in Fig. 371, at _a_). If examined with a low power soon after the eggs
are deposited, they appear to consist simply of a pellucid protoplasmic
substance. In about twenty-four hours a very minute yellowish spot,
the nucleus, is discovered near the cell-wall. In another twenty-four
hours the nucleus referred to is seen to have assumed a somewhat deeper
colour and to contain within it a minute spot--a nucleolus.
On the fourth day the nucleus has changed its position, and is
enlarged to double the size; a slightly magnified view is seen at _b_.
On a closer examination a tranverse fissure is seen; this on the eighth
day divides the small mass as at _c_, and the outer wall is thickened.
The embryo becomes detached from the side of the cell, and moves with
a rotatory motion around the interior; the direction of this motion
is from the right to the left, and is always increased when sunlight
falls upon it. The increase is gradual up to the eighteenth day, when
the changes are more distinctly visible, and the ova crowd down to
the mouth of the ova-sac, as at _d_. By employing a higher magnifying
power a minute black spec, the future eye (_e_) and tentacles of the
snail, is quite visible. Upon closely observing it, a fringe of cilia
is noticed in motion near the edge of the shell. It is now apparent
that the rotatory motion first observed must have been in a great
measure due to this; and the current kept up in the fluid contents of
the cell by the ciliary fringes. For days after the young animal has
escaped from the egg, this ciliary motion is carried on, not alone by
the fringe surrounding the mouth, but by cilia entirely surrounding the
tentacles themselves, which whips up a supply of nourishment, and at
the same time aeration of the blood is effected. From the twenty-sixth
to the twenty-eighth day it appears actively engaged near the side of
the egg, using force to break through the cell-wall, which at length
it succeeds in accomplishing; leaving its shell in the ova-sac, and
immediately attaching itself to the side of the glass its ciliary
action recommences, and it appears to have advanced a stage, as at _f_.
It is still some months before the embryo grows to the perfect form,
Fig. 372; the animal is here shown with its sucker-like foot adhering
closely to the glass of the aquarium. A single snail will deposit from
two to three of these ova-sacs a week, producing, in the course of six
weeks or two months, from 900 to 1,000 young.
[Illustration:
Fig. 372.--_Limnæus stagnalis_ (natural size).]
The shell itself is deposited in minute cells, which take up a circular
position around the axis; on its under-surface a hyaline membrane is
secreted. The integument expands, and at various points an internal
colouring-matter or pigment is deposited. The increase of the animal
goes on until the expanded foot is formed, the outer edge of which
is rounded off and turned over by condensed tissue in the form of a
twisted wire; this encloses a network of small vessels filled with a
fluid in constant and rapid motion. The course of the blood or fluid,
as it passes from the heart, may be traced through the larger branches
to the respiratory organs, consisting of branchial-fringes placed
near the mouth; the blood may also be seen returning through other
vessels. The heart, a strong muscular apparatus, is pear-shaped, and
enclosed within a pericardium or extremely thin and pellucid enveloping
membrane. The heart is seen to be furnished with muscular bands of
considerable strength, the action of which appears like the alternate
to-and-fro motion occasioned by drawing out a band of indiarubber, and
which, although so minute, are clearly analogous to the muscular fibres
of the mammal heart; it beats or contracts at the rate of about sixty
times a minute, and is placed rather far back in the body, towards the
axis of the shell. The nervous system is made up of ganglia, or nervous
centres, and distributed throughout the various portions of the body.
The singular arrangement of the eye cannot be omitted; it appears at an
early stage of life to be within the tentacle, and consequently capable
of being retracted into it. In the adult animal the eye is situated at
the base of the tentacle; and although it can be protruded at pleasure
for a short distance, it seems to depend much upon the tentacle for
protection as a coverlid--it invariably draws down the tentacle over
the eye when that organ needs protection. The eye itself is pyriform,
somewhat resembling the round figure of the human eye-ball, with its
optic-nerve attached. In colour it is very dark, having a central
pupillary-opening for the admission of light. The tentacle, which is
cylindrical in the young animal, becomes flat and triangular in shape
in the adult. The tentacles serve in some respect to distinguish
species. In Limnœa they are, as I have said, compressed and triangular,
with the eyes at their inner base. In Physa they are cylindrical and
slender and without lateral mantle lobes. The development of the
lingual membrane is delayed; consequently, the young animal does not
early take to a vegetable sustenance: in place of teeth it has two rows
of cilia, as before stated, which drop off when the teeth are fully
formed. The lingual band bearing the teeth, or the “tongue,” as it is
termed, consists of several rows of cutting spines, pointed with silica.
It is a fact of some interest, physiologically, to know that if the
young animal is kept in fresh water alone, without vegetable matter
of any kind, it retains its cilia, and arrest of development follows,
and it more slowly acquires gastric teeth, and attains to perfection
in form or size. If, at the same time, it is confined within a narrow
cell or space, it grows only to such a size as will enable it to move
about freely; thus it is made to adapt itself to the necessities
of a restricted state of existence. Some young animals in a narrow
glass-cell, at the end of six months, were alive and well; the cilia
were seen to be retained around the tentacles in constant activity,
whilst other animals of the same brood and age, placed in a situation
favourable to growth, attained their full size, and produced young,
which grew in three weeks to the size of their elder relations.[71]
My experimental investigations were further extended to the development
of the lingual membrane, or teeth, of Gastropoda, as well as the jaw
and radula. In Limnœa, the teeth when fully developed resemble those
of Helix; that is to say, in the fully grown animal are found several
rows or bands of similar teeth, with simple obtuse cusps and a much
suppressed central tooth. In the young snail a high power of the
microscope is required to make them out. The dental band, however,
in most Mollusca is disposed in longitudinal series, but varies a
good deal in this respect, as will be seen on reference to my several
papers, with illustrations of upwards of a hundred different species,
published in “Linnæan Transactions” of 1866, and in the “Microscopical
Society’s Transactions” of 1868. By way of example I may say, in the
Pulmonata the lingual band usually consists of a single median row, the
laterals on each side being broad and similar. But in many other groups
the teeth are arranged in three, five, or seven dissimilar series.
Taking Nerita as a type, the broad teeth on each side of the median
are termed _laterals_; and the numerous small teeth on the outside of
the band, known as the _pleuræ_, are termed _uncini_.
Since the investigations of Lovén into the lingual dentition of the
Mollusca, various observers have studied the subject, with great
advantage to our knowledge of the affinities of these animals. That
these investigations have proved of value is shown by the light which
has been shed on the true position of many species. When once we have
ascertained the homology of a genus, whose relations were otherwise
somewhat doubtful, it is surprising how other characteristics, even
of the shell, probably misunderstood before, concur to bear out
the affinities indicated by the lingual band. These tooth-bearing
membranes, armed with sharp cutting points, admirably adapted for the
division of the food on which they feed, are most of them beautiful
objects for the microscope.
[Illustration: Fig. 373.
