24, 27, and 28. At the point of union of the oviduct (Fig. 28, _ovd_), the
vitelline ducts (_yd_), and the commencement of the uterus (_ut_), a
slender duct is given off which opens into the intestine, and is known as
the "vitello-intestinal canal" (Fig. 24, _dvi_; Fig. 28, _gic_). This duct
has apparently the same relations as the "canal of Laurer" of Digenea,[73]
except only that the latter opens to the exterior directly. In connexion
with this vitello-intestinal canal a "vagina" is present, which in
_Polystomum_ and most Monogenea is paired (Fig. 24, _vag_), in _Diplozoon_
and in one {58}or two other forms, however, unpaired. The vagina receives
the penis of another individual during copulation (Fig. 26), and does not
appear to have an homologue in the liver-fluke or other Digenea.
[Illustration: FIG. 25.—Eggs of Monogenea. A, Eggs of _Encotylabe pagelli_
v. Ben.-Hesse; B, eggs of _Udonella pollachii_ v. Ben.-Hesse (with young
forms just hatching out); C, egg of _Microcotyle labracis_ v. Ben.-Hesse.
(After van Beneden and Hesse.) × 50.]
LIFE-HISTORIES OF THE POLYSTOMATIDAE.[74]—_Polystomum integerrimum._ After
the mutual fertilisation of two individuals, the eggs are laid in the water
by the protrusion of the body of the parent through the urinary aperture of
the Frog. About 1000 eggs are laid in the spring at the rate of 100 a day
for ten days. After about six weeks, the larva (.3 mm. long) hatches out,
and swims about freely by means of bands of large ciliated cells (Fig. 26,
A); but if it does not meet with a tadpole within twenty-four hours, it
dies. Should it, however, encounter one, the larva creeps along it in a
looping fashion until it approaches the opercular spout, or opening of the
branchial chamber, on the left side; into this it darts suddenly, fixes
itself, and throws off its cilia. Here it remains eight or ten weeks,
feeding, increasing in size, and forming the suckers from behind forwards.
{59}At the time of the tadpole's metamorphosis, the young _Polystomum_
works its way down the pharynx into the oesophagus and along the intestine,
till it reaches and enters the opening of the bladder. Three years
afterwards it becomes mature.
Sometimes, however, _Polystomum_ experiences another fate. The larvae
settling down on the external gills of a young, recently-hatched tadpole,
and obtaining a richer supply of blood than in the previous case, grow far
more rapidly, so that in five weeks they are mature, although still in the
branchial chamber of the tadpole. They do not then wander into the
alimentary canal, but usually, having discharged their eggs, die at the
time of the tadpole's metamorphosis. Still more interesting, however, is
the difference between the genitalia in these and in the normal
_Polystomum_. In contrast with the latter, these possess (1) one testis and
a rudimentary penis; and their spermatozoa differ in structure and shape
from those of the normal _Polystomum_. (2) The vaginae are absent, a fact
connected with the absence of a functional copulatory organ. (3) In
compensation for the loss of these, a duct connects the single testis and
the point of union of oviduct and yolk-ducts, and by this
self-fertilisation occurs. (4) The uterus is absent; the "ootype" or duct
into which the shell-gland opens, communicating directly with the exterior.
In (1) and (4) these aberrant _Polystomum_ resemble _P. ocellatum_, from
the Tortoise _Emys europaea_.
[Illustration: FIG. 26.—_Polystomum integerrimum._ A, Free-swimming larva,
seen from the ventral surface. × 80. B, Two mature individuals in mutual
coition attached to the bladder of a Frog. × 5. (After Zeller.) _d_,
Intestine; _ex.o_, excretory pore, dorsal in position, seen here by
transparency; _ey_, eye-spots; _gl_, frontal glands; _m_, mouth; _ph_,
pharynx; _sd_, adhering disc; _vag_, vagina.]
{60}[Illustration: FIG. 27.—A, Egg of _Diplozoon paradoxum_ v. Nord.,
consisting of a shell enclosing _ov_, the actual ovum, surrounded by _yc_,
the yolk-cells; B, larva just hatched (× 125); C, two _Diporpa_ (I and II)
about to unite; D, conjugation in progress but not yet complete. _dt_,
Dorsal papilla; _e_, eye; _g_, intestine; _m_, mouth; _sc_, ad-oral sucker;
_th_, spirally-wound thread attaching the egg to the gill of the Minnow;
_vs_, ventral sucker; (in D) I, I, one _Diporpa_, ventral view; II, II, the
other, dorsal view. (After Zeller.)]
[Illustration: FIG. 28.—Hinder part of the body of _Diplozoon paradoxum_.
The fusion of the two _Diporpa_, where they come into contact, is now
complete. They now cross each other like an X, and are twisted, so that
_Diporpa_ I, in front of the point of fusion, is seen from the dorsal
surface; behind, from the ventral surface; and the reverse is the case with
_Diporpa_ II. The compound animal is seen from the opposite surface to that
shown in Fig. 27, D. The digestive and excretory organs are omitted. (After
Zeller.) I _Ant. dorsal_, dorsal surface of _Diporpa_ I, facing the
anterior end; I _Post. ventral_, ventral surface of _Diporpa_ I, posterior
end; and similarly for II _Ant. ventral_ and II _Post. dorsal_. _d_, Piece
of the intestine showing opening of, _gic_, vitello-intestinal canal; _ov_,
ovary; _ovd_, point of union of female genital ducts; _sc_, suckers; _te_,
testis; _ut_ (in _Diporpa_ I), "ootype" or chamber into which shell-glands
open. This is continuous with the uterus (_ut_) of _Diporpa_ I; _uto_,
ventral opening of uterus; _vag_, vagina, with _vd_, vas deferens,
permanently inserted into it through the genital pore; _yd_, yolk-ducts;
_yg_, yolk-glands.]
{61}_Diplozoon paradoxum._—The life-history of _Diplozoon_ is unique. For
whereas the larvae of most animals grow up, each into a single adult, in
_Diplozoon_, of the few larvae that survive the dangers of their
free-swimming existence, only those become mature which conjugate
permanently with another individual. But although there are thus only half
as many adult _Diplozoon_ as there were conjugating larvae (or _Diporpa_,
as they were called when they were considered distinct forms), yet the
total number of eggs produced is probably as great as if each larva became
individually mature.
[Illustration: FIG. 29.—_Gyrodactylus elegans_ v. Nord., from the fins of
the Stickleback. (After v. Nordmann.) × 125. _emb_, Embryo.]
_Diplozoon paradoxum_ lays its eggs on the gills of the Minnow, which it
frequently infests in great numbers. The ovum divides rapidly at the
expense of the yolk-cells, and in a fortnight a larva (.2 mm. long) of the
shape and complexity shown in Fig. 27, B, hatches out, which, however,
succumbs if it does not meet with a Minnow in five or six hours. Should it
survive, a dorsal papilla, a median ventral sucker, and a second pair of
posterior suckers develop. Thus the _Diporpa_ stage is attained. These
_Diporpa_ may acquire a third and even a fourth pair of suckers, and
continue to live three months, but they only develop and mature their
reproductive organs, if each conjugates with another _Diporpa_ (Fig. 27, C,
D), and this only occurs in a small percentage of instances. Each grasps
the dorsal papilla of the other by its own ventral sucker, thus undergoing
a certain amount of torsion. Where the two bodies touch, complete fusion
occurs, and, as shown in Fig. 28, the united _Diporpa_ (or _Diplozoon_, as
the product is now called) decussate, each forming one limb of the X-shaped
_Diplozoon_, within which the two sets of complex genitalia develop (Fig.
28).
IV. _Gyrodactylidae._—_Gyrodactylus_ (Fig. 29), the structure of which is
in many ways peculiar, produces one large egg at a time. An embryo, in
which the large and smaller hooks of the adhesive disc can be seen (_emb_),
develops from this egg while still within the body of the parent, and may
give rise to yet another generation within itself. The details of the
process have not, however, been well ascertained.
{62}TREMATODA DIGENEA (_endoparasitica_).
OCCURRENCE AND HABITS OF DIGENEA.—Endoparasitic Trematodes have been found
in almost all the organs of Vertebrate hosts excepting in the nervous,
skeletal, and reproductive systems. The alimentary canal, however, is the
most usual habitat. From the buccal cavity to the large intestine, or even
to the cloaca, its different regions are the resorts of various Trematodes.
No Digenea have been found in the mouth, pharynx, or oesophagus of Mammals;
but in Birds, Reptiles, Amphibia, and especially in Fishes, these parts are
largely affected. It is a striking fact that Trematodes should occur in the
stomach of (chiefly) large predaceous fishes, such as the Pike, Sharks, the
Angler-fish, and others, considering the powerful digestive action of the
gastric juice of these carnivores. The peculiar nature of the defence which
must be employed by the parasites against this digestive action, becomes
still more marked when it is considered that if a Trematode normally living
in the stomach of one host be transferred to that of another, it is usually
speedily digested, as is shown (p. 65) in the case of _Distomum
macrostomum_. From these considerations the suggestion has been made that
the cutaneous secretions of these Trematodes must act, not only as a
protection against digestive or other ferments, but that the action in each
case must be a specific one (Frenzel, Braun).
[Illustration: FIG. 30.—_Distomum luteum_ v. Baer (immature), to show the
arrangement of the excretory vessels. × 50. _ex.o_, Excretory aperture by
which the terminal contractile duct opens—the finer vessels end in
flame-cells; _int_, intestine; _m_, mouth-sucker; _ph_, pharynx; _vs_,
ventral sucker. (After la Valette.)]
It is, however, in the small intestine that most Trematodes occur, as the
examination of the common Frog[75] will readily demonstrate. Both this and
the edible Frog are attacked by a dozen Distomatidae, only a few of which,
however, are common {63}to both hosts, and a number of Holostomatidae also
pass a stage of their development within these Amphibia. Some idea of the
extent to which animals, whose habits lead to infection, may be attacked by
Trematodes (to say nothing of Cestodes and Nematodes, which often occur
also) may be gathered from the fact that in dissecting a black stork,
Nathusius found several hundred _Holostomum excavatum_ and about a hundred
_Distomum ferox_ in the small intestine, twenty-two _D. hians_ in the
oesophagus, five others in the stomach, and one _D. echinatum_ in the
intestine. Snipe, Woodcock, Sandpipers, Dunlin, Gulls, Bittern, Geese, and
Wild Ducks are, to mention a few cases, greatly infested by members of this
group.
The following Trematodes have occurred in man[76]:—
_Distomum hepaticum_ Abild.
" _lanceolatum_ Mehlis.
