal rows; they are usually of two kinds, but in

_E. proteus_ of three. They have a certain specific value, but not much

stress can be laid on the number of rings, _e.g._ in _E. angustatus_ the

number varies from eight to twenty-four. The recurved hooks serve to fasten

the parasite very firmly to the tissues of the host. The proboscis is

hollow and retractile; it can be withdrawn into the body by means of

muscles attached internally to its tip. It does not, however, pass straight

into the body-cavity, but is retracted into a special cavity—the proboscis

sheath—with a double muscular wall. The proboscis sheath may perhaps be

looked upon as a septum, such as is found in some of the Nematomorpha,

dividing the body-cavity into two parts. It is inserted into the body-wall

at the junction of the neck and trunk or of the proboscis and trunk. In

addition to the muscles which withdraw the proboscis into its sheath, there

are two retractors running from the {175}outside of the sheath to the

body-wall; these serve to retract the whole sheath and its contents into

the body-cavity of the trunk.

The structure of the skin is essentially like that of Nematodes, but the

details are much more complicated. The whole body is covered by a thin

cuticle secreted by the epidermis, which, as in the other groups, breaks

down and forms a syncytium called the sub-cuticle. The minute fibrils which

penetrate this layer are much more definitely arranged than in Nematodes;

the largest of them run from without inwards, others run concentrically

round the body. Large oval or spherical nuclei are scattered in the

sub-cuticle, which is further honeycombed by a number of lacunae or spaces

which are described below.

[Illustration: FIG. 93.—A, Five specimens of _Echinorhynchus acus_ Rud.

attached to a piece of intestinal wall, × 4; B, the proboscis of one still

more highly magnified.]

Within the sub-cuticular layer is found a sheath of circularly-arranged

muscle-fibres, and within this again a sheath of longitudinal muscles which

do not extend into the proboscis; this inner layer lines the body-cavity,

there being no epithelium within it. In their minute structure the

muscle-cells resemble those of Nematodes.

The canals in the sub-cuticle form a very curious system of anastomosing

spaces, in which a clear fluid containing fat globules circulates. The

extent to which the system is developed varies in different species, but in

all there is a pair of longitudinal canals which are situated laterally,

and which give off the subsidiary channels in their course. The above

description applies to the lacunar spaces in the skin of the trunk; those

of the proboscis are quite distinct, and there is no communication between

the two sets of spaces; in fact, the sub-cuticle in which the lacunae are

formed is not continuous across the line of junction of the proboscis and

the neck, or, when the latter is absent, of the proboscis and the trunk,

but it is interrupted by the ingrowth of a thin ring of cuticle which

reaches down to the muscular layers (Fig. 94).

{176}[Illustration: FIG. 94.—A longitudinal section through the anterior

end of _Echinorhynchus haeruca_ Rud. (From Hamann.) _a_, The proboscis not

fully expanded; _b_, proboscis-sheath; _c_, retractor muscles of the

proboscis; _d_, cerebral ganglion; _e_, retinaculum enclosing a nerve; _f_,

one of the retractors of the sheath; _g_, a lemniscus; _h_, one of the

spaces in the sub-cuticular tissue; _i_, longitudinal muscular layer; _j_,

circular muscular layer; _k_, line of division between the sub-cuticular

tissue of the trunk and that of the proboscis with the lemnisci.]

All the spaces in the skin of the proboscis open ultimately into a circular

canal situated round its base; on each side the canal opens into a sac-like

structure which extends through the body-cavity towards the posterior end

of the animal. These two lateral diverticula are termed the _lemnisci_.

They have always attracted considerable attention from the workers at the

group, and numerous functions have from time to time been attributed to

them. They are more or less hollow, and their walls consist of

sub-cuticular tissue surrounded with a scanty muscular coat; they contain

the same fluid as the lacunae of the skin of the proboscis, with which they

are placed in communication by means of the circular canal; and it seems

most probable that, as Hamann[216] suggests, they act as reservoirs into

which the lacunar fluid retires when the proboscis is retracted, {177}and

which, by means of the contractions of their muscular coat, force the fluid

into the lacunae when the proboscis is everted, and thus aid in its

protrusion.

The parasitic habits of _Echinorhynchus_ have had a deeper influence on the

structure of the body than is the case with the Nematoda. All traces of an

alimentary canal have disappeared, and the animals live entirely by the

imbibition through the skin of the already elaborated fluids of their

hosts. The power of absorbing fluids is shown by the fact that they swell

up and become tense when placed in fresh water.