1. Palate of _Buccinum undatum_, common Whelk, seen under polarised
light; 2. Palate of _Doris tuberculata_, Sea-slug.]
The two ends of each longitudinal row of teeth are connected with
muscles attached to the upper and lower surfaces of cartilaginous
cushions; the alternate contractions and extensions of the muscles
cause the bands of teeth to work backwards and forwards, after the
fashion of a chain-saw, or rather of a rasp, upon any substance to
which it is applied, and the resulting wear and tear of the anterior
teeth are made good by a development of new teeth in the secreting
sac in which the hinder end of the band is lodged. Besides the
chain-saw-like motion of the band the lingual membrane has a kind of
licking or scraping action as a whole. With the constant growth of the
band new teeth are developed, when the teeth on the extreme portion of
the band differ much in size and form from those in the median line.
As I have shown in the papers already referred to, that as each row
is a repetition of the first, the arrangement of teeth admits of easy
representation by a numerical formula, in which, when the uncini are
very numerous, they are indicated by the sign ∞ (infinity), and the
others by the proper figure. Thus, ∞ · 5 · 1 · 5 · ∞, which, in the
genus Trochus, signifies that each row consists of one median, flanked
on both sides by five lateral teeth, and these again by a large number
of uncini. When only three areas are found, the outer ones must be
considered the pleuræ, inasmuch as there is frequently a manifest
division in the membrane between them and the lateral areas.
Most of the Cephalopod molluscs are provided with well-developed
teeth, and they are, as we know, carnivorous. The teeth of the
cuttle-fish, _Sepia officinalis_ (Plate V., No. 111), resemble those
of the Pteropoda, and have the same formula, 3 · 1 · 3. Sepia are also
furnished with a retractile proboscis, and a prehensile spiny collar,
apparently for the purpose of seizing and holding prey while the teeth
are tearing it to pieces. In the squid Loligo (Plate V., No. 113) the
median teeth are broad at the base, approach the tricuspid form with
a prolonged acute central cusp, while the uncini are much prolonged
and slightly curved. The lingual band increases in breadth towards
the base, sometimes to twice that of the anterior portion. This band,
mounted dry, forms an attractive object for black-ground illumination.
In another family, that of the rock-limpet, _Patella radiata_, the
lingual band (Plate V., No. 116) well serves to distinguish it from
the better-known common limpet. It is furnished with a remarkable long
ribbon, studded by numerous rows of strong dark-brown tricuspid teeth.
The lingual membrane when not in use lies folded up in the abdominal
cavity. The teeth of Acmæa are somewhat differently arranged (Plate V.,
No. 117); their formula is 3 · 1 · 3.
_Testacella maugei_, belonging to Pulmonifera, is slug-like in
appearance, and subterranean in its habits, chiefly feeding on
earth-worms. During winter and in dry weather it forms a kind of
cocoon, and thus completely encloses itself in an opaque white mantle;
in this way it protects itself from frost and cold. Its lingual
membrane is large, and covered with about fifty rows of divergent
teeth, gradually diminishing in size towards the median row; each
tooth is barbed and pointed, broader towards the base, and with an
articulating nipple set in the basement membrane. A few rows are
represented slightly magnified (Plate V., No. 121). Their formula is 0
0 · 1 · 0 0.
[Illustration: TONGUES, ETC., OF GASTEROPODS.
Tuffen West, del. W. F. Maples, ad. nat. del. Edmund Evans.
PLATE V.]
The boat-shell, _Cymba olla_, belonging to the Velutinidæ, formula 0 ·
1 · 0, or 1 · 1 · 1. The lingual band (Plate V., No. 118) is narrow and
ribbon-like in its appearance, with numerous trident-shaped teeth set
on a strong muscular membrane. The end of the band and its connection
with the muscles at the extremity of the cartilaginous cushion is shown
in the drawing. The blueish appearance is produced by a selenite film
and polarised light. In _Scapander ligniarius_ the band (Plate V.,
No. 119) is also narrow, but the teeth are bold and of extraordinary
size; their formula is 1 · 0 · 1. This mollusc is said to be eyeless.
_Pleurobranchus plumula_ belongs to the same family; its teeth are
simple, recurved, and convex, and arranged in numerous divergent rows,
the medians of which are largest. The mandible (Plate V., No. 122)
presents an exceedingly pretty tesselated appearance, and the numerous
divergent rows of teeth are tricuspid.
The velvety-shell, _Velutina lævigata_, formula 3 · 1 · 3. The teeth
(Plate V., No. 108) are small and fine; medians recurved, with a series
of delicate denticulations on either side of the central cusp, which is
much prolonged: 1st laterals, denticulate, with outer cusp prolonged;
2nd and 3rd laterals, simple curved or hooked-shaped. The mandible
(No. 109), divided in the centre, forms two plates of divergent
denticulations.
The ear-shell, _Haliotis tuberculatus_, is a well-known beautiful
shell, much used for ornamental purposes. The lingual band (Plate V.,
No. 114), is well developed. The medians are flattened-out, recurved
obtuse teeth; 1st laterals, trapezoidal or beam-like; uncini numerous,
about sixty, denticulate, the few first pairs prolonged into strong
pointed cusps.
The top-shell, _Turbo marmoratus_. After the outer layer of shell is
removed, it presents a delicate pearly appearance. Its lingual band
(No. 123) closely resembles Trochus; it is long and narrow, the median
teeth are broadest, with five recurved laterals, and numerous rows of
uncini, slender and hooked. A single row only is represented in the
plate.
_Cyclotus translucidus_, a family of operculate land-shells, belongs
to the Cyclostomatidæ. The teeth shown in No. 110, formula 3 · 1 · 3,
are arranged in slightly divergent rows on a narrow band; they are more
or less subquadrate, recurved, with their central cusps prolonged.
_Cistula catenata_, one of the family Cyclophoridæ; its band (No. 115)
formula, 2 · 1 · 2. Its teeth resemble those of Littorina. The lingual
band of Cyclostomatidæ points out a near alliance to the Trochidæ;
but this question can only be determined by an examination of several
species, when it may, perhaps, be decided to give them rank as a
sub-order. They are numerous enough; the West Indian islands alone
furnish 200 species.
The length of the lingual band, and number of rows of teeth borne on
it, vary greatly in different species. But it is among the Pulmonifera
we meet with the most astonishing instances of large numbers of teeth.
_Limax maximus_ possesses 26,800, distributed through 180 rows of 160
each, the individual teeth measuring only one 10,000th of an inch.
_Helix pomatia_ has 21,000, and its comparatively dwarfed congener, _H.
absoluta_, no less than 15,000.