" _conjunctum_ Cobbold.
" _spathulatum_ Leuckart (= _D. sinense_ Cobb.,
_D. japonicum_ R. Blanch.).
" _rathouisi_ Poir. (probably = _D. crassum_ Busk,
_D. buskii_ Lank.).
" _heterophyes_ v. Sieb.
" _pulmonale_ Bälz (= _D. ringeri_ Cobb., _D. westermanni_ Kerb.).
" _oculi humani_ Ammon (= _D. ophthalmobium_ Dies.).
_Monostomum lentis_ v. Nord.
_Amphistomum hominis_ Lewis and M‘Connell.
_Bilharzia haematobia_ Cobb.
LIFE-HISTORIES OF THE DIGENEA.—The classification of Trematodes according
to their life-histories, expressed in the divisions Monogenea and Digenea,
though a very useful one, breaks down entirely in the case of certain
forms. Thus the life-history of _Gyrodactylus_ is probably digenetic rather
than monogenetic. _Aspidogaster conchicola_,[77] which lives in the
pericardial cavity of the fresh-water mussel (possibly the only case of a
Trematode becoming normally mature in an Invertebrate host, since other
species of _Aspidogaster_ live in Chelonia), produces larvae which enter
another _Anodonta_ and develop directly into the sexual form. In other
words, _Aspidogaster_, though structurally a digenetic form, possesses a
life-history which is direct and simple, _i.e._ monogenetic.
The Holostomatidae, which live in birds of prey and aquatic birds, give
rise to eggs from which a minute larva escapes. The fate of this aquatic
larva is not directly known, {64}but in all probability after entering a
host (Fish, Amphibian, Mollusc), it undergoes a gradual change into what
has long been known as a _Tetracotyle_, from the frequent presence of four
(sometimes only three) adhering organs. Fig. 31 exhibits a species which is
abundant in the lens and vitreous humour of the eye of the Perch. Its
further history is not known, but presumably the Perch is presently
devoured by the final host in which the _Diplostomum_ attains maturity.
Thus the Holostomatidae are "metastatic" (Leuckart), their (probably)
direct development requiring the presence of two hosts.[78]
The other Digenea, the life-histories of which are known, belong to the
Distomatidae and Amphistomatidae, and we may distinguish the steps by which
the complex life-history of the liver-fluke (_Distomum hepaticum_) has been
brought about, by a consideration of that of _Distomum macrostomum_.
[Illustration: FIG. 31.—_Diplostomum (Tetracotyle) volvens._ (After
v. Nordmann.) × 130. _cv_, Contractile excretory vesicle; _d_, intestine;
_e_, calcareous bodies in excretory tubules; _ex.o_, excretory aperture;
_gl_, glandular adhesive body; _ms_, oral sucker; _ph_, pharynx; _vs_,
ventral sucker.]
_Distomum macrostomum._—This form occurs in the intestine of several common
Passerine birds. It is remarkable not only for the large oral sucker, but
also on account of the position of the common genital pore at the hinder,
and not as usual, at the anterior, end of the body (Fig. 32, A). The eggs
pass out through this pore, and are discharged with the bird's excrement.
Should a certain snail (_Succinea putris_) happen to rasp off the epidermis
of a leaf upon which the faeces have fallen, the eggs are swallowed and a
minute active larva is set free (Fig. 32, B). This penetrates through the
thin wall of the digestive tract of the snail, and passing into the
connective tissue, throws off its cilia and assumes the shape of Fig. 32,
C. This _sporocyst_, as the larva is now termed, grows rapidly in all
directions (Fig. 32, D) at the expense of the snail's tissues, until it
becomes impossible to separate parasite and host completely.
{65}[Illustration: FIG. 32.—Life-history of _Distomum macrostomum_ Rud. A,
Immature _Distomum_ (really a tailless _Cercaria_) found in the swollen
terminal parts of _Leucochloridium_ (Fig. 33, B) and enclosed in two
protective membranes, × 40; B, larva which hatches out of the egg of _D.
macrostomum_, × 125; C, the metamorphosed larva (sporocyst) fourteen days
after having entered _Succinea putris_, and pierced through its intestinal
wall; D, actively growing sporocyst. (After Heckert.) _go_, Genital
aperture; _int_, intestine; _ms_, mouth sucker; _n_, nervous system; _ov_,
ovary; _ps_, ventral sucker; _te_, testis.]
Those branches which lie superficially in the cephalic region of the snail
become greatly swollen, cylindrical, and contractile. They are banded with
green and white, ornamented with red terminal spots, and pulsate rapidly.
Hence these fertile branches of the sporocyst (which in this condition was
known as _Leucochloridium paradoxum_, Fig. 33, B) naturally attract the
attention of insectivorous birds, which peck off the tentacles of the
snail, and with it the swollen sporocyst-branch. A sphincter muscle closes
the cut end of the fertile sac when the bird's bill nips it off. The sac
contains large numbers of young _D. macrostomum_ (Fig. 32, A), produced by
the division of embryonic cells of the larva (Fig. 32, B), which are
apparently blastomeres of the egg reserved for this future use. It is a
remarkable circumstance that the old bird itself is immune from infection,
and if it swallows these young Distomes, they are digested. Should,
however, the snail's tentacle and its contents be offered as food to the
nestlings, their weaker digestive powers merely set the Distomes free from
the protective membranes (Fig. 32, A), and thus the Blackcaps, Sparrows,
and other birds infested by _D. macrostomum_ have acquired the parasite
when they were {66}nestlings by the unintentional agency of their
parents.[79] The snail regenerates its lost tentacles only for the
sporocyst to again bud off fertile branches into them.
[Illustration: FIG. 33.—A, _Succinea putris_, infested by B,
_Leucochloridium paradoxum_, or the fully-formed sporocyst of _Distomum
macrostomum_. (After Heckert.) A, Natural size; B, × 7.]
The egg of this Distome thus gives rise to a larva which enters the tissues
of one particular Mollusc. Here it becomes a branched sporocyst within
which the sexual worms are formed, apparently each from a single embryonic
blastomere ("Keimzelle"), by a process comparable with the development of a
parthenogenetic ovum, and the whole cycle has been termed _Alloiogenesis_,
_i.e._ alternation of sexual and parthenogenetic generations (Grobben).[80]
Leuckart[81] and Looss,[82] however, consider that what was once a
metamorphosis of an individual (as in the {67}Holostomatidae) has now
become, by maturation of the Cercaria in the comparatively modern
warm-blooded bird, a metamorphosis extending over two or more generations.
_Distomum (Fasciola) hepaticum._—The liver-fluke of the Sheep, which
produces the disastrous disease, liver-rot, has a distribution as wide as
that of a small water-snail, _Limnaea truncatula_, the connexion between
the two being, as Thomas[83] and Leuckart discovered, that this snail is
the intermediate host in which the earlier larval, sporocyst, and redia
stages are passed through, and a vast number of immature flukes (Cercariae)
are developed. These leave the snail and encyst upon grass, where they are
eaten by the sheep. Over the whole of Europe, Northern Asia, Abyssinia, and
North Africa, the Canaries, and the Faroes, the fluke and the snail are
known to occur, and recently the former has been found in Australia and the
Sandwich Islands, where a snail, apparently a variety of _Limnaea
truncatula_, is also found.[84] Over these vast areas, however, the disease
usually only occurs in certain marshy districts and at certain times of the
year. Meadows of a clayey soil, liable to be flooded (as in certain parts
of Oxfordshire), are the places where this _Limnaea_ occurs most
abundantly, and these are consequently the most dangerous feeding-grounds
for sheep. The wet years 1816, 1817, 1830, 1853, and 1854—memorable for the
occurrence of acute liver-rot in England, Germany, and France—showed that
the weather also plays a considerable part in extending the suitable ground
for _Limnaea_ over wide areas, which in dry years may be safe pastures. In
1830 England lost from this cause,[85] one and a half million sheep,
representing some four millions of money, while in 1879-80 three millions
died. In 1862 Ireland lost 60 per cent of the flocks, and in 1882 vast
numbers of sheep perished in Buenos Ayres from this cause. In the United
Kingdom the annual loss was formerly estimated at a million animals, but is
now probably considerably less. After infection during a wet autumn, it is
usually in the succeeding winter that the disease reaches its height.
{68}The symptoms of "rot" appear about a month after infection, more
acutely in lambs than in sheep, and again, less in oxen than in sheep. At
first, death may result from cerebral apoplexy, but if the first few weeks
are passed through, a pernicious anaemia sets in, the sheep are less lively
and fall at a slight touch, the appetite diminishes, and rumination becomes
irregular. The conjunctiva is of a whitish-yellow colour, the dry, brittle
wool falls off, and there is sometimes fever and quickened respiration. In
January, about three months after infection, the wasting, or fatal, period
sets in. Oedemas or swellings, usually visible before, become larger at the
dependent parts of the body, a large one in the submaxillary region being
especially well marked, and this is considered one of the most
characteristic symptoms ("watery poke"). Through this period few of the
infected sheep survive, but should they do so, the flukes begin to migrate,
though some remain much longer within the liver. Migration is effected
through the bile-duct into the duodenum and outwith the faeces, in which
the altered remains of the _Distomum_ are sometimes scarcely recognisable.
Under these circumstances (or owing to death of the fluke _in situ_) the
sheep recover more or less fully.
The preventive measures seem to be: (1) Destruction of the eggs and of the
manure of rotten sheep; (2) slaughter of badly fluked sheep; (3) adequate
drainage of pastures; (4) an allowance of salt and a little dry food to the
sheep; and (5) dressings of lime or salt on the ground to destroy the
embryos.[86]
_Distomum hepaticum_, contrary to most Trematodes, enjoys a wide range of
hosts. Man himself occasionally falls a victim; thus in Dalmatia, in the
Narenta Valley, the disease is endemic but slight in its effects. The
horse, deer, camel, antelopes, goat, pig, rabbit, kangaroo, beaver, and
squirrel have all been known to harbour this fluke occasionally. In the
Italian deer-parks at Mandria a large species, _D. magnum_, decimated the
herds some years ago; and this species, probably imported from Italy, is
now almost as dangerous a parasite on the western plains of the United
States as _D. hepaticum_.
_Bilharzia haematobia._[87]—This formidable parasite was discovered by
Bilharz in 1853 in the veins of the bladder of patients {69}at the Cairo
Hospital, and is remarkable from its abundance on the east coast and inland
countries of Africa from Egypt to the Cape, as well as in the districts
bordering Lake Nyassa and the Zambesi river, while westwards it occurs on
the Gold Coast. Mecca is a source of infection whence Mohammedans carry the
disease to distant places. In Egypt about 30 per cent of the native
population is affected by the serious disease known as Haematuria,
resulting from the attacks of _Bilharzia_, so that, of the many scourges
from which in Africa man suffers, this one is perhaps the most severe.