Until recently no definite excretory organs had been recognised, and the

function of excreting the nitrogenous matter was by some assigned to the

lemnisci. In 1893 Kaiser[217] described in _G. gigas_ two organs which he

called nephridia, placed dorsally to the ducts of the male and female

reproductive organs. Each nephridium, which somewhat resembles a

cauliflower, consists of a stalk or duct, opening at one end into the

reproductive ducts, and at the other branching and breaking up into a

number of secondary and tertiary twigs. The end of each twig is closed by a

membrane pierced with a number of most minute pores, by means of which it

communicates with the body-cavity; on the inner side the membrane bears a

number of long cilia, which keep up an active flickering. The presence of

these cilia is interesting, as elsewhere they are unknown throughout the

Nemathelminthes.

[Illustration: FIG. 95.—A, A longitudinal section through the terminal

twigs of the nephridium of _Gigantorhynchus gigas_. (From J. E. Kaiser.)

Highly magnified. _a_, Nucleus. B, A terminal twig more highly magnified;

_b_, the porous membrane.]

The nervous system consists of a central ganglion situated in the proboscis

sheath; it is oval and flattened in shape. The ganglion gives off nerves to

the proboscis, and two main trunks which pierce the proboscis-sheath and

run backward surrounded by a cluster of muscle-fibres, the whole being

termed the {178}_retinaculum_; in the male they are in connexion with a

special genital ganglion which lies near the ductus ejaculatorius.

With the exception of certain sensory papillae in the neighbourhood of the

male genital orifice, and of three similar papillae mentioned by Kaiser on

the proboscis, the Acanthocephala are devoid of sense organs.

The Acanthocephala are dioecious; their generative organs are developed in

connexion with the _ligament_, a cord-like structure which arises between

the inner and outer layer of the hinder end of the proboscis sheath and

traverses the body-cavity, ending posteriorly in connexion with the genital

ducts. The testes lie in this ligament; they are paired oval bodies which

open each into a vas deferens. The vasa deferentia each bear three lateral

diverticula, the vesiculae seminales; and three pairs of cement glands pour

their secretion into a duct which opens into the vasa deferentia; the

latter unite and open by a penis which is withdrawn into a genital bursa,

but is capable of being extruded.

[Illustration: FIG. 96.—An optical section through a male _Neorhynchus

clavaeceps_ Zed. (From Hamann.) _a_, Proboscis; _b_, proboscis sheath; _c_,

retractor of the proboscis; _d_, cerebral ganglion; _f_, _f_, retractors of

the proboscis sheath; _g_, _g_, lemnisci, each with two giant nuclei; _h_,

space in sub-cuticular layer of the skin; _l_, ligament; _m_, _m_, testes;

_o_, glands on vas deferens; _p_, giant nucleus in skin; _q_, opening of

vas deferens.]

The two ovaries are formed in the ligament of the female in a corresponding

position to that occupied by the testes in the male, but at an early stage

they break down into packets of cells, of which those of the peripheral

layer develop into ova at the cost of the central cells, which serve them

as a food supply. As these masses grow and increase in number they rupture

the walls of the ligament, and escape into the body-cavity, in which they

float. The ova are {179}fertilised whilst floating in the fluid of the

body-cavity. The eggs segment and the embryo is formed whilst still in the

body of the mother.

The embryos escape by means of a complicated apparatus the details of which

vary in the different species, but which, like many of the organs in these

animals, consists of very few cells with very large nuclei. This apparatus

consists of three parts: the bell, the uterus, and the oviduct. The bell is

a large funnel-shaped structure, which opens into the body-cavity, and is

connected with the end of the ligament; near its lower end, where it is

continuous with the uterus, is a second smaller opening situated dorsally.

By the contraction and expansion of its lips the oval embryos are swallowed

and pass on through the uterus to the oviduct, which opens at the posterior

end of the body. If the bell takes in any of the less mature eggs which are

spherical in shape, they are passed back into the body-cavity through the

above-mentioned dorsal opening, and the same orifice permits the passage of

the spermatozoa even when the bell is full of embryos.

[Illustration: FIG. 97.—An egg of _Echinorhynchus acus_ Rud. surrounded by

three egg-shells. Highly magnified. The egg has segmented, and the cells

are differentiated into _a_, the entoblast, and _b_, the ectoblast; _c_,

spines. (From Hamann.)]