_Structure of the Shell of Mollusca._--In my opening sketch of the
sub-order Mollusca an idea may have been gathered of the general
character of the shell covering of these animals. The simplest form of
shell occurs in the rudimentary oval plate of the common slug, _Limax
rufus_. It is embedded in the shield situated at the back, near the
head of the animal. In the Chitons, a small but singular group of
molluscs allied to the univalve limpets, we have an ovoid shell, made
up of eight segments, or movable plates, which give them a resemblance
to enormous woodlice. These have been regarded as forming a transition
series--a link between one division and the other. The shell in by
far the greater portion of all the molluscs is developed from cells
that in process of growth have become hardened by the deposition
of calcareous matter in the interior. This earthy matter consists
principally of calcium carbonate deposited in a crystalline state; and
in certain shells, as in that of the oyster (Plate XVIII., Fig. 8),
from the animal cell not having sufficiently controlled the mode
of deposition of the earth particles, they have assumed the form of
perfect rhomboidal crystals.[72]
[Illustration: PLATE XVIII.
SECTIONS OF SHELL-STRUCTURE.]
The shell of the wing-shells, _Pinna ingens_ (Plate XVIII., No. 7),
is composed of hexagonal cells, filled with partially translucent
calcareous matter, the outer layer of which can be split up into
prism-like columns. Figs. 3 and 6 are horizontal sections of the
_Haliotis splendens_, with stellate pigment in a portion of the
section, and wavy lines, as in the dentine of the human tooth, and of
_Terebratulata rubicuna_, showing radiating perforations. Nos. 4 and
5, sections of the shell of a crab, show pigment granules beneath the
articular layer and the general hexagonal structure of the next layer.
Some difference of opinion has been expressed with regard to the
formation of pearls, but it is now generally understood to be a
diseased condition. Pearls are matured on a nucleus, consisting of the
same matter as that from which the new layers of shell proceed at the
edge of the mussel or oyster. The finest kinds are formed in the body
of the animal, or originate in the pearly-looking part of the shell. It
is from the size, roundness, and brilliancy of pearls that their value
is estimated.
The microscope discloses a difference in the structure of pearls: those
having a prismatic cellular structure have a brown horny nucleus,
surrounded by small imperfectly-formed prismatic cells; there is
also a ring of horny matter, followed by other prisms, and so on, as
represented in Fig. 374; and all transverse sections of pearls from
oysters show the same successive rings of growth or deposit.
[Illustration: Fig. 374.
1. A transverse section of a Pearl from Oyster, showing its prismatic
structure 2. A transverse section of another Pearl, showing its
central cellular structure, with outside rings of true pearly matter.
(Magnified 50 diameters.)]
In a segment of a transverse section of a small purple pearl from a
species of Mytilus (Fig. 375), all trace of prismatic structure has
disappeared, and only a series of fine curved or radiating lines is
seen. This pearl consists of a beautiful purple-coloured series of
regular laminæ, many of which have a series of concentric zones, and
are of a yellow tint. The most beautiful sections for microscopic
examination are obtained from Scotch pearls.
_Preparation of the Teeth and Shell of Mollusca for Microscopical
Examination._--The method of preparing lingual membranes of Mollusca
is as follows: Under a dissecting microscope, and with a large bull’s
eye lens, cut open and expose to view the floor of the mouth; pin back
the cut edges throughout its length, and work out the dental band with
knife and forceps. The band being detached, place it in a watch-glass,
and boil in caustic potash solution for a few minutes. Having by this
process freed the tongue from its integuments, remove it, wash it well,
and place it for a short time in a dilute acid solution, either acetic
or hydrochloric. Wash it well and float it upon a slide; with a fine
sable brush open it out flat, and remove whatever dirt or fibre may be
adhering to it. Lastly, place it in weak spirit and water, and there
let it remain for a few days before mounting in formalin. Canada balsam
renders them rather too pellucid, and the finer teeth are thereby lost.
[Illustration: Fig. 375.
1. Transverse section of a small Pearl from a Mytilus; 2. Horizontal
section magnified 240 diameters to show prismatic structure and
transverse striæ.]
The preparation of shell structure must be proceeded with with some
amount of care and caution, or the delicate reticulated network
membrane will be destroyed. If any acid solvent be used to remove the
calcareous structure it should be much diluted, so that the action
may proceed slowly rather than hastily. In the young hermit-crab, for
example, where the calcareous and membranous portions of the shell
are continuous, and the calcium carbonate in a relatively small
proportion, a strong acid solution would entirely destroy the specimen.
In the case of nacreous shells the process of cutting and grinding
must also be proceeded with with some amount of caution. The operation
should be examined as the process proceeds, and under polarised light.
Sections of shell structure are usually mounted in Canada balsam. Under
the heading _Technique_ much useful information on this and kindred
subjects will be found in the “Journal of the Royal Microscopical
Society.”
Annulosa, Worms, and Entozoa.
The Annulosa of Huxley embraces the lowest grade of articulated
animals, most of which are now grouped with Metazoa, while some
writers place them in a sub-kingdom Vermes. It appears to me then only
possible to describe this heterogeneous group of worm-like animals
among those which resemble each other in certain negative features, but
not possessing any of the distinctive characters of those previously
described. There are numerous species among Entozoa, every one of which
is of the highest interest to mankind in general, and to animal life as
a whole. To these I shall devote some attention, from the wide-spread
importance attached to them. They are characterised by having a soft
absorbent body with little or no colour, in consequence of being
excluded from light, living within the bodies of animals and absorbing
their vital juices, thereby inflicting a large amount of injury and
death upon the whole vertebrate kingdom. They bear in this respect a
close analogy to parasitic Fungi in the nature of their destructive
action upon plant life, which I have fully discussed in a previous
chapter.
The relations which obtain between parasites and their hosts are in all
respects conditioned by their natural history; and without a detailed
knowledge of the organisation, the development, and the mode of life
of the different species, it is impossible to determine the nature and
extent of the pathological conditions to which they give rise, and at
the same time find means of protection against guests in every way so
unwelcome.
The nutritive system of the entozoa must be regarded as in the lowest
state of development, yet there are some among them of a higher grade,
as will be seen as we proceed. All are remarkable alike for their
vast productiveness and for their peculiar metamorphoses. For example,
the greater number of the Tænia begin their lives as sexless, encysted
larvæ, and on entering their final abode, segments are successively
added, until the worm has finally reached the adult stage. Again, the
tapeworm of the cat has its origin in the encysted larvæ found in the
livers of the mouse and rat. Another species of entozoa inhabit the
stomach of the stickle-back, and only attain their perfect form in
the stomachs of aquatic birds that feed exclusively on fish. Another
infests the mantle of pond-snails, and through their agency, the
embryos pass into the stomach of sheep.
An almost endless number of similar transformations take place in other
genera. The simplest form among internal parasites is the Gregarinæ,
formerly grouped among Protozoa. They consist of a simple limiting
membrane, with a mass of granular matter enclosed and surrounding a
nucleus (Plate III., No. 53). These parasites pass through a crystoid
stage in the body of one of the lower animals, usually the earthworm,
_Lumbricus agricola_. In the more mature organism an envelope,
differentiated from the protoplasm within, can be made out (No. 54);
this affords an indication of greater differentiation in the subjacent
layer of protoplasm. An anterior portion is in many cases separated
by a constriction from the cylindrical or band-like body (No. 56).