[Illustration: FIG. 34.—_Bilharzia haematobia_ Cobb. × 10. The female (♀)
lying in the gynaecophoric canal of the male (♂). _d_, Alimentary canal;
_ms_, oral sucker of male; _vs_, ventral suckers. (After Leuckart.)]
The worm is found usually in couples, which have been proved to be male and
female individuals (Fig. 34), often in considerable numbers in the veins of
the pelvic region, chiefly the veins of the bladder and of the large
intestine, and it is tolerably certain that _Bilharzia_ enter these vessels
from the portal vein. Their long slender bodies enable them to penetrate
into the finer vessels, which get partially or entirely choked up, and the
circulation accordingly impeded. But the most serious consequences are
observed in the urinary bladder. The mucous membrane is swollen and
inflamed here and there, chiefly on the dorsal surface, the capillaries
appear varicose and covered with mucus, mixed with blood-extravasations in
which _Bilharzia_-eggs are noticeable. The eggs also cause numerous swollen
knots in the submucous tissue. Should the disease not pass beyond this
stage (and such is usually the case, especially in South Africa), a
temporary haematuria ensues. The urine, which is only expelled with great
effort, accompanied by intense pain, is mixed with blood, mucous clots, and
masses of _Bilharzia_-eggs, from which some of the embryos have already
hatched out. The symptoms, however, may gradually pass away, and a more or
less complete recovery accomplished. The disease may indeed be of a far
less severe character, and may not interfere with the usual occupations of
the patient; but, on {70}the other hand, a far more extensive thickening of
the wall of the bladder sometimes occurs; hard masses of eggs, uric acid
crystals, and other deposits, may lead to the formation of stones,
degeneration of the substance of the ureter, and eventually to that of the
kidney itself. The stone, indeed, has long been known to be a prevalent
disease in Egypt, and it is now known to arise from concretions formed
round masses of _Bilharzia_ eggs. From the portal vein, again, other
_Bilharzia_ may gain access to the rectum, or the liver, and it has also
been found in the lungs, and may give rise to most serious complications,
if indeed the patient lives.
How infection occurs is a question to which at present no satisfactory
answer can be made. The attempt to introduce embryos of _Bilharzia_ into
the common fresh-water animals of Alexandria has hitherto proved fruitless
(Looss[88]), although there seems little doubt that the comparative
immunity of Europeans from the disease is in some way owing to their
drinking purer water than the natives. Possibly, as Leuckart suggests, the
embryo becomes a sporocyst in man himself, somewhat as _Taenia murina_ is
known to develop in the rat without an intermediate host.[89] The immense
numbers of the parasite in one host would then readily receive an
explanation.
A _Bilharzia_, possibly _B. haematobia_, was found by Cobbold in the portal
vein of _Cercopithecus fuliginosus_; and _B. crassa_ infests the cattle of
Egypt, Sicily, and certain parts of India, but does not produce haematuria.
Of the other Trematodes of man and domestic animals there is not room to
speak fully. _Distomum pulmonale_, which occurs in the lungs of the cat,
tiger, and dog, as well as in man, is especially common in Japan, China,
Corea, and Formosa. _D. sinense_ and _D. rathouisi_ have been also found in
inhabitants of these countries.
BISEXUAL TREMATODES.—Zoologically, _Bilharzia_ is interesting from its
bisexual condition. It is not, however, the only bisexual Trematode. In
cysts in the branchial chamber of Ray's bream, _Brama raii_, two worms are
found, which are probably the slender male and the swollen female of the
same species (_Distomum okenii_). The only doubt that can arise proceeds
from the tendency in all Trematodes for the male organs to ripen before
{71}the female organs. Until we certainly know that the swollen egg-bearing
form (♀) does not arise from a previously male form (♂), the case is open
to suspicion. Since, however, Kölliker[90] never found intermediate
hermaphrodite conditions, this _Distomum_ may be almost certainly regarded
as of distinct sexes. _Didymozoon thynni_ (_Monostomum bipartitum_), from
cysts on the gills of the Tunny (_Thynnus_), is another case. Two slender
worms flattened posteriorly, come together, and the body of one becomes
folded to receive that of the other. They fuse completely except for a
small lateral opening through which the anterior parts of both worms may
freely protrude. The enclosing individual contains a coiled uterus filled
with eggs, and is the female, whereas the smaller individual never
possesses eggs, and is probably the male.[91] _Nematobothrium_ (Fig. 22,
A), which occurs also in the Tunny, in the form of two immensely long
individuals intricately wound about each other in a cyst, is, however, not
bisexual.
[Illustration: FIG. 35.—_Distomum okenii_ Köll. Showing male and female as
they occur together in the branchial cavity of _Bramaraii_ (Ray's bream).
(From Bronn, after Kölliker.) Nat. size.]
TABLE OF DIGENETIC TREMATODES AND THEIR LIFE-HISTORIES.[92]
Host into which the Host into which the
larva enters, and in Cercariae migrate and
Species. Final host. which Cercariae are encyst; eaten by
eventually formed. final host.
_Diplodiscus_ Smaller species Insect-larvae, _Rana_,
(_Amphistomum_) _Rana_, _Bufo_, of _Planorbis_ _Bufo_, but frequently
_subclavatus_ _Triton_ and _Cyclas_ omitted
Göze
_Distomum advena_
Duj. _Sorex araneus_ Not known _Limax_
(_D. migrans_ Duj.)
_D. appendiculatum_ _Clupea alosa_ Not known _Lucullus acuspes_,
Rud. _Centropages hamatus_
(Copepoda)
_Limnaea
_D. ascidia_ Species of Bats stagnalis_ _Ephemera_, _Perla_,
v. Ben. _Planorbis _Chironomus plumosus_
corneus_
_D. atriventre_ Frogs and Toads _Physa heterostropha_
Weinl. of N. America Not known
_D. brachysomum_ The Dunlin Not known _Anthura gracilis_
Crepl. (_Tringa alpina_)
_D. caudatum_ Hedgehog _Helix hortensis_
v. Linst. (_Erinaceus europaeus_)
_D. clavigerum_ _Limnaea ovata_ Not known {72}
Rud. _Rana_ _Planorbis corneus_
_D. cygnoides_ _Pisidium_, _Limnaea_ sp.
Zed. _Rana_ _Cyclas_ (_Cercaria macrocerca_
Fil.)
_D. cylindraceum_ _Limnaea ovata_
Zed. _Rana_ _Ilybius fuliginosus_
_D. dimorphum_ _Ardea_, Different species of
Dies. _Ciconia_ Not known Fishes
(Brazil)
_D. echinatum_ _Cygnus_, Species of Species of _Limnaea_,
Zed. _Anser_, Limnaea _Paludina vivipara_
_Anas_
_D. endolobum_ _Rana_ _Limnaea _L. stagnalis_,
Duj. stagnalis_ _Gammarus pulex_,
larvae of
_Limnophilus rhombicus_
_Limnaea stagnalis_,
_L. ovata_,
_D. globiporum_ Not known _Succinea pfeifferi_,
Rud. _Perca fluviatilis_ _S. putris_,
_Physa fontinalis_,
_Planorbis marginatus_
_D. hepaticum_ Sheep, Oxen, _Limnaea truncatula_
Abild. Man, etc. Omitted
_D. hystrix_ _Lophius piscatorius_ Marine Fishes
Duj. Not known
_D. macrostomum_ Warblers, Tits, Omitted
Rud. Woodpeckers, etc. _Succinea putris_
_D. militare_ Common Snipe _Paludina vivipara_ _P. vivipara_
v. Ben.
_D. nodulosum_ _Bithynia tentaculata_ _Cyprinus_,
Zed. _Perca fluviatilis_ _Acerina cernua_
_D. ovocaudatum_ Species of _Planorbis_ Probably omitted.
Vulp. _Rana esculenta_ (_Cercaria_ known
as _C. cystophora_
Wag.)
_D. retusum_ _Rana_ _Limnaea stagnalis_ _L. stagnalis_, larvae
Duj. of Phryganeidae
_D. squamula_ Polecat Unknown _Rana temporaria_
Dies.
_D. signatum_ _Tropidonotus Unknown _Rana_
Duj. natrix_
_D. trigonocephalum_ _Paludina vivipara_
Rud. Badger, Polecat Unknown
Dogfish, Rays _Ostrea edulis_, _Belone vulgaris_
_Gasterostomum_ sp. _Cardium rusticum_,
_C. edule_
_Unio_, _Anodonta_
_G. fimbriatum_ (_Cercaria_ known _Leuciscus
v. Sieb. _Perca_, _Esox_ as _Bucephalus erythrophthalmus_
polymorphus_)
_G. gracilescens_ Unknown Species of _Gadus_
Rud. _Lophius piscatorius_ (e.g. _G. aeglefinus_),
_Molva_, _Lophius_
_Monostomum flavum_ _Planorbis corneus_
Mehl. _Anas_ Omitted
CLASSIFICATION OF TREMATODES.—We have seen (p. 63) that it is hardly
possible to carry out fully the division of Trematodes into Monogenea and
Digenea. Nevertheless, pending further investigation on the doubtful
points, this classification may still be {73}used. Monticelli[93] has
proposed the main divisions of a new classification, which has been also
adopted by Braun, and is based on the nature of the suckers. These
divisions are indicated below in brackets.
A. MONOGENEA v. Ben. (HETEROCOTYLEA Mont.).
1. Fam. TEMNOCEPHALIDAE Hasw.
Gen. _Temnocephala_ Hasw.
2. Fam. TRISTOMATIDAE Tschbg.
Sub-Fam. 1. Tristomatinae Mont.
Gen. _Tristomum_, _Nitzschia_, _Epibdella_, _Trochopus_,
_Acanthocotyle_, _Phyllonella_, _Placunella_,
_Encotylabe_.
Sub-Fam. 2. Monocotylinae Tschbg.
Gen. _Pseudocotyle_, _Calicotyle_, _Monocotyle_.
Sub-Fam. 3. Udonellinae v. Ben.-Hesse.
Gen. _Udonella_, _Echinella_, _Pteronella_.
3. Fam. POLYSTOMATIDAE Tschbg.
Sub-Fam. 4. Octocotylinae v. Ben.-Hesse.
Gen. _Octobothrium_, _Pleurocotyle_, _Diplozoon_,
_Anthocotyle_, _Vallisnia_, _Phyllocotyle_,
_Hexacotyle_, _Platycotyle_, _Plectanocotyle_,
_Diclidophora_.