EMBRYOLOGY.—After fertilisation the egg surrounds itself with several

egg-shells, three of which are usually distinguished; the embryo is already

far advanced in its development by the time it leaves the body of the

mother and passes out into the alimentary canal of the Vertebrate host. It

leaves the body of this second host with the faeces, and is eaten by the

first or larval host, usually a small Crustacean or water-insect, but in

some cases a fish, within whose alimentary canal it casts its membranes and

{180}becomes actively mobile. By means of a ring of hooks developed round

the anterior end it bores its way through the wall of the alimentary canal,

and after some time—three weeks in _E. proteus_—comes to rest in the

body-cavity of its host. By this time most of the organs of the adult, with

the exception of the reproductive glands, are already well established; the

latter only attain maturity when the first host is eaten by the second, and

the larvae find themselves in the intestine of a Vertebrate.

[Illustration: FIG. 98.—A, A larval _Echinorhynchus proteus_ Westrumb.

further developed than in Fig. 97. Highly magnified. The entoblast has

developed inside it the proboscis _a_; _b_, _b_, the giant nuclei of the

ectoblast. B, The entoblast at a more advanced stage, the ectoblast is not

shown. The outermost layer of cells will form the muscles of the body-wall;

the body-cavity has appeared; _a_, proboscis; _b_, cerebral ganglion; _c_,

body-cavity; _d_, _d_, the testes beginning to appear in the ligament; _e_,

cells which will form the generative ducts.]

Some of the details of the development are very remarkable, and a short

account of them may be given. The segmentation of the egg is unequal; it

results in the formation of a central biscuit-shaped mass of small cells

and a peripheral mass of larger cells; the former is called by Hamann[218]

the entoblast, the latter the ectoblast. From the entoblast arise all the

organs of the body but the sub-cuticle and the associated lemnisci, which

are formed from the ectoblast. The latter has a remarkable history; the

cells begin to break down and lose their outlines, whilst their nuclei fuse

together and form a small number of giant nuclei, which lie scattered

throughout the syncytium thus formed. The syncytium surrounds the entoblast

on all sides; by this time the anteriorly-placed hooks have appeared; in

_E. proteus_ there {181}are ten of these, but the number is not the same in

all species. The syncytium is in a fluid state, with a few gigantic nuclei

floating in it; these now lose their spherical shape, and throwing out

processes become amoeboid; in this way they bud off small portions of their

substance, and from these the oval nuclei of the sub-cuticle and the

lemnisci arise. The rest of the syncytium hardens into the fibrillar matrix

of the sub-cuticle, leaving, however, scattered spaces which form the

sub-cuticular sinuses of the adult. An interesting feature of _N.

clavaeceps_ and _Arhynchus hemignathi_ is that the skin of the adult

retains the larval features, and it and the lemnisci consist of a syncytium

with a very few giant nuclei scattered through it. Hamann counted only

eight in the skin and two in each lemniscus in the example figured on p.

178.

[Illustration: FIG. 99.—A, The larva of _Echinorhynchus proteus_ from the

body-cavity of _Phoxinus laevis_, with the proboscis retracted and the

whole still enclosed in a capsule. B, A section through the same; _a_, the

invaginated proboscis; _b_, proboscis sheath; _c_, beginning of the neck;

_d_, lemniscus. Highly magnified. (Both from Hamann.)]

The whole of the rest of the body is formed by the entoblast. Within the

latter a circular split arises which separates a single layer of outermost

cells from an axial strand of many cells (Fig. 98, B). The split is the

future body-cavity; the axial strand forms the proboscis, its sheath, the

cerebral ganglion, muscles, etc., and the ligament with the contained

generative organs; the outermost layer of cells forms the muscular lining

to the skin. It is interesting to note that these cells destined to become

muscle-fibres are at first arranged as a single layer of cubical epithelial

cells lining the body-cavity; most of them become circular muscle-fibres,

but a few are pushed inwards so as to lie next the body-cavity, and these

become the longitudinal fibres.

CLASSIFICATION.—Until recently the Acanthocephala were supposed to include

but one genus, _Echinorhynchus_, with several hundred species, but

Hamann[219] has pointed out that these species {182}present differences

which enabled him to divide the group into three families, each with a

corresponding genus. To these I have ventured to add a fourth family, to

include a remarkable species, _Arhynchus hemignathi_, described below. The

characters of the first three families in the account given below are taken

from Hamann's paper.

[Illustration: FIG. 100.—Fully formed larva of _Echinorhynchus proteus_

from the body-cavity of _Phoxinus laevis_. (From Hamann.) Highly magnified.

_a_, Proboscis; _b_, bulla; _c_, neck; _d_, trunk; _e_, _e_, lemnisci.]