Gregarinæ multiply when encysted, and divide into a multitude of
minute _pseudo-navicula_, so named from their resemblance in shape to
a well-known form of Diatomaceæ. When a young pseudo-navicule escapes
it behaves somewhat like an amœba, and if perchance it is swallowed by
an appropriate host, it develops at once into the higher stage. The
various forms are represented in Plate III., Nos. 53--61. Miescher, in
1843, described suchlike bodies, taken from the muscles of a mouse.
A good account of specimens obtained from the muscles of a pig was
published by the late Mr. Rainey in the “Philosophical Transactions,”
1857. He regarded them as cestoid entozoa. They have been described
under a variety of names, as worm-nodules, egg-sacs, eggs of the fluke,
young measles, &c. M. Lieberkühn carefully traced the pseudo-naviculæ
after leaving the perivisceral cavity of the earth-worm; he found
large numbers of small corpuscles, exhibiting amœba-like movements,
as well as pseudo-naviculæ, containing granules, formed in an encysted
Gregarinæ. He imagines that these latter bodies burst, and that their
contained granules develop into the amœbiform bodies which subsequently
become Gregarinæ.
Professor Ray Lankester made a careful examination of more than a
hundred worms for the purpose of studying these questions, but he
succeeded in arriving at no other conclusion than that certain forms
may be the by-products of encysted Gregarinæ. The _G. lumbricus_ is one
of those forms which are unilocular. The vesicle is not always very
distinct, and is sometimes altogether absent; occasionally it contains
no granules, sometimes several, one of which is generally nucleated. In
other of these cysts a number of nucleated cells may be seen developing
from the enclosed Gregarina, which gradually become fused together and
broken up, until the entire mass is converted into nucleated bodies,
often seen in different stages of development, assuming the form of
a double cone, as that presented by certain species of Diatomaceæ.
At length the cyst contains nothing but pseudo-naviculæ, sometimes
enclosing granules; these gradually disappear, and finally the cyst
bursts. Encystation seems to take place much more rarely among the
bilocular forms of Gregarinæ than in the unilocular species found in
the earthworm and other Annelids.[73]
Dr. J. Leidy published in the “Transactions of the Philadelphia
Society,” 1853, the results of his examinations of several new species
of Gregarinæ. He described a double membrane “within the parietal tunic
of the posterior sac, this being transparent, colourless, and marked
by a most beautiful set of exceedingly regular parallel longitudinal
lines.”
Professor R. Leuckart is the latest writer on the parasites of animals,
and to him we are indebted for a more systematic account of the whole
group, and their life-history, than to any previous investigator. I
can only attempt to give a mere outline of the developmental stages
of a few typical forms of parasites, commencing with the cystic
tapeworm, Tænia. These worms are ribbon-like in appearance, and are
divided throughout the greater part of their length into segments,
and their usual habitation is the intestinal cavity of vertebrate
animals. The anterior extremity of a tænoid worm is usually called
the head, and bears the organ by which the animal attaches itself to
the mucous membrane of the creature which it infests. These organs
are either suckers, or hooks, or both conjoined. In Tænia, four
suckers are combined with a circlet of hooks, disposed around a median
terminal prominence. The embryo passes through certain stages of
development--viz., four forms or changes: but the embryo itself is
very peculiar, consisting of an oval non-ciliated mass, provided with
six hooks, three upon each side of the middle line. Tænia are found
enclosed in various situations besides that of the alimentary canal:
the eye, the brain, the muscular tissues, the liver, &c., of animals.
The following cystic worms are usually included in this genera,
_Cysticercus Anthocephalus_, _Cœnurus_, and _T. Echinococcus_. Plate
IV., No. 100, shows an adult specimen of the latter with rostellum
suckers, and three successive segments, the last of which is the ova
sac. The water-vascular system is represented coloured by carmine. This
parasite infests the human body as frequently as many other species. My
accurately-drawn figure is copied from Cobbold’s “Introduction to the
Study of Entozoa.”
_Cysticercus fasciolaris_ is developed within the liver of white
mice; _Cysticercus cellulosæ_ in the muscles of the pig; hence we
have the diseased state of pork familiarly known as “_measly pork_.”
Should a lamb become infested with Tænia the final transformation
will be different; within a fortnight symptoms of a disease known as
“staggers” manifest themselves, and in the course of a few weeks the
_Cœnurus cerebralis_ will be developed within the brain. Von Siebold
pointed out the bearing of this fact upon the important practical
problem of the prevention of “staggers.” Others belonging to the same
class of parasites are quite as remarkable in their preference for the
alimentary canal of fishes. The Echinorhynchus is developed in the
intestinal canal of the flounder, _Triænophorus nodulus_ in the liver
of the salmon. Thus, by careful and repeated observation with the
microscope, a close connection is found to exist between the cystic and
cestoid entozoa.
The Echinococcus (Plate IV., No. 101) infests the human liver. These
parasites are always found in cysts, and in closed cavities in the
interior of the body. They are united in fours by a very short stalk
or pedicle, common to the whole. By an increase of magnification the
contents of a cyst present the several structures represented in Fig.
376.
Echinorhynchus, or spiny-headed threadworms, constitute a group
of entozoa which undergo a metamorphosis hardly, perhaps, less
remarkable than that known to take place in other Nematode worms.
Leuckart instituted, in 1861, a series of experiments with the ova of
_Echinorhynchus proteus_ found parasitic upon the _Gammarus pulex_.
The ova of _E. proteus_ resemble in form and structure those of allied
species. They are of a fusiform shape, surrounded with two membranes,
an _external_ of a more albuminous nature, and an _internal_ chitinous
one. When the eggs reach the intestine the outer of these membranes is
absent, being in fact digested, while the inner remains intact until
ruptured by the embryo.
[Illustration: Fig. 376.--Cystic Disease of Liver (Human).
_a._ Cyst with Echinococcus enclosed; _b._ detached hooklets from the
head of Echinococcus, magnified 250 diameters; _c._ crystals found in
cyst, chiefly cholesterine; _d._ cylindrical epithelium, some enclosed
in structureless vesicles; _e._ Puro-muculent granules, fat and blood
corpuscles.]
The typical Threadworm belonging to the order Nematoidea infest the
intestines of children, and are a source of much suffering. The egg is
elliptical, and contains a mass of granular protoplasm, the external
wall of which soon becomes marked out into a layer of cells. The mouth
of the worm appears as a depression at the end of the blunt head. When
the muscular system and alimentary canal are developed the embryo
hatches out, some few of which are free living forms; most of them
lead a parasitic life. Their reproduction is enormous, representing
thousands of eggs and embryos.