Sub-Fam. 5. Polystomatinae v. Ben.
Gen. _Polystomum_, _Onchocotyle_, _Erpocotyle_,
_Diplobothrium_, _Sphyranura_.
Sub-Fam. 6. Microcotylinae Tschbg.
Gen. _Microcotyle_, _Gastrocotyle_, _Axine_, _Pseudaxine_.
4. Fam. GYRODACTYLIDAE v. Ben.
Sub-Fam. 7. Gyrodactylinae Par. et Per.
Gen. _Gyrodactylus_, _Dactylogyrus_, _Tetraonchus_,
_Diplectanum_.
Sub-Fam. 8. Calceostominae Par. et Per.
Gen. _Calceostomum_, _Anoplodiscus_.
5. Fam. ASPIDOBOTHRIDAE Burm. (= Aspidocotylea Mont.).
Gen. _Aspidogaster_, _Platyaspis_, _Cotylogaster_,
_Macraspis_.
B. DIGENEA v. Ben. (MALACOCOTYLEA Mont.).
6. Fam. HOLOSTOMATIDAE Brandes (= Metastatica Leuckart).
Gen. _Diplostomum_, _Polycotyle_, _Hemistomum_,
_Holostomum_.
7. Fam. AMPHISTOMATIDAE Mont.
Gen. _Amphistomum_, _Diplodiscus_, _Gastrodiscus_,
_Homalogaster_, _Gastrothylax_, _Aspidocotyle_.
8. Fam. DISTOMATIDAE Mont.
Gen. _Distomum_ (and sub-genera), _Rhopalophorus_,
_Koellikeria_, _Bilharzia_.
9. Fam. GASTEROSTOMATIDAE Braun.
Gen. _Gasterostomum_.
10. Fam. DIDYMOZOONTIDAE Mont.
Gen. _Didymozoon_, _Nematobothrium_.
11. Fam. MONOSTOMATIDAE Mont.
Gen. _Monostomum_, _Notocotyle_, _Ogmogaster_,
_Opisthotrema_.
{74}CHAPTER III
CESTODA
INTRODUCTION—NATURE OF CESTODES—OCCURRENCE OF CESTODES—THE TAPE-WORMS OF
MAN AND DOMESTIC ANIMALS—TABLE OF THE LIFE-HISTORIES OF THE PRINCIPAL
CESTODES OF MAN AND DOMESTIC ANIMALS—STRUCTURE AND DEVELOPMENT OF
CESTODES—TABLE FOR THE DISCRIMINATION OF THE MORE USUAL CESTODES OF MAN AND
DOMESTIC ANIMALS—CLASSIFICATION.
The Cestodes or Tape-worms are exclusively endoparasitic Platyhelminthes
living, in the adult condition, in the alimentary canal of Vertebrates,
with the exception of _Archigetes_ (Fig. 37), which may become mature in
the body-cavity of _Tubifex_. In relation with this wholly parasitic
existence, the Cestodes exhibit certain characteristic modifications in
structure and mode of development, such as the formation, by the
segmentation of the "neck," of a (usually) long chain of "proglottides" or
joints, which form the "body" of the Cestode; and the entire absence of an
alimentary tract, both in the larva and adult. As an adaptation to the
fixed mode of life, the anterior end (head, scolex) is modified to form an
adhering organ. Various adaptive forms of larvae are known. These live in
the internal organs of one or more intermediate hosts, and are transferred
to the final host passively during a meal. Lastly, there is the curious
metamorphosis by which the adult is formed from a portion (scolex) of the
larva.[94]
{75}[Illustration: FIG. 36.—_Echinobothrium affine_ Dies., from the
intestine of _Torpedo_, × 43. _hd_, Head; _hk_, hooks; _hl_, lobes of the
head; _ov_, ovary; _pe_, penis; _ps_, penis-sheath; _te_, testes; _ut_,
uterus; _vag_, vagina; _yg_, yolk-glands. (After Pintner.[95])]
_Taenia solium_, from man (Fig. 39, B), or _Echinobothrium_ (Fig. 36), from
an Elasmobranch fish, is fixed to the mucous lining of the intestine of its
host by means of a radially-constructed apparatus of four suckers and a
circlet of hooks (Fig. 39), which are borne by the "head" or "scolex,"
being that part of the worm which is directly derived from part of the
larva, and which contains the central, commissural portion of the nervous
system. Firm adhesion to the host's intestine is necessary, in order to
avoid the loosening action of the peristaltic movements of the intestine as
the food passes along. The heads of different Cestodes exhibit a marvellous
variety of suckers and hooks, from a mere muscular depression in
_Schistocephalus_, to the compound proboscides of _Tetrarhynchus_[96] which
is found in Elasmobranchs. The jointed body, often of enormous length (up
to 20 yards in _Bothriocephalus latus_), is usually separated from the head
by a slender neck, from which the proglottides are segmented off from
behind forwards, and become more and more individualised as they recede
farther away from the neck by the intercalation of younger joints. Thus in
Fig. 36 the mature, distal proglottis has passed through all the stages
represented by the other segments.
{76}[Illustration: FIG. 37.—_Archigetes sieboldii_ (_appendiculatus_), from
the coelom of _Tubifex rivulorum_. × 40. _app_, Persistent larval
appendage; _go_, genital pore; _hk_, persistent larval hooks; _ov_, ovary;
_sc_, sucker; _te_, testes; _yg_, yolk-glands. (After Leuckart.)]
The longitudinal muscles, the nerves, and excretory vessels which supply
the proglottides are continuous throughout and with those of the head. Each
joint contains at first male genitalia comparable with those of a
Trematode; then the female organs develop, and finally self-fertilisation
follows. The Cestodes feed through their skin, probably by the aid of fine
protoplasmic processes, which penetrate the tough investing membrane and
absorb the already digested food which bathes them. When a proglottis of
_Calliobothrium_ is approaching maturity it separates from the parent, the
broken ends of muscles, nerves, and excretory vessels speedily heal, and it
is now capable of continued growth and of fairly active movement if it
remains in the intestine of the host. According to van Beneden, it may even
attain a size equal to, or exceeding, that of the whole parent or
"strobila."[97] These considerations led Leuckart, von Siebold, P. J. van
Beneden, and others, to Steenstrup's conclusion that a jointed tape-worm is
really a colony composed of two generations—the head and neck derived from
the larva, and the proglottides produced by the segmentation of the
neck.[98] This view of the colonial nature of jointed Cestodes was
generally adopted from 1851 to 1880. During the last fifteen years,
however, the varied interpretations of the facts of the ontogeny of this
group have led some authors to adopt the monozootic view (that a Cestode is
one individual), others are still of the older opinion, and Hatschek
(_Lehrbuch_, p. 349) and Lang take up intermediate positions. Lang
considers that the formation of the joints of a tape-worm from a small
fixed "scolex," is not only largely comparable with the strobilation of a
_scyphistoma_ and the consequent formation of a pile of medusae, as in the
life-history of _Aurelia_, but {77}that both processes have arisen from the
power of regenerating the necessary organs in each of the new segments. The
result in both cases is the rapid formation of a number of joints, which
gradually separate from the parent, to carry the eggs and young to new
stations. Just as some Coelenterata (_Lucernaria_) may be regarded as not
having advanced much beyond a scyphistoma stage, so there are unisegmental
Cestodes (e.g. _Archigetes_, Fig. 37) which have remained as a slightly
altered but sexual scolex, directly comparable with a Trematode, and, as
all authors are agreed, representing one generation only. Such monozootic
forms are now classed as a special family, the Cestodaria or Monozoa, of
which _Caryophylleus mutabilis_, from the intestine of various Cyprinoid
fish, is the most abundant representative, while _Amphiptyches_
(_Gyrocotyle_) _urna_, from _Chimaera monstrosa_ of the northern
hemisphere, is paralleled by _A. rugosa_, found in _Callorhynchus
antarcticus_ of the southern seas.
[Illustration: FIG. 38.—_Scolex polymorphus_ Rud. (larva of _Calliobothrium
filicolle_ Zschokke), from the muscles of _Apogon_, a Mediterranean fish;
also found in many Invertebrates (e.g. _Sepia_). A, Inverted scolex, with
calcareous bodies; B, everted older larva. _br_, Brain; _exo_, terminal
excretory aperture; _fc_, flame-cells; _for.sec_; secondary excretory
pores; _hk_, hooks of the adult Cestode; _inrag_, pit at the bottom of
which the head is developed; _msc_, anterior sucker; _nl_, lateral nerve;
_sc_, suckers; _tl_, _tp_, lateral and main excretory vessels. (After
Monticelli.)]
OCCURRENCE OF CESTODES.—The distribution of Cestodes and their larvae is
analogous to that of the digenetic Trematodes, although the absence of an
alimentary canal limits the habitat of the mature worms to certain sites,
such as the blood-vessels, the lymphatic and coelomic spaces, and the
digestive system, where their body may be bathed by a nutritive fluid.
Almost all groups of Vertebrates are attacked by Cestodes. Those of fishes,
and particularly of Elasmobranchs, are distinguished by certain structural
and developmental features; those of birds by {78}others; those of mammals,
by a third set of characters. The young stages of the Cestodes of Sharks
and Rays occur encysted in the body-cavity, or in the pyloric appendages,
of Teleosteans, which probably swallow them along with those invertebrate
animals upon which they prey. The larvae of the Cestodes of carnivorous
mammals or piscivorous birds, live respectively in herbivores and fishes,
but how the latter are infected we know in very few instances. Cestode
larvae are known to occur in many Invertebrates, and occasionally are taken
free swimming in the sea, presumably crossing from one host to the next.
Ctenophores, Siphonophores, Copepods, Ostracods, Decapods, various Molluscs
especially Cephalopods, Earthworms, and other Annelids, are the
intermediate hosts of these larvae (see Fig. 38), the fate of which,
however, has been determined in but few cases.
OCCURRENCE OF CESTODES IN MAN.[99]—Tape-worms, either in the adult or
larval stages (bladder-worms), have, from ancient times, been known to
occur in man, and in the animals that serve him as food. Until
comparatively recent times, however, the true nature of these parasites,
and particularly of "hydatids" (cystic larvae), was unrecognised. Up to the
seventeenth century the larvae were regarded as abscesses or diseased
growths of the affected organs, and it was only at the close of that
century that their animal nature was even suggested. Even at the beginning
of the nineteenth century, three modes of origin of Cestodes—by "generatio
aequivoca" from the tissues of the body, or by the union of previously
distinct proglottides, or again by metamorphosis of free-living worms drunk
with water by cattle or birds (as Linnaeus suggested)—were still variously
held, at a time when Malpighi, Pallas, and Goeze had recognised the true
connexion between the cystic and segmented states of _Taenia crassicollis_
(the cat tape-worm), and when Goeze had seen the eggs of _Taeniae_, and
Abildgaard[100] had even conducted the first helminthological experiments
(conversion of the larval _Schistocephalus_, Fig. 40, into the adult form).