FAMILY I. ECHINORHYNCHIDAE.—The body is elongated and smooth. The

proboscis-sheath has a double wall, and the proboscis is invaginated into

it. The central nerve-ganglion lies in the middle line, as a rule on the

posterior blind end of the proboscis-sheath. The papillae which bear the

hooks are only covered with a chitinous cap at their apex, and the hooks

have a process below. This family is by far the largest; a few species only

can be mentioned. _Echinorhynchus proteus_ lives in its mature form in

fishes; the young forms, up to a centimetre in length, are found living

freely in the intestine of numerous fresh-water fishes. Those found in

_Gobio fluviatilis_, the gudgeon; _Leuciscus virgo_; _Lota vulgaris_, the

burbot or eel-pout; young trout; _Thymallus vulgaris_, the grayling, seldom

surpass this size, but those found in _Acerina cernua_, the pope fish; in

_Abramis bipunctatus_; in _Esox lucius_,the pike, and in older trout,

attain or surpass double the length. As the parasites grow older they bury

their proboscis and neck in the wall of the intestine, the inner surface of

which is studded with the orange-coloured bodies of the parasites. The

proboscis is so deeply sunk in the wall of the alimentary canal as to form

a papilla on its outer surface (Fig. 92). The larvae of _E. proteus_ are

found in the body-cavity of _Gammarus pulex_, one of the Amphipod

Crustacea, and also in the same position in numerous fresh-water fishes;

they must have passed into this first host by the {183}mouth and alimentary

canal. If the liver of an infested minnow, _Leuciscus phoxinus_, be

examined, it will be found to contain on its surface numerous spherical or

egg-shaped capsules of an orange colour, 2 to 2.5 mm. in length; these

contain the larval forms of the parasite. They develop into the adult form

when the first host is eaten by a carnivorous fish, but a complication may

take place when the larval form is found in _Gammarus_, as the latter, the

first host, may be eaten by a fish (intermediate host) in which the larva

does not become mature, and only develops sexual organs when eaten by a

carnivorous fish (second host). The larval form is also found in

_Nemachilus barbatulus_, _Gobio fluviatilis_, and the sticklebacks

_Gasterosteus aculeatus_ and _G. pungitius_.

_E. clavula_ Duj. is found in _Salmo fario_, _Abramis brama_, _Cyprinus

carpio_, _Gobius niger_, _Lepadogaster gouanii_, etc.; _E. linstowi_ Ham.

in _Leuciscus idus_, _Abramis ballerus_, _Abramis bipunctatus_, and

_Acipenser huso_; _E. lutzii_ Ham. was found by Dr. Lutz in Brazil in the

intestine of _Bufo agua_; _E. angustatus_ Rud. occurs in such numbers in

the perch, _Perca fluviatilis_, as to almost occlude the lumen of the

intestine, and one out of every three or four fish in certain districts is

infested by it. It is also found in the pike, _Esox lucius_, and the

barbel, _Barbus vulgaris_. The first or larval host of this species is the

Isopod _Asellus aquaticus_. _E. moniliformis_ Brews. is stated to attain

maturity in the human intestine. Except for the fact that _G. gigas_ has

once been observed in the same place, this is the only human parasite

amongst the Acanthocephala. Its normal second hosts are _Mus decumanus_ and

_Myoxus quercinus_, and its first or larval host, the larvae of the beetle

_Blaps mucronata_. _E. porrigens_ Rud. is found in considerable numbers in

the small intestine of a fin-whale (_Balaenoptera sibbaldii_), and _E.

strumosus_ Rud., in the small intestine of a seal (_Phoca vitulina_), and

in the body-cavity of the angler fish (_Lophius piscatorius_). _E. acus_ is

common in the whiting, _Gadus merlangus_.

FAMILY II. GIGANTORHYNCHIDAE.—Large forms with ringed, flattened, and

_Taenia_-like bodies. The hook-papillae are covered all over with

transparent chitinous sheaths with two root-like processes. The

proboscis-sheath is muscular and without a lumen. The central nervous

system is excentrically placed below the middle of the so-called sheath.

The lemnisci are long twisted tubes with a central canal.