Of the non-parasitic species of thread-worm, the common vinegar
eel, Anguillula,[74] affords an example. This is found in polluted
water, bog-moss, and moist earth, as well as in vinegar; also in the
alimentary canal of the pond-snail, the frog, fish, &c. Another species
is met with in the ears of wheat affected with a blight termed the
“cockle”; another, the _A. glutinis_, in sour paste. If grains of the
affected wheat are soaked in water for an hour or two before they are
cut open, the so called “eels” will be found. The paste-eel makes its
appearance spontaneously just as the pasty mass is turning sour; the
means of securing a supply for microscopical examination consists in
allowing a portion of the paste in which they show themselves to dry
up, and laying it by for stock; if at any time a portion of this is
introduced into a little fresh-made paste, and the whole kept warm and
moist for a few hours, it will be found to swarm with these wriggling
little worms. A small portion of paste spread over the face of a
Coddington lens is a ready way of viewing them.
_Trichina spiralis._--One of the smallest and most dangerous of all
human internal parasites is _T. spiralis_, since it finds its way into
the muscles throughout the human body. The young animal presents the
form of a spirally-coiled worm in the interior of a minute oval-shaped
cyst (Plate IV., No. 104), a mere speck scarcely visible to the naked
eye. In the muscular structure it resembles a small millet seed,
somewhat calcareous in composition. The history of the development of
Trichina in the human muscle is briefly that in a few hours after the
ingestion of infected pork, Trichina, disengaged from the muscle, will
be found in the stomach: hence they pass into the small intestine,
where they are further developed. Continuing their migrations, they
penetrate far into the interior of the primitive muscular fasciculi,
where they will be found, in about three days after ingestion, in
considerable numbers, and so far developed that the young entozoa have
almost attained a size equal to that of the full-grown Trichina (Plate
IV., No. 105). They quickly advance into the interior of the muscular
fasciculi, where they live and multiply in continuous series, while
the surrounding structures as well as the muscular tissue undergo a
process of histolysis. The destructive nature of the parasite is very
great.
The number of progeny produced by one female may amount to several
thousands, and as soon as they leave the egg they either penetrate
through the blood-vessels, or are carried on by the circulation, and
ultimately become lodged in the muscles situated in the most distant
parts of the body. Here, as already explained, they become encysted.
[Illustration:
Fig. 377.--Monads in Rat’s Blood, stained with methyl violet, showing
membrane under different aspects; blood-corpuscles, some crenated and
others with stained discs (× 1,200).--(Crookshank.)]
Professor Virchow draws the following conclusions:--“1. The ingestion
of pig’s flesh, fresh or badly dressed, containing Trichinæ, is
attended with the greatest danger, and may prove the proximate cause of
death. 2. The Trichinæ maintain their living properties in decomposed
flesh; they resist immersion in water for weeks together, and when
encysted may, without injury to their vitality, be plunged in a
sufficiently dilute solution of chromic acid for at least ten days. 3.
On the contrary, they perish and are deprived of all noxious influence
in ham which has been well smoked, kept a sufficient length of time,
and then well boiled before it is consumed.”
A more minute Filarian worm has been detected in the human
blood-vessels, known as _Filaria sanguinis hominis_. This worm carries
on its work of destruction throughout the night; during the day it
remains perfectly passive. It increases rapidly, and produces swellings
of the glandular structures of the body, somewhat after the nature of
those characteristic of the Bombay plague, with a slight difference,
that after death the swellings are seen to be due to the vast
accumulations of the _Filaria sanguinis_ blocking the blood-vessels.
The accompanying Fig. 377 shows a similar infiltration of monads in the
blood of rats dying of plague in Bombay.
_Trematode Worms._--In the order Trematoda, to which the fluke belongs,
the body is unsegmented, and to the naked eye smooth throughout, with
a blood circulatory system, and two suctorial discs at the hinder end.
There is a distinct digestive canal, usually forked, furnished with
only one aperture, the mouth. The excretory organs open out as in tape
worms, and the male and female organs co-exist in the same individual.
The Fluke (shown in Plate IV., No. 103) is cone-shaped, and is the
_Amphistome conicum_ of Rudolphi. This parasite is common in oxen,
sheep, and deer, and it has also been found in the Dorcas antelope.
It invariably takes up its abode in the first stomach, or rumen,
attaching itself to the papillated folds of the mucous membrane. In the
full-grown, adult stage, it rarely exceeds half an inch in length. It
is certainly one of the most remarkable in form and organisation of any
of the internal parasites.
The larger fluke (_Fasciola hepatica_) often attains to an inch or
more in size. It is not only of frequent occurrence in all varieties
of grazing cattle, but has likewise been found in the horse, the ass,
and also in the hare and rabbit and other animals. Its occurrence
in man has been recorded by more than one observer. The oral sucker
forming the mouth leads to a short œsophagus, which very soon divides
into two primary stomachal or intestinal trunks, the latter in their
turn sending off branches; the whole together forming that attractive
dendritic system of vessels so often compared to plant-venation.
This remarkably-formed digestive apparatus is represented in Plate
IV., Nos. 106 and 107, _Fasciola gigantea_ of Cobbold, and should
be contrasted with the somewhat similarly racemose character of the
water-vascular system. Let it be expressly noted, however, that in the
digestive system the majority of the tubes branch out in a direction
obliquely downwards, whereas those of the vascular system slope
obliquely upwards. A further comparison of the disposition of these
two systems of structure, with the same systems figured and described
as characteristic of the Amphistoma, will at once serve to demonstrate
the important differences which subsist between the several members
of the two genera, if we turn to the consideration of the habits of
_Fasciola hepatica_, which, in so far as they relate to excitation of
the liver disease in sheep, acquire the highest practical importance.
Intelligent cattle-breeders, agriculturists, and veterinarians have
all along observed that the _rot_, as this disease is commonly called,
is particularly prevalent after long-continued wet weather, and more
especially so if there have been a succession of wet seasons; and
from this circumstance they have very naturally inferred that the
humidity of the atmosphere, coupled with a moist condition of the soil,
forms the sole cause of the malady. Co-ordinating with these facts,
it has likewise been noticed that the flocks grazing in low pastures
and marshy districts are much more liable to the invasion of this
endemic disease than are those pasturing on higher and drier grounds;
a noteworthy exception occurring in the case of those flocks feeding
in the salt-water marshes on our eastern shores. Plate IV., No. 106,
_Fasciola gigantea_: the anterior surface is exposed to display oral
and ventral suckers, and the dendriform digestive apparatus injected
with ultra-marine; No. 107 shows the dorsal aspect of the specimen and
the multiramose character of the water-vascular system, the vessels
being injected with vermilion.