{79}[Illustration: FIG. 39.—A, _Taenia saginata_ Goeze. Nat. size. (From a
specimen in the Cambridge Museum.) The approximate lengths of the portions
omitted in the drawing are given. At * (after Leuckart) the branched uterus
and the longitudinal and transverse excretory vessels are shown. The
genital apertures are seen as a lateral opening on each of the larger
proglottides. B, Head (scolex) of _T. solium_ Rud. × 12. (After Leuckart.)]
Generally speaking, "a tape-worm" in Western Europe will prove to be
_Taenia saginata_ Goeze (the beef tape-worm, Fig. 39, A), exceedingly
prevalent also in the East, and indeed cosmopolitan, occurring wherever the
infected flesh of the ox is eaten in a raw or half-cooked state. Its
attacks are fortunately not usually severe. _Taenia solium_ Rud. (the pork
tape-worm) is found wherever the pig is kept as a domestic animal, and has
consequently a world-wide distribution. Its size (6-9 feet long) and powers
of adhesion would alone render _T. solium_ a formidable parasite. But the
danger of its presence in the body of man, or in the flesh of pigs, lies in
the fact that the larva or bladder-worm (known as _Cysticercus cellulosae_)
can live in the most varied organs. Thus if by accident a mature proglottis
be eaten, the embryos escape, bore their way into the wall of the stomach,
and entering the portal vein, may reach in time the muscles, the brain, the
eye, or even the heart itself, and attain the cystic condition. Even more
disastrous may be the result, should some ripe joints of a mature worm work
their way from the intestine back towards the stomach. Should this happen
(and though it has not been directly proved, the possibility is to be
reckoned with), the result would be the release of vast numbers of embryos
capable of inflicting fatal injury on the host. An abnormal _Cysticercus_
of this species is probably the _Taenia_ {80}(_Cysticercus_) _acanthotrias_
Weinl. (see, however, Leuckart, _loc. cit._ p. 711).
_Taenia_ (_Hymenolepis_) _nana_ v. Sieb.[101] is found in man in Egypt,
Italy, England, Servia, Argentine Republic, and the United States. Though
small (¾-1 inch long), its numbers usually excite digestive and nervous
disorders of considerable severity, more serious, indeed, than those caused
by the commoner tape-worms. _H. diminuta_ Rud. (_flavopunctata_ Weinl.),
normally found in Rodents, has been rarely recorded in man. _Taenia_
(_Dipylidium_) _caninum_ L. (= _T. cucumerina_ Bloch = _T. elliptica_
Batsch), the commonest parasite of pet cats and dogs, and _T._ (_Davainea_)
_madagascariensis_ Davaine, have occasionally been recorded from infants
and young children. But the attacks of these species are insignificant in
comparison with those of the cystic stage (_Echinococcus polymorphus_) of a
tape-worm (_T. echinococcus_ v. Sieb.) which lives when mature in the dog.
_Echinococcus_ is most frequent in Iceland, where it affects 2 to 3 per
cent of the population, and a still larger proportion of sheep; while in
Copenhagen, Northern Germany, some districts of Switzerland, and Victoria
it is not uncommon, but is frequently found during _post-mortem_
examinations when no definite symptoms of its presence had been previously
noticed. _Echinococcus_[102] varies greatly in size, form, and mode of
growth, but is distinguished in the formation not of one scolex only, as in
the _Cysticercus_, but in the production of a number of vesicles, usually
from the inner wall. Within these, large numbers of scolices may be
developed. The whole organism continues to swell by the formation of a
watery liquid within it, and if its growth be rapid the fluid tension may
cause the rupture of the enclosing connective-tissue capsule formed around
the parasite, at the expense of the host, and the protrusion of the
daughter vesicles. It is the consequent injury to the surrounding organs of
the host, at this critical stage, often only reached after the lapse of
several years, that occasions serious or even fatal results. Zoologically,
_Taenia echinococcus_ and _T. coenurus_ are interesting, since they exhibit
an {81}indubitable alternation of asexual generations in the larval state,
with a sexual adult stage.
_Bothriocephalus latus_ Brems., the broad tape-worm, which attains a length
of 20-30 feet, or even more, occurs in man endemically in the eastern
Baltic provinces, certain parts of Switzerland, generally throughout Russia
(especially near Kasan), in North America, and commonly in Japan,—that is,
in districts where the population partake largely of pike or other fish in
a raw or partially-cooked state. Elsewhere it occurs sporadically, and in
Munich, where it was unknown before 1880, its presence has been traced to
emigrants from infected districts, who settled on the shores of the
Starenberger Lake, from which Munich was supplied with fish. How the pike,
the usual but not invariable intermediate host, becomes infested (and its
musculature is frequently riddled with the larvae) we do not accurately
know, but some Invertebrate, the prey of the pike, is probably the first
host into which the free-swimming ciliated larva (Fig. 42) finds its way.
In Greenland, _B. cordatus_ is very common in the dog, and probably also in
man, though few cases have been recorded. _B. mansoni_ Cobb. (= _B.
liguloides_ Leuck.) was, till recently, known only in the larval state from
China and Japan. Iijima, however, has found older specimens in the latter
country. _B. cristatus_ Dav. is a species founded somewhat doubtfully on
two fragments found, one in a child, the other in a man, in France.
OCCURRENCE OF CESTODES IN DOMESTIC ANIMALS.[103]—Among domestic animals,
the dog is, undoubtedly, the most frequently attacked by Taeniae. Six
species of _Taenia_ (_T. serrata_, _marginata_, _coenurus_, _echinococcus_,
_krabbei_, and possibly _T. serialis_), _Dipylidium caninum_ (the commonest
form), _Mesocestoides lineatus_, and three or four species of
_Bothriocephalus_ have been found in the dog. The table of life-histories
(p. 83) shows that sheep, rabbits and other Rodents serve as the
intermediate hosts, in which the cystic stages of the species of _Taenia_
are found. Hence the prevalence of _T. serrata_ in a given locality is
connected with the abundance there of the rabbit and hare, in which the
larva (_Cysticercus pisiformis_) occurs. _Bothriocephalus cordatus_
develops from the young stage present in the fish which the Icelanders give
to their dogs. In Iceland and certain parts of {82}Australia _T.
echinococcus_ infests one-third to one-half the number of dogs examined; a
fact connected with the frequency of _Echinococcus_ in man in these
countries.
In sheep the most noteworthy and dangerous parasite is _Coenurus
cerebralis_ (or the cystic stage of the dog-taenia, _T. coenurus_), which
gives rise to the disease known as "gid" or "staggers." It is found in
various parts of the brain or spinal cord, and the symptoms differ
according to the position of the parasite. If this presses upon one
hemisphere the sheep describes circles and finally falls: if on the optic
lobes, the eyes are affected: if the pressure affects the cerebellum the
movements of the sheep are uncertain and incoordinated. Four or six weeks
after the appearance of the symptoms, death results from cerebral
paralysis, or from general debility, and the loss of sheep incurred by this
disease (happily less frequent in England than formerly) has been
calculated by Youatt at a million for France annually; at 35 per cent of
the flocks for England in bad seasons; and about 2 per cent for Germany.
Besides sheep, which are most subject to "gid" during their first year,
various ruminants—Goat, Ox, Moufflon, Chamois, Roe, Antelope, Reindeer,
Dromedary—are attacked in the same way. A similar form, _Coenurus serialis_
Baill., is common in the wild rabbit in this country, and in Australia in
the hare and squirrel. It forms large swellings in the connective tissue of
various parts of the body, but usually does not affect the health of the
host. It is not known in what carnivore _Taenia serialis_ Baill. normally
occurs. Experiments have, however, shown that it develops rapidly in dogs.
The preventive measures which are steadily diminishing the prevalence of
the Cestode parasites in man in some parts of Western Europe cannot be
dealt with here, but it may be noticed that the Jewish observance with
regard to swine is the surest preventive measure against taeniasis and
trichinosis. Careful inspection of meat and general cleanliness, are the
leading measures that in these hygienic matters secure the greatest
immunity from disease.
{83}TABLE OF THE LIFE-HISTORIES OF THE PRINCIPAL CESTODES OF MAN AND THE
DOMESTIC ANIMALS.
Cestode. Final host. Larva. Intermediate host.
_Taenia serrata_ _Cysticercus Rabbit, Hare, Mice
Goeze Dog pisiformis_ (liver and peritoneum)
Zed.
_T. marginata_ _Cyst. Monkeys, Ruminants,
Batsch Dog, Wolf tenuicollis_ ungulates (in
Rud. peritoneum)
_T. saginata_
Goeze (= _T. Man _Cyst. bovis_ Ox, Giraffe
mediocanellata Cobb. (in muscles)
_Küch.)
_Cyst.
_T. solium_ Man cellulosae_ Pig, Man, Monkeys,
Rud. Rud. (? _Cyst. Bear, Dog, Cat, Black
acanthotrias_ Rat (in various
Weinl.) organs)
_T. crassicollis_ Cat and other _Cyst. fasciolaris_
Rud. Felidae, Rud. Rat, Mouse, Bat (liver)
Stoat
_T. coenurus_. Dog, Arctic _Coenurus Brain of Sheep, Ox,
Küch Fox cerebralis_ Goat, Dromedary, Camel,
Rud. Antelope, Horse
_T. serialis_ ? Dog _Coenurus Rabbit (connective
Baill. serialis_ Baill. tissue)
_Echinococcus Man, Monkeys, many
_T. echinococcus_ Dog, Dingo, polymorphus_ Carnivores, Rodents,
v. Sieb. Jackal, Wolf Dies.(incl. Ungulates, Ruminants,
_E. multilocularis_ and Marsupials; also
found in Man) in Turkey and other
birds
_Moniezia expansa_ Sheep, Ox, Unknown
Rud. Goat, etc.
_Thysanosoma Sheep, Unknown
fimbriata_ Dies. Cervidae
_Stilesia
globipunctata_ Sheep Unknown
Riv.