{184}Hamann places three species in this family: _Gigantorhynchus

echinodiscus_, _G. spira_, and _G. taenioides_; but as he points out that

_E. gigas_ resembles these in its more important structural features, it

seems advisable to include it here under the name _G. gigas_. The members

of the first family often present a transversely ringed appearance after

death, but the Gigantorhynchidae are ringed when alive, and the circular

canals in the skin show a certain regularity, being arranged one between

each two rings. There is no lumen in the proboscis-sheath, which is not

attached to the boundary between the proboscis and the trunk, but to the

inner surface of the proboscis, and the whole can be retracted within the

anterior portion of the body, which is invaginable. There are always eight

cement-glands, and other differences exist in the musculature, hooks, and

position of the nervous system.

_G. gigas_ occurs in the adult state in the small intestine of swine; in

Europe its first or larval host is believed to be the grubs of _Melolontha

vulgaris_ and _Cetonia aurata_, but these beetles are absent from America,

though the parasite infests American hogs. Stiles[220] has recently made

some experiments which tend to show that in the United States the source of

infection is some species of the beetle _Lachnosterna_, and he has

succeeded in infecting the grub of _L. arcuata_ by feeding it on the eggs

of the parasite; from one larva he took 300 parasites six weeks after

feeding it. _L. arcuata_ is, like _M. vulgaris_, phytophagous, but the

grubs of both the beetles are fond of frequenting manure heaps and patches

of dung, and thus are much exposed to the dangers of infection.

_G. echinodiscus_ inhabits the intestine of ant-eaters, having been found

in _Myrmecophaga jubata_ and _Cycloturus didactylus_. _G. spira_ lives in

the king vulture _Sarcorhampus papa_, and _G. taenioides_ in _Dicholophus

cristatus_, a species of Cariama.

FAMILY III. NEORHYNCHIDAE.—Sexual maturity is reached in the larval stage.

The proboscis-sheath has a single wall. A few giant nuclei only are found

in the sub-cuticle and in the lemnisci. The circular muscle layer is very

simply developed. The longitudinal muscle-cells are only present in certain

places.

This family includes two species, _Neorhynchus clavaeceps_ and _N. agilis_,

which afford interesting examples of paedogenesis. The sub-cuticle and the

lemnisci are dominated by a few giant {185}nuclei, which remain in the

embryonic state and do not break up into numerous nuclei as in other forms.

The musculature is but little developed and the longitudinal sheath hardly

exists. The proboscis-sheath consists of a simple muscular layer, and the

short proboscis has few hooks and presents an embryonic appearance.

The sexually-mature form lives in the carp, _Cyprinus carpio_; the larval

form is found, according to Villot,[221] encysted in the fat bodies of the

larva of _Sialis lutaria_, one of the Neuroptera, and in the alimentary

canal of the leech _Nephelis octocula_, and successful experiments have

been made in infecting some species of the water snail _Limnaea_. _N.

agilis_ occurs in _Mugil auratus_ and _M. cephalus_.

FAMILY IV. ARHYNCHIDAE.—Short forms with the body divided into three

well-marked regions—head, collar, and trunk. The head is pitted, the collar

smooth, and the trunk wrinkled, not annulated, in spirit specimens. There

is no eversible introvert, and no introvert sheath and no hooks. The

sub-cuticle and the lemnisci have a few giant nuclei, and the lemnisci are

long and coiled.[222]

This family resembles the Gigantorhynchidae in the length and curvature of

its lemnisci, and the Neorhynchidae in the persistence of the embryonic

condition of the nuclei in the sub-cuticle and the lemnisci; but in the

shape of the body, its division into three well-marked regions, the absence

of eversible proboscis, proboscis sheath, and hooks it stands alone, though

it is nearer to the Neorhynchidae than to either of the other families.

The single species _Arhynchus hemignathi_ was found attached to the skin

around the anus of a Sandwich Island bird, _Hemignathus proceros_. The bird

is a member of a family Drepanididae, which is entirely confined to the

Sandwich Island group. Professor Newton tells me that it is probable that

the "food of _Hemignathus_ consists entirely of insects which it finds in

or under the bark of trees," hence it is probable that the second host of

this parasite, if such exists, must be looked for amongst the Insecta.

{186}CHAPTER VII

CHAETOGNATHA

STRUCTURE—REPRODUCTION—HABITS—FOOD—CLASSIFICATION TABLE OF IDENTIFICATION

At certain seasons and at certain times of the day the naturalist who is

investigating the fauna of the surface of the sea is apt to find his

tow-net crammed with innumerable transparent spindle-shaped animals, which

by their number and the way in which they become entangled with rarer

objects, often render useless the result of his labours. These animals

belong to the class Chaetognatha, which includes three genera, _Sagitta_,

_Spadella_, and _Krohnia_. Amongst them are divided about twenty species,

some of which, however, are of doubtful value.