In their larval condition the Amphistoma live in or upon the body of
the pond-snail. This we infer from the circumstance that the larvæ, or
cercariæ, of a closely-allied species, the _Amphistoma subclavatum_,
are known to infest the alimentary canal of frogs and newts, and have
also been found on the body of the Planorbis by myself. The cercariæ
larvæ are taken, it is believed, by the sheep and the cattle while
drinking. The earliest embryotic stage in which I have found the embryo
fluke is represented at Fig. 378, No. 1. In the year 1854, whilst
observing the habits of Limnœa and other water-snails, I brought home
specimens from the ornamental water in the Botanic Gardens; upon these
were discovered thousands of minute thread-like worms, subsequently
met with on other embryos, and at first taken to be simple infusorial
animals, but upon placing them in a glass vessel these minute bodies
were observed to detach themselves and commence a free-swimming
existence. A fringe of cilia was seen to surround the flask-shaped body
(No. 1).
[Illustration: Fig. 378.--Forms of Cercaria; stages in the development
of the Fluke.
1. An infusorial embryo; 2. a Trematode embryo having quite recently
escaped from the egg; 3. embryo cercaria; 4. fully-formed cercaria,
showing alimentary canal and sucker-like head; 5. encysted form of
same; 6. _Cercaria furcata_, with the nervous system and forked tail
displayed; 7. in the act of breaking up; 8. tail portion half an hour
after division; 9. parasitic worm of another species of Trematoda.
(Magnified from 10 to 25 diameters.)]
The study of these embryos throws a flood of light upon the obscure
history of Cercariæ. After a short period of wandering, their embryos
fasten upon the water-snail, and compel it to act as a wet-nurse,
and prepare it for a further and higher stage of life. The earliest
condition in which I have discovered them concealed about the body of
the water-snail is shown at No. 2; in appearance, a simple elongated
sac filled with ova or germs, and which in a short time develop into
the caudate worms already spoken of; their tails gradually attaining
to the length of the mature embryos, Nos. 3 and 4, the latter being a
full-grown _Cercaria ephemera_.
Diesing described no less than twelve species of Cercariæ, some of
the most curious of which live on the puddle-snail, in colonies of
thousands. All throw off their tails at the moment of changing into
a fluke. On placing some _Cercaria furcata_ (Nos. 6 and 7) under
the microscope, they were seen to plunge about in frantic attempts
to escape from confinement. Suddenly I saw them shed their tails
and their bodies divide into two parts, each half swimming about as
vigorously as before, quite indifferent as to the severance, and
apparently dying from exhaustion. Those represented in Nos. 6 and 7
have a highly-organised nervous system, forming a continuous circuit
throughout the body and tail. The mouth is furnished with a sucker and
hooklets, which can be projected out some distance, while a digestive
apparatus and ventral opening or sucker can be differentiated.
The tail is bifurcated and articulated with the body by a sort of
ball-and-socket joint, and when broken off, the convexity of one part
is seen to accurately fit into the concavity of the other; it lashes
about this appendage with considerable dexterity, rarely attaching
itself to any of the small aquatic plants.[75]
There is yet another Filarian worm, a pest to the poultry-yard, the
Gape-worm, _Sclerostoma syngamus_. This parasite is widely distributed,
and is invested with special interest, since it produces disease, and
kills annually thousands of young chickens, pheasants, partridges,
and many of the larger kinds of wild birds. The worms find their way
into the windpipe or tracheæ, through the drinking water, while in
the embryotic or cercarian stage of existence, and their increase is
so rapid, the birds quickly die of suffocation. The female gape-worm
often attains to a considerable size, and when full grown resembles the
well-known mud-worm of the Thames (_Gordius aquaticus_). She measures
full six-eighths of an inch in length, while the male only measures
one-eighth. So insignificantly small is he that the female carries him
about tucked into a side pocket. The ova sac occupies a considerable
portion of the internal body space, and is always found loaded with
eggs in all stages of development, numbering some five hundred or a
thousand. In shape these are ovoid. On cutting open the windpipe of
chicken and partridges, I have found their tracheæ literally swarming
with the gape-worm.[76]
A remarkable form of the Trematode worm is _Bilharzia hæmatobra_ of
Cobbold, _Distomia hæmatobium_ of other authors (Plate IV., No. 102).
This genus of fluke, discovered by Dr. Bilharz in the human portal
system of blood vessels, gives rise to a very serious state of disease
among the Egyptians. So common is the occurrence of this worm, that
this physician expressed his belief that half the grown-up population
of Egypt suffer from it. Griesinger conjectures that the young of the
parasite exist in the waters of the Nile, and in the fish which abound.
Dr. Cobbold thinks “it more probable that the larvæ, in the form of
cercariæ, rediæ, and sporocysts, will be found in certain gasteropod
mollusca proper to the locality.” The anatomy of this fluke is fully
described by Küchenmeister in his book on parasites, by Leuckart,[77]
and by Cobbold. The eggs and embryos of Bilharzia are peculiar in
possessing the power of altering their forms in both stages of life;
and it is more than probable that the embryo form has been mistaken for
some extraordinary form of ciliated infusorial animal, its movements
being quick and lively. We cannot fail to notice the curious form of
the male animal, and, unlike the Filarian previously described, it is
he who carries the female about and feeds her. The whip-like appendage
seen in the figure is a portion of the body of the female. The disease
produced by this parasite is said to be more virulent in the summer
months, probably owing to the greater abundance of cercarian larvæ at
this period of the year.
[Illustration: Fig. 379.--The double parasitic worm (_Diplozoum
paradoxum_).]
There are also double parasitic worms, which may be described as a
sub-order of Trematoda, differing very much from those previously
described. These live on the gills of several species of fresh-water
fish, the gudgeon and minnow, for instance. Among them is a most
remarkable creature well deserving the name of _Diplozoum paradoxum_
which has been bestowed upon it. It consists of two complete mature
similar halves, each possessing every attribute of a perfect animal
(_a_). Each of the pointed front ends has a mouth aperture, and close
to it two small sucking discs; while each individual has a separate
intestine, consisting of a medium tube and innumerable side-branches.
At the hinder end of the body are two suckers sunk in a depression, and
protected by four hard buckle-shaped organs. The eggs are elongated,
and provided at one end with a fine thread-like appendage (_b_). In
this egg the young (_c_)--which at the time of hatching is only about
one-hundredth of an inch--takes about a fortnight to develop. It is
covered with cilia, has two eyes and two suckers; after quitting the
egg, the larvæ are very lively and restless in their movements,
gliding about and then swimming off with rapidity. If unable to find
the fish into which they are destined to live, they grow feeble and
perish, but if successful they grow into the Diporpa (_d_), which is
flattened and lancet-shaped, and bears a small sucking disc on the
under surface and a conical excrescence on the back. After living in
this state for some weeks, and gaining nourishment by sucking the blood
from the fish’s gills, the worms begin to join together in pairs, one
specimen seizing the conical excrescence of another by its ventrical
sucker; then, by a truly acrobatic feat, the second twists itself to
the dorsal excrescence of the first, and in this state an inseparable
fusion takes place between the suckers and the excrescences involved in
the adhesion.[78]
In the group Vermes, the more highly-organised Annelida must be
included. These, for the most part, live either in fresh or salt water.