_Anoplocephala
perfoliata_ Horse Unknown
Goeze
_Dipylidium Cysticercoid larva
caninum_ L. Man, Dog, (Fig. 43), Body-cavity of
(= _Taenia Cat _Cryptocystis_ _Trichodectes_ and
cucumerina_ trichodectis_ _Pulex_ of Dog
Bloch = _T. Vill.
elliptica_ Batsch)
_Hymenolepis_ _Cercocystis_ Usually absent
_murina_ Duj. Mouse, Rat Vill.[104] (develops
in parental host)
_H. nana_ Man Unknown
v. Sieb.
_H. diminuta_ Meal-moth, _Asopia_
Rud.(= _Taenia Man, Mouse, _Cercocystis_ (_Pyralis_)
flavopunctata_ Rat Vill. _farinalis_; also
Weinl.) certain Orthoptera
and Coleoptera
_Drepanidotaenia_ Duck, Goose, _Cercocystis_ The Ostracods {84}
_gracilis_ Zed. Wild Duck Vill. _Candona rostrata_ and
_Cypris compressa_,
and also
_Cyclops viridis_
_D. anatina_ Duck " " _Cypris incongruens_,
Krabbe and also Perch
_D. setigera_ Goose " " _Cyclops brevicaudatus_
Fröh.
_D. infundibuliformis_
Goeze Common Fowl " " House-fly
_Dicranotaenia Duck " " _Cypris ovum_
coronula_ Duj.
_Davainea proglottina_ " " ? _Limax cinereus_,
Dav. Fowl _L. agrestis_
_D. madagascariensis_ Unknown
Dav. Children
_D. friedbergeri_ Unknown ? Ants
v. Linst. Pheasant
_Mesocestoides
lineatus_ Dog Unknown
Goeze
Plerocercoid, Probably first enters
_Bothriocephalus Man, Dog, i.e. solid, an Invertebrate host,
latus_ Brems. ? Cat elongate larva, which is eaten by
with no bladder Pike, Perch, Trout,
etc.
[Illustration: FIG. 40.—A, Stickleback (_Gasterosteus aculeatus_) infested
by an advanced larva of _Schistocephalus solidus_ Crepl. B, The larva. All
× 1½. (From specimens in the Cambridge University Museum.)]
STRUCTURE AND DEVELOPMENT OF CESTODA.[105]—Of the unsegmented Cestodes,
_Caryophyllaeus mutabilis_, from the intestine of carp and other Cyprinoid
fishes, is the most easily accessible form. _Triaenophorus nodulosus_,
which is very useful for the study of the excretory system, occurs mature
in the pike. In the body-cavity of the Stickleback (Fig. 40) a large,
broad, yellow worm may sometimes {85}be found, the larva of
_Schistocephalus solidus_ Crepl., which occurs in the intestine of Terns,
Storks, Mergansers, and other birds. Species of _Ligula_ are found in the
same birds. The intestine of a _Lophius_ or _Cyclopterus_ ("lump-fish")
contains, usually, the early and intermediate stages of various Cestodes,
while the alimentary canal of Elasmobranchs often contain many peculiar
Tetrarhynchidae and other forms. For the study of development, the _Taenia
anatina_ from the duck may be used. The ripe proglottides are collected,
and the eggs placed with _Cypris ovum_ in an aquarium, with the probability
that some of the embryos will enter the Ostracod, and the peculiar
Cysticercoid may be bred.[106] _Cysticercus pisiformis_ and _Coenurus
serialis_, which occur commonly in rabbits, are also suitable objects for
examination.
A Cestode such as _Echinobothrium_ (Fig. 36) is divisible into head and
proglottides. Moniez has suggested that the head is really the
morphologically hinder end of the body, in which case the formation of
proglottides would closely resemble the mode of segmentation of an Annelid
larva. The close similarity, however, between the Cysticercoid larva (Fig.
43, F) and the Cercaria of a liver-fluke, seems to show that the anterior
end is the same in both cases, and since it bears the central part of the
nervous system, we may reasonably call it the "head." Moreover the hinder
end of a Platyhelminth usually possesses the chief excretory pore. Another
difficulty is the determination of dorsal and ventral surfaces. Authors are
agreed,—on the analogy of Trematodes, in which the testes are usually
dorsal and the ovaries ventral,—that the dorsal and ventral aspects of a
Cestode are determined by the position of these organs, although the often
radially formed "head," the lateral or superficial position of the genital
apertures, and the variability of these features, render it a matter of
considerable doubt whether "dorsal" and "ventral" are more than useful
conventional terms. The suckers and hooks are borne on a muscular cap, the
"rostellum," which is only slightly developed in the _Ichthyotaeniae_. The
body is solid, and is divisible into an outer muscular coat—enveloped in a
(possibly epidermal) investing membrane—and an inner parenchymatous tissue
containing the chief part of the excretory, nervous, and reproductive
systems. One or two pairs {86}of longitudinal excretory vessels are
present, usually connected by transverse ducts and opening by a single
terminal pore. Occasionally a regularly paired arrangement of lateral or
secondary pores is present (Figs. 38 and 41, _for.sec_). Flame-cells occur
at the end of the fine tubules (Fig. 38), and the whole system is well
developed, but may undergo degenerative changes in the older proglottides.
The central nervous system varies according to the degree of
differentiation of the rostellum; and, owing to the difficulty of staining
the nerves and the contradictory statements of authors, we do not yet
possess a fully reliable account of the nervous system even of the commoner
Taeniae. Free nerve endings and other sensory terminations have been
recently stated to exist in the cuticle of Cestodes and Trematodes. If
true, this would tend to show that the parasitic mode of life of these
animals demands a complex nervous system comparable with that of the
Turbellaria.
[Illustration: FIG. 41.—Diagrammatic transverse section of _Schistocephalus
solidus_ Crepl., from the Wild-duck, illustrative of the Cestodes with
uterine aperture (_uto_). × 12. _cs_, Cirrus-sac; _for.sec_, one of the
paired lateral openings of the excretory vessels; _ln_, longitudinal nerve;
_ov_, ovary; _ovd_, oviduct; _par.m_, parenchymatous muscles; _r.sem_,
receptaculum seminis; _sh.gl_, shell-gland; _te_, testes; _ut_, uterus;
_uto_, uterine pore; _vag_, vagina; _vd_, vasa deferentia; _yd_, yolk-duct;
_yg_, yolk-glands (black); ♂, male, ♀, female genital aperture. (After
Riehm.)]
The reproductive organs, unlike the preceding systems, are discontinuous
from one proglottis to the next. The male and female organs and their
mutual connexions, especially in the unsegmented Cestodes, may be compared
in detail with those of Trematodes, but the difference between the
arrangement of the generative organs of various Cestodes is very
great.[107] The penis (Fig. 41, _cs_) is evaginated through the male pore
(Fig. 41, ♂), and inserted far into the vagina (♀, _vag_) of the same or
another segment of the tape-worm.
{87}[Illustration: FIG. 42.—A, Free-swimming, six-hooked larva of
_Bothriocephalus latus_ Brems. (the broad tape-worm of Man), still enclosed
in a ciliated (possibly cellular) double membrane or mantle. In this
condition it may continue to live in water for a week or more, but
eventually throws off its ciliated coat (as in B) and commences to creep
about vigorously by the aid of its hooks, in search of its first host,
which is at present unknown. (After Schauinsland.) × 600.]
From this fact and the anatomical relations of the vagina, it is becoming
increasingly probable that the so-called uterus of Trematodes is an organ
corresponding to the vagina of Cestodes, and not to the uterus of Cestodes.
The latter opens to the exterior in _Schistocephalus_, _Bothriocephalus_,
and some other Cestodes of fishes by a special pore (Fig. 41, _uto_).
Through this, some of the eggs (which in these genera give rise to ciliated
larvae) are enabled to escape, and need not wait for the detachment of the
proglottis, as must happen in the Taeniidae, where the uterus is closed.
This uterus, a true physiological one, is probably the homologue of the
"canal of Laurer" ("Laurer-Stieda canal," or "vagina") of Trematoda. The
fertilised ovum and yolk are brought together into the "ootype," where the
shell-gland forms the egg-shell around them (Fig. 41, _sh.gl_) and the egg
is then passed into the uterus. The ovum segments to form a minute
six-hooked larva, which may (Bothriidae, Fig. 42) or may not (Taeniidae) be
ciliated. Thus in _Taenia serrata_ the proglottides are shed with the
faeces of the host (dog), and they protect the young from the desiccating
influence of the surroundings. If inadvertently eaten by a rabbit along
with herbs, the proglottis and larval envelope are digested, and by its six
hooks the tiny larva bores through the gastric wall into the portal vein,
and so into the liver. Here the hooks are thrown off, and the solid mass of
cells becomes vacuolated.
{88}[Illustration: FIG. 43.—Stages in the development of _Dipylidium
caninum_ L. (= _Taenia elliptica_ Batsch, _T. cucumerina_ Bloch), the
commonest of the Dog-Taeniae; compare Fig. 44. A, Six-hooked larva (now
often spoken of as an "Onchosphaera"); B, larva elongating; formation of a
central lacuna; C, larva further advanced; D, distinction between body and
tail is visible; E, invagination of the rostellum is commencing; F,
Cysticercoid larva with four suckers, invaginated rostellum, and excretory
vessels. _c_, Calcareous concretions in cells of the larva; _ex.o_,
excretory aperture; _ex.v_, excretory vessels; _inv_, invagination
commencing; _rost_, rostellum; _sc_, suckers. (After Grassi and Rovelli;
highly magnified.)]
[Illustration: FIG. 44.—Schematic longitudinal sections through the larvae
of _Dipylidium caninum_ L. All these stages are passed in the body-cavity
of the Dog-flea (_Pulex serraticeps_). (Compare Fig. 43 for further
details.) A, Six-hooked larva with developing rostellum (shaded) and
suckers (black). In this species the invagination (C, _invag_.) occurs
after the formation of these organs, and not, as in most Taeniae, before
it. B, Invagination commencing; the hooks are developing above the
rostellum, while beneath it the nervous system (dotted) is seen. C, The
invagination has now carried the suckers inwards. The tail has become
distinct, and the whole larva at this stage is known as a Cysticercoid.
_hk_, Larval hooks; _invag_, mouth of the invagination; _n_, central
nervous system; _rost_, rostellum and hooks; _sc_, suckers, of which only
two can be seen in a longitudinal section; four are really present. (After
Grassi and Rovelli.)]
{89}At one pole an invagination occurs, at the bottom of which the
rostellum, suckers, and hooks are gradually formed, but inside out as
compared with the head of the _Taenia serrata_. At this stage the larva
(_Cysticercus pisiformis_) has usually issued from the liver and attached
itself to the omentum. The invagination projects into the cavity of the
bladder, within which a watery fluid accumulates. Thus the "bladder worm"
is formed, the head of which is evaginated if the larva be introduced into
the digestive system of a dog. The bladder and neck of invagination are
digested, while the head, protected by these, remains, and forms the neck,
from which the proglottides are afterwards segmented off. In _Taenia_
(_Hymenolepis_) _murina_ the whole development may take place in the
parental host, the larva living in the villi, the adults in the cavity of
the same rat's intestine (Grassi). The different forms of Cestode larvae
depend largely upon the presence and degree of development of the caudal
vesicle or bladder, which in _Scolex polymorphus_ (Fig. 38) (the young
stage of _Calliobothrium filicolle_ Zsch.) is practically absent. If the
bladder be small, the larva is known as a Cysticercoid. For example, the
common _Dipylidium caninum_, which lives in the dog, has such a larva, the
development of which is explained and illustrated by Figs. 43 and 44. The
bladder becomes exceeding capacious in _Coenurus_ and _Echinococcus_.
TABLE FOR THE DISCRIMINATION OF THE MORE USUAL CESTODES OF MAN AND DOMESTIC
ANIMALS.[108]
I. Scolex in most cases with hooks; uterus with a median and lateral
branches; yolk-glands simple, median; genital pore single; dorsal
excretory vessel narrower than the ventral, without a circular
commissural trunk; eggs without pyriform apparatus (processes of the
ovarian membrane) Gen. TAENIA L. (s. str.)
A. Genital ducts pass on the ventral side of the nerve and of the two
longitudinal excretory vessels _T. crassicollis_ Rud.
B. Genital ducts pass between the dorsal and ventral longitudinal
vessels.
_a._ Nerve present on dorsal side of genital ducts.
α. Head armed _T. solium_ Rud.
β. Head unarmed _T. saginata_ Goeze.
_b._ Nerve on ventral side of genital ducts.
DOG-TAENIAE[109] {90}
Head armed; genital pore marginal and
— Single
Many proglottides; strobila several centimetres long; small
hooks with guard.
Bifid hooks, which are
— 230µ-260µ long[110]; genital pore very distinct
_T. serrata_ Goeze.
— 136µ-157µ long; genital pore not very salient
_T. serialis_ Ball.
Entire large hooks, which are
— 180µ-220µ long; length of mature segments double their width
_T. marginata_ Batsch.
— 150µ-170µ long; length of mature segments treble their width
_T. coenurus_ Küch.
3-4 segments; a few mm. long _T. echinococcus_ v. Sieb.
— Double and bilateral _Dipylidium_ caninum L.
Head unarmed; two genital pores on ventral surface
_Mesocestoides lineatus_ Goeze.
II. Scolex without hooks; one or two transverse uteri present; one or two
genital pores and yolk-glands, the latter never median; genital ducts
pass on the dorsal side of the nerve; eggs with pyriform apparatus.
A. One transverse uterus present.
_a._ Uterus with bullate egg-sacs; pyriform apparatus without horns;
genital ducts between dorsal and ventral vessels
THYSANOSOMA Dies.
α. Head large (1.5 mm.); square lobed testes in median field;
posterior margin of segments fimbriated; genital pore double
_T. fimbriata_ Dies.
β. Head small; no fimbriae; pore rarely double
_T. giardii_ Riv.
_b._ Uterus without saccular dilatations; segments short, thick, and
slightly imbricate ANOPLOCEPHALA E. Blanch.
HORSE-TAENIAE.
α. Head very large
— No posterior lobes _A. plicata_ Zed.
— Four posterior lobes _A. perfoliata_ Goeze.
β. Head small, without posterior lobes
_A. mamillana_ Mehl.
B. Two uteri and two genital pores present; horns of pyriform apparatus
well developed; genital ducts pass on the dorsal side of the
longitudinal vessels MONIEZIA R. Bl.
_a._ Interproglottidal glands[111] arranged in linear series
(_planissima_ group)
_M. planissima_ S. and H. _M. benedeni_ Mz. _M. neumani_ Mz.
_b._ Interproglottidal glands saccular (expansa group)
_M. expansa_ Rud. _M. oblongiceps_ S. and H.
_M. trigonophora_ S. and H.
_c._ Interproglottidal glands absent (_denticulata_ group)
_M. denticulata_ Rud. _M. alba_ Perr.
C. Uterus single or double, without spore-like egg-sacs; eggs with a
single shell; genital pores irregularly alternate; strobila narrow;
testes absent from median part of the field STILESIA Raill.
_a._ A transverse uterus in middle part of median field; {91}
head 2 mm. diameter _S. centripunctata_ Riv.
_b._ Two lateral uteri in each segment; head less than 1 mm. in
diameter _S. globipunctata_ Riv.
III. Scolex almost invariably provided with hooks; genital pores on left
border of segment; eggs with three shells but no cornua. Segments
broader than long; posterior angles salient. HYMENOLEPIS Weinl.
_a._ Scolex with a single series of 24-30 hooks, each 14-18µ long
_H. nana_ v. Sieb. _H. murina_ Duj.
_b._ Scolex very small, unarmed _H. diminuta_ Rud.
IV. Scolex provided with two elongated muscular pits. Body segmented;
three genital apertures in middle of ventral surface
BOTHRIOCEPHALUS Rud.
Body 2-20 metres in length _B. latus_ Brems. _B. cristatus_ Dav.
(doubtful species). _B. cordatus_ Leuck. _B. mansoni_ Cobb.
(= _B. liguloides_ Leuck.)
CLASSIFICATION OF CESTODES.—The following classification, which, so far as
the Taeniidae are concerned, follows that employed by Railliet, Blanchard,
and most recent writers, includes only a few representative genera:—
1. Fam. CESTODARIIDAE Mont. (MONOZOA Lang).
Gen. _Caryophyllaeus_, _Archigetes_, _Gyrocotyle_, _Amphilina_.
2. Fam. BOTHRIOCEPHALIDAE.
Sub-Fam. 1. Bothriocephalinae. Gen. _Bothriocephalus_, _Schistocephalus_,
_Triaenophorus_ (= _Tricuspidaria_).
Sub-Fam. 2. Ligulinae. Gen. _Ligula_.
Sub-Fam. 3. Solenophorinae. Gen. _Solenophorus_, _Duthiersia_.
Sub-Fam. 4. Diphyllinae. Gen. _Echinobothrium_.
3. Fam. TETRARHYNCHIDAE.
Gen. _Tetrarhynchus_.
4. Fam. TETRAPHYLLIDAE.
Sub-Fam. 1. Phyllobothrinae. Gen. _Phyllobothrium_, _Echeneibothrium_,
etc.
Sub-Fam. 2. Phyllacanthinae. Gen. _Calliobothrium_, _Anthobothrium_,
etc.
5. Fam. TAENIIDAE.
Sub-Fam. 1. Cystotaeninae. Gen. _Taenia_ s. str.
Sub-Fam. 2. Anoplocephalinae. Gen. _Moniezia_, _Thysanosoma_, _Stilesia_,
_Anoplocephala_.
Sub-Fam. 3. Cystoidotaeninae. Gen. _Dipylidium_, _Hymenolepis_,
_Drepanidotaenia_, _Dicranotaenia_, _Echinocotyle_, _Davainea_.
Sub-Fam. 4. Mesocestoidinae. Gen. _Mesocestoides_, _Dithyridium_.
Sub-Fam. 5. Ichthyotaeninae. Gen. _Ichthyotaenia_, _Corallobothrium_.
{92}CHAPTER IV
MESOZOA
DICYEMIDAE—STRUCTURE—REPRODUCTION—OCCURRENCE: ORTHONECTIDAE—
OCCURRENCE—STRUCTURE: TRICHOPLAX: SALINELLA.
The Mesozoa are an obscure group, the position of which in the animal
kingdom is still doubtful. The name Mesozoa was given to the group by its
discoverer, E. van Beneden,[112] as he concluded that they were
intermediate between the Protozoa and the higher Invertebrates. Recent
authors, however, have called attention to the resemblance existing between
them and the "sporocysts" of Trematodes, and though we still are ignorant
of certain important points in their life-histories, the Mesozoa are most
conveniently (and probably rightly) considered as an appendix to the
Platyhelminthes.
[Illustration: FIG. 45.—A, B, C, Stages in the development of the vermiform
larva in _Dicyema typus_ van Ben. (After Ed. van Beneden.) _cal_,
"Calotte"; _gc_, germinal cell; _n_, nucleus of endodermal cell.]
The animals composing this group are minute and parasitic, and are composed
of a small number of cells. They may be divided into two families: the
_Dicyemidae_, which occur exclusively in the kidneys of certain Cephalopods
(cuttle-fish); and the _Orthonectidae_, which live in the brittle-star
_Amphiura squamata_, the Nemertine _Nemertes lacteus_, or the {93}Polyclad
_Leptoplana tremellaris_. In addition to the undoubted Mesozoa, certain
anomalous forms—_Trichoplax adhaerens_ and _Salinella salve_—may be
referred to this group.
[Illustration: FIG. 46.—_Dicyemennea eledones_ Wag., from the kidney of
_Eledone moschata_. A, Full-grown Rhombogen with infusoriform embryos
(_emb_); B, one of the latter developing; C, fully formed; D, calotte,
composed of the upper nine cells shown in the figure. (After Ed. van
Beneden and Whitman.) _emb_, Infusoriform embryo; _g_, part of
endoderm-cell where formation of these embryos is rapidly proceeding;
_n.ect_, nucleus of ectoderm-cell; _n.end_, nucleus of endoderm-cell; _p_,
"calotte."]
DICYEMIDAE.—If the kidney of _Eledone moschata_, a Cephalopod common on our
south-western shores, be opened, a number of fine, yellowish, hair-like
filaments may be seen attached at one end to its inner surface, floating in
the fluid contained in the renal cavity. These may be _Dicyemennea
eledones_ Wag., although another form, _Dicyema moschatum_ Whit., also
occurs in the same host. _D. eledones_ (Fig. 46) is 7 to 9 mm. long,
transparent, and is composed of one large inner cell with a simple nucleus
(Fig. 46, _n.end_), and of an outer layer of ciliated cells, nine of which
form the "calotte" or pole by which the animal is attached. Within the
former (endodermal) cell the formation of urn-shaped "infusoriform embryos"
takes place (B and C), the fate of which is not known, but they are
possibly the males. The individual which produces these larvae is called a
{94}"Rhombogen." Other individuals which produce a more elongated larva
("vermiform larva," Fig. 45) are called "Nematogens," and Whitman has
described a third kind, which produce first infusoriform, and then
vermiform, larvae (Secondary Nematogens).[113]
The occurrence of the known species of Dicyemids (a group which has not
been investigated on our coasts) is as follows:—
Species. Host.