The Annelids are various, while the Planaria, a genus of Turbellaria,
are very common in pools, and resemble minute leeches; their motion is
continuous and gliding, and they are always found crawling over the
surfaces of aquatic plants and animals, both in fresh and salt water.
The body has the flattened sole-like shape of the _Trematode entozoa_
(Fig. 378, No. 9), the mouth is surrounded by a circular sucker; this
is applied to the surface of the plant from which the animal draws its
nourishment; it is also furnished with a rather long proboscis, which
is probably employed for a similar purpose.
Planariæ multiply by eggs, and by spontaneous fissuration in a
transverse direction, each segment becoming a perfect animal. Professor
Agassiz believes that the infusorial animals, Paramæcium and Kolpoda,
are simply planarian larvæ.
Hirudinidæ, the leech tribe, are usually believed to form a link
between the Annelida on the one hand, and the Trematoda on the other;
their affinities place them closer with the latter than the former.
Although deprived of the characteristic setæ of the Annelida, and
exhibiting no sectional divisions, they are provided with a sucker-like
mouth possessed by Trematoda, but they present no resemblance to them
in their reproductive organs. On the other hand, in the arrangement
of the nervous system and in their vascular system, the Hirudinidæ
resemble Annelida. The head in most of the Annelida is distinctly
marked, and furnished with eyes, tentacles, mouth, and teeth, and
in some instances with auditory vesicles, containing otolithes. The
nervous system consists of a series of ganglia running along the
ventral portion of the animal, and communicating with a central mass of
brain.
_Hirudina medicinalis_ puts forth a claim for special attention on
the ground of services rendered to mankind. The whole of the family
live by sucking the blood of other animals; and for this purpose the
mouth of the leech is furnished with a number of strong horny teeth,
by which they cut through the skin. In the common leech three rows of
teeth exist, arranged in a triangular, or rather triradiate form, a
structure that accounts for the peculiar appearance of leech bites.
The most interesting part of the anatomy of the leech to microscopists
is certainly the structure of the mouth (Fig. 380). This is a muscular
dilatable orifice, within which three beautiful little semi-circular
saws are situated, arranged so that their edges meet in the centre. It
is by means of these saws that the leech makes the incisions whence
blood is to be procured, an operation which is performed in the
following manner. No sooner is the sucker firmly fixed to the skin,
than the mouth becomes slightly everted, and the edges of the saws are
thus made to press upon the tense skin, a sawing movement being at the
same time given to each, whereby it is made gradually to pierce the
surface, and cut its way to the capillary blood-vessels beneath.
[Illustration: Fig. 380.--Mouth of Leech.]
In Clepsinidæ the body is of a leech-like form, but very much narrowed
in front, and the mouth is furnished with a prehensile proboscis. These
animals live in fresh water, where they may often be seen creeping over
aquatic plants. Their prey is the pond-snail.
_Tubicola._--The worms belonging to this series of branchiferous
Annelida are all marine, and distinguished by their invariable habit
of forming a tube or case, within which the soft parts of the animal
can be entirely retracted. This tube is usually attached to stones or
other submarine bodies. Externally it is composed of various foreign
materials, sand, crystalline bodies, and the _débris_ of shells;
internally it is lined with a smooth coating of sarcode, sometimes of a
harder consistency. The Tubicola generally live in societies, winding
their tubes into a mass which often attains a considerable size; only a
few are solitary in their habits. They retain their position in their
cases by means of tufts of bristles and spines; the latter, in the
tubicular Annelids, are usually hooked, so that by applying them to
the walls of the case, the animal is enabled to oppose a considerable
resistance to any effort made to withdraw it. In the best known family
of the order (Sabellia), the branchiæ are placed in the head, and form
a circle of plumes, or a tuft of branched organs. The Serpulidæ form
irregularly twisted calcareous tubes, and often grow together in large
masses, when they secure themselves to shells and similar objects;
other species, Terebellidæ, which build their cases of sand and stones,
appear to prefer a life of solitude. The best known form is _Terebella
littoralis_.[79] The curious little spiral shells seen upon the fronds
of seaweeds are formed by an animal belonging to the Spirorbis.
[Illustration: Fig. 381.--Serpula with extended tentacles and body
protruding from calcareous case.]
If the animals be placed in a vessel of sea-water a very pleasing
spectacle will soon be witnessed. The top part of the tube is seen to
open, and the creature cautiously protrudes a fringe of tentacles;
these gradually spread out two beautiful fan-like rows of tentacles,
surrounded by cilia of a rich purple or red colour. These serve the
double purpose of breathing and feeding organs. When withdrawn from its
calcareous case, the soft body is seen to be constructed of a series of
rings, with a terminal prehensile foot by which it attaches itself.
Many Annelids are without tubes or cells of any kind, simply burying
their bodies in the sand near tidal mark. The Arenicola, lob-worm,
is a well-known specimen of the class; its body is so transparent
that the circulating fluids can be distinctly seen under a moderate
magnifying power. Two kinds of fluids flow through the vessels, one
nearly colourless, the other red; the vessels through which the latter
circulate are described as blood-vessels.
Not very much interest attaches to the developmental stage of the
Annelida. They issue forth from ova, and the embryo so closely resemble
ciliated polypes, that competent observers have mistaken them for
animals belonging to a lower class; a few hours’ careful watching
is sufficient to dispel a belief of the kind, when the embryonic,
globular, or shapeless mass is seen to assume a form of segmentation,
and soon the various internal organs become more and more developed,
eye spots appear, and the young animal arrives at the adult stage of
its existence.
Crustacea.
The crustaceans comprise a large assemblage of Arthropods, presenting
great diversity of structure. Some of the parasitic species have
become so simplified in organisation that they appear to present
no relationship with the higher members of the class, yet it is
certain that all the species, whether terrestrial or aquatic, belong
to the same stock, and may have had origin in the same fundamental
plan of structure. Essentially, the body consists of a large number
of segments, to each of which is attached a pair of two-branched
appendages; the external branch is termed the exopodite and the
internal the endopodite. Five segments at the front end of the body
unite to form a head, the appendages of the first two being situated in
front of the mouth, and performing the office of feelers or antennæ,
while those of the remaining three segments are transformed into jaws,
the first pair of jaws being the mandibles and the following two
pairs the maxillæ. The rest of the appendages are variously modified
and to some are attached respiratory organs in the form of gills.
Crustaceans are broadly divided from Centipedes, Millipedes, Insects,
&c., by the presence of two pairs instead of one pair of antennæ, and
by the possession of branchial and not tubular (tracheal) respiratory
organs. Arachnida and some other species are again widely separated.
The majority of the young on leaving the egg are quite unlike the
parent, and only acquires their definite form after undergoing a series
of changes. The earliest stage, which has been called the Nauplius,
already referred to in connection with the barnacle, is a minute body
showing no trace of segmentation, and provided with a single eye, and
three pairs of swimming appendages, which become the two pairs of
antennæ and the mandibles of the adult. This stage is by no means of
invariable occurrence, but is chiefly characteristic of the lowest
members, the Entomostraca, and is rare in the higher, Malacostraca. The
typical crustaceans are shrimps, crayfish, &c., so familiarly described
by Huxley. The zoæa stage of the crab, a minute transparent creature,
which undergoes several changes, swims about flapping its long jointed
abdomen, like some of the Entomostraca, and the shrimp in particular.
The larva of crayfish, the so-called glass-crab, is very peculiar and
interesting. The sessile-eyed series, in which the compound eyes are
never mounted on a movable stalk, and to which the Isopoda belong,
exhibits great diversity of structure as well as of habits and habitat.
Some live in fresh water, most are marine, while others live on land
and take to a parasitic life.
[Illustration: Fig. 382.--Male Gnathia, enlarged.]
This genus contains Gnathia, in which the male and female are so
dissimilar, that they are frequently referred to as members of two
families. In the adult male the mandibles are powerful and prominent,
and the head is large, squared, and as wide as the thorax. In the
female, on the contrary, the head is curiously small and triangular,
without visible mandibles, and the thorax is much dilated. The
creatures are about one-sixth of an inch long, and of a greyish colour,
and the destruction they bring about is due to their habit of boring
into timber below water mark. Fig. 382 represents an enlarged view
of the male Gnathia. These crustaceans are vegetarians, and feed on
wood. Other members of the group, known as fish lice, are much larger
in size, and chiefly infest the cetacea, and bear in addition two
large eyes. By means of their powerful fore feet the Cymothordæ attach
themselves to both marine and fresh-water fish, showing a preference
for the inside of the mouth of their host.
[Illustration: Fig. 383.--1. Cypris; 2. Cyclops; 3. _Branchipus
grubei_.]
The bar-footed group Copepoda are free living, and the thorax bears
four or five swimming feet; the abdomen is without appendages. The best
known fresh-water form is Cyclops, the structure of which serves as
a type of the order. The body is, as is well known to microscopists,
broad in front and tapering behind, being thus, when viewed swimming,
pear-shaped in outline. The dorsal elements of the head are fused to
form a carapace, which bears a single eye, from which circumstance it
derives its name. The eggs are carried by the female in a couple of
ova-sacs attached to the last segment of the thorax, and so prolific
are these creatures that a female will produce over four thousand
million young. The young when hatched is an oval Nauplius, which after
two or three moults acquires the adult state. In the family of the
Apodidæ we have an equally well-known crustacean, the Branchipus.
In the Branchipodidæ the body is also elongated, but there are no
appendages to the abdomen, which consists of nine segments, while
there are eleven pairs of thoracic appendages. The head shield is not
developed backwards, and the large separated eyes are supported on
distinct stalks. In the male the second antennæ are converted into
claspers. These crustaceans swim upside down (Fig. 383).
=Cladocera= (_Daphniadæ_ of Dr. Baird).--The water-flea (_Daphnia
pulex_) may be taken as the best known example of the order. The
body of this little active animal is narrowed in front, and at the
posterior end, where the carapace is deeply notched, is the tip of the
abdomen bearing the pair of rigid barbed setæ from which the genus
takes its name. At the front of the head is a large compound eye and
two pairs of branched plumed appendages, antennæ. The first pair of
these are small and simple. The jaws consist of the mandibles and the
first pair of maxillæ, the second pair of maxillæ being obsolete in
the adult. The thorax comprises five segments, each bearing a pair of
leaf-like swimming limbs. The abdomen consists of three segments, and
is destitute of limbs. The males are usually smaller than the females,
and much rarer, being rarely met with before the end of summer.
Eggs are laid both in summer and winter, and are passed into a
brood-pouch, separating the upper surface of the thorax from the
backward extension of the carapace. Here the summer eggs hatch, but
the winter set are enclosed in a kind of capsule developed from the
carapace. This capsule, termed the _ephippium_, is cast off with the
next moult of the mother’s integument (a process necessary for the
gradual growth of the crustacean), and falling to the bottom of the
water, gives exit to the embryos, which hatch in its interior, and the
young born from these “ephippial” eggs produce young, which in their
turn become mothers. It appears, then, the winter eggs are enclosed
in capsules of more than usual hardness to enable them to withstand
any degree of cold that might otherwise prove fatal to the parent.
Dr. Baird found, on examining ponds that had been again filled up by
rain after remaining two months dry, numerous specimens of Daphnia and
_Cyclops quadricornis_ in all stages of growth.[80]
We learn also from his investigations that the Daphnia have many
enemies. “The larva of the _Corethra plumicornis_, known to
microscopical observers as the skeleton larva, is exceedingly rapacious
of Daphnia. Pritchard says they are the choice food of a species
of Nais; and Dr. Parnell states that the Lochleven trout owes its
superior sweetness and richness of flavour to its food, which consists
of small shell-fish and Entomostraca.” These crustaceans abound in
fresh and salt water. Artemiæ are formed exclusively in salt water, in
salt marshes, and in water highly charged with salt. Myriads of these
Entomostraca are found in the salterns at Lymington, in the open tanks
or reservoirs where the brine is deposited previous to boiling. A pint
of the fluid contains about a quarter of a pound of salt, and this
concentrated solution destroys most other marine animals. During the
fine days in summer Artemiæ may be observed in immense numbers near
the surface of the water, and, as they are frequently of a lively red
colour, the water appears tinged with the same hue. The movements of
this little animal are peculiar. It swims about on its back, and by
means of its tail, its feet being at the same time in constant motion.
They are both oviparous and ovoviviparous, according to the season of
the year. At certain periods they only lay eggs, while during the hot
summer months they produce their young alive. In about fifteen days
the eggs are expelled in numbers varying from 50 to 150. As is the
case with many of the Entomostraca, the young present a very different
appearance from the adult animals; and they are so exactly like the
young of _Chirocephalus_, that with difficulty are they distinguishable
one from the other. The ova of other species are furnished with
thick capsules, and imbedded in a dark opaque substance, presenting
a minutely cellular appearance, and occupying the interspace between
the body of the animal and the back of the shell; this is called the
ephippium. The shell is often beautifully transparent, sometimes
spotted with pigment; it consists of a substance known as chitine,
impregnated with a variable amount of calcium carbonate, which produces
a copious effervescence on the addition of a small quantity of a strong
acid to the water in which the shell is immersed. When boiled, Artemiæ
turn red as their congeners, lobsters. Their shells may be said to
consist of two valves united at the back, resembling the bivalve shell
of a mussel, or simply folded at the back to appear like a bivalve,
but are really not so; or they may consist of a number of rings or
segments. The body of Cypris presents a reticulated appearance,
somewhat resembling cell structure. Entomostraca should be narcotised
and prepared for examination under the microscope as directed by Mr.
Rousselet at pages 345, 346.
INSECTS’ EGGS, ETC.
[Illustration:
Tuffen West, del. Edmund Evans.
PLATE VI.]