_Dicyema typus_ van Ben. _Octopus vulgaris._
_D. clausianum_ van Ben. _O. macropus._
_D. microcephalum_ Whit. _O. de Filippi._
_D. moschatum_ Whit. _Eledone moschata._
_D. macrocephalum_ van Ben. _Sepiola rondeletii._
_D. truncatum_ Whit. _Rossia macrosoma_, _Sepia elegans_,
_S. officinalis._
_D. schultzianum_ van Ben. _S. biseralis, Octopus vulgaris._
_Dicyemennea eledones_ Wag. _Eledone moschata, E. aldrovandi._
_D. mülleri_ Clap. _E. cirrosa._
_D. gracile_ Wag. _Sepia officinalis._
_Conocyema polymorphum_ van Ben. _S. officinalis, Octopus vulgaris._
ORTHONECTIDA.[114]—Two species of Orthonectids are fairly well known,
_Rhopalura giardii_ Metschn. from _Amphiura squamata_, and _R. intoshii_
Metschn. from _Nemertes lacteus_. The latter appears to be very rare, the
former occurring in 2 to 5 per cent of the number of hosts examined. The
parasites occur in a granular "plasmodium," the nature of which is
uncertain. Metschnikoff regards it as formed by the Orthonectids, and he
considers that the cellular envelope, by which it is sometimes enclosed, is
developed from the neighbouring tissue of the host. These granular,
sometimes nucleated, plasmodial masses, which can perform active amoeboid
movements in sea-water, occur attached to the ventral part of the
body-cavity of _Amphiura_, and between the gut-branches and body-wall in
_Nemertes_. Should these hosts be infected by great numbers of the
Orthonectids, their sexual organs degenerate (as is the case with
pond-snails attacked by sporocysts[115]), and it is possible that the
remains of these organs may constitute the "plasmodia" (Braun).
{95}_Rhopalura giardii_ is of distinct sexes. Either males or females are
found in one _Amphiura_. Two kinds of females, flattened unsegmented, and
cylindrical segmented forms, are known. They consist of a ciliated
ectodermal layer enclosing an endodermal mass of eggs, between which is a
fibrillar layer usually considered to be of a muscular nature. The
cylindrical female gives rise to eggs which develop, probably exclusively,
into males. The flattened female produces eggs from which females alone
arise, though the origin of the two forms of this sex is not well
ascertained. The males contain spermatozoa which fertilise the eggs of the
cylindrical female, whereas the ova of the flat form probably develop
parthenogenetically.
[Illustration: FIG. 47.—_Rhopalura giardii_ Metschn. (from the brittle-star
_Amphiura squamata_). ♂, Full-grown male (× 800); ♀_{1}, flattened form of
female (× 510); ♀_{2}, cylindrical female (× 510). (After Julin.)]
TRICHOPLAX.[116]—This anomalous animal has only been found in aquaria,
originally in the marine aquarium at Graz by {96}Schulze. It has the
appearance of a large, flattened, ciliated _Amoeba_ (1.5-3 mm. in
diameter), but is distinguished by its structure. The upper surface is
composed of a flattened epithelium. The lower surface is made up of
cylindrical ciliated cells, which pass imperceptibly into the branched
cells, embedded in a hyaline matrix, which compose the middle layer of the
body. No distinct organs, and beyond simple fission, no mode of
reproduction, have been observed. One species, _T. adhaerens_, is known,
but has never been met with in a free state.
SALINELLA.[117]—This is another aquarium-animal, found by Frenzel in the
Argentine, in an artificial saline solution with which he filled some
aquaria. It measures .2 mm. in length, and has a somewhat flattened,
barrel-shaped appearance. A single layer of ciliated cells bounds a central
cavity opening at each end. Fission, and conjugation followed by
encystment, have been observed. One form, _S. salve_, is known from salines
taken from Cordova.
NEMERTINEA
BY
LILIAN SHELDON
Staff Lecturer in Natural Science, Newnham College, Cambridge.
{99}CHAPTER V
NEMERTINEA
INTRODUCTORY—EXTERNAL CHARACTERS—ANATOMY—CLASSIFICATION—DEVELOPMENT—HABITS—
REGENERATION—BREEDING—GEOGRAPHICAL DISTRIBUTION—LAND, FRESH-WATER, AND
PARASITIC FORMS—AFFINITIES
The Nemertinea form a compact group, the affinities of which have not been
at present clearly determined. Several species were mentioned and described
in the works of various naturalists during the latter half of the
eighteenth century, though their anatomy was not understood until
considerably later. The first mention of any member of the group was made
by the Rev. W. Borlase in his _Natural History of Cornwall_, published in
1758. He gives a short description and a rough figure of _Lineus marinus_.
From that time the increase in the knowledge of the group was very gradual.
New species were from time to time described, but few of the descriptions
could boast of much completeness, and many erroneous views were held until
comparatively recent years. The group was very variously classified, but
the general arrangement in early times seems to have been to unite it with
the Planarians. Valuable contributions to the history of the development
were made in 1848 and the few subsequent years by Desor,[118]
Gegenbaur,[119] Krohn,[120] and Leuckart and Pagenstecher[121]; and more
recently by Metschnikoff[122] and Salensky.[123]
{100}Nemertines for the most part closely resemble one another in all
essential points, though they differ considerably in size, colour, and
external details. They vary in length from less than an inch to thirty
yards, this extreme size being attained by _Lineus marinus_.
[Illustration: FIG. 48.—_Lineus marinus_ Mont., from the living specimen in
the coiled condition. Plymouth. × 1. _a_, Anterior end; _b_, posterior
end.]
[Illustration: FIG. 49.—_L. marinus_, from the same specimen as Fig. 48, in
the expanded condition. _a_, Anterior end; _b_, posterior end.]
Nemertines are common on the British coasts; about forty species have been
recorded from this area. On turning over a stone on a sandy or muddy shore
in a pool left by the receding tide, there may often be seen a coiled mass,
having the appearance of a uniform slimy string twisted into a
complicated knot. If it be carefully removed, the ends can generally be
made out, one bluntly rounded and the other slightly tapering (Fig. 48,
_a_ and _b_). Occasionally there may be seen attached to the blunter end a
fine thread, which moves about freely. This thread may, by an instantaneous
movement, be drawn into the body, no trace of its existence being left
except at the tip of the head, where a small pore is visible; this is the
orifice through which it was withdrawn. Shortly afterwards the thread may
be again shot out, the process being instantaneous and often accomplished
with {101}great force. This thread (Fig. 50, _p_) is the proboscis, a very
important and characteristic organ in Nemertines.
Most Nemertines are marine; they are mostly indifferent to climate and to
the nature of the soil on which they live.
A few forms live on land (e.g. _Tetrastemma agricola_,[124] _Geonemertes
palaensis_,[125] and _G. chalicophora_[126]) or in fresh water (e.g.
_Tetrastemma aquarum dulcium_[127] and _T. lacustre_[128]) in various parts
of the globe. There are also parasitic forms; the best known of which is
_Malacobdella_.[129] A pelagic form, _Pelagonemertes_,[130] has been
described by Moseley.
[Illustration: FIG. 50.—Side view of head of _Cerebratulus_ (_Micrura_)
_tristis_ Hubr., showing the everted proboscis. Naples. × 2. Drawn from a
spirit specimen. _c.s_, Cephalic slit; _m_, mouth; _p_, proboscis.]
EXTERNAL CHARACTERS.—A typical Nemertine possesses an elongated worm-like
body (Fig. 49), which is usually thrown into numerous close coils (Fig.
48). In section it may be either round or more or less flattened, with the
lateral edges in some cases quite thin and almost fin-like. One or two
broad, flattened, and leaf-shaped forms are known, but such a condition is
exceptional, and the forms in which it occurs have probably assumed it
owing to the adoption of special modes of life.
In the ordinary forms the posterior end of the body is pointed either
bluntly or sharply. The head is somewhat broader than the rest of the body,
and often assumes a spatulate form. Eyes (Fig. 51, _e_) are usually present
either in one or several pairs, or in symmetrically-arranged groups on each
side of the head. The mouth (Fig. 58, _m_) is situated near the front end
of the body on the ventral surface, and is usually rendered conspicuous by
being surrounded by thick tumid lips. It varies in form from being
slit-like to elliptical. At the anterior end of the body a {102}small
terminal pore occurs; this is the external opening of the proboscis (Fig.
51, _p.p_).
Nemertines are often very diversely and brilliantly coloured, the hues most
commonly found being white, yellow, green, deep purple, and various shades
of red and pink. The ventral surface is usually paler in colour than the
dorsal, and the latter is often marked by longitudinal and transverse
stripes (Fig. 59) in contrasting colours.
The whole animal is enveloped in a layer of mucus, which sometimes becomes
hardened to form a tube, and this may be still further strengthened by an
admixture of particles of sand or earth.
The body is capable to a great extent of contraction and extension, a
Nemertine many inches long being apt, when irritated or alarmed, to
contract itself to the length of not more than half an inch. Hence, unless
the animal is kept and carefully watched, a very erroneous idea may be
conceived as to its size.
ANATOMY.—The body-wall consists of several layers (Fig. 52), which in a
typical highly-developed Nemertine are as follows:—
1. An external epidermic layer (_ep_), consisting of ciliated cells, among
which are placed numerous unicellular glands. These glands probably
secrete the mucus in which the Nemertine is usually enveloped; their
contents when in the body are very highly refracting. The epidermis rests
on a basement membrane (_b.m_).
2. The two or three muscular layers, arranged as either an external
circular and an internal longitudinal, or an inner and an outer circular
separated by a longitudinal layer, or, as in the figure (_c.m_ and _l.m_),
two longitudinal separated by a circular layer.
[Illustration: FIG. 51.—_Amphiporus lactifloreus_ Johnst., drawn from the
living specimen, from the dorsal surface. Plymouth. × 2. _e_, Eyes; _g_,
generative organs; _n.g_, nerve ganglion; _p.p_, proboscis pore; _p_,
proboscis.]
3. A fairly thick connective-tissue layer often found between the epidermis
and the muscles, into which latter it gradually merges (_s.t_).
{103}THE DIGESTIVE SYSTEM.—The mouth is placed on the ventral surface near
the anterior end of the body (Figs. 53, 58, _m_). It leads into a straight
oesophagus (Fig. 53, _oes_), whence passes off the intestine (_int_), which
is continued as a straight non-convoluted tube to the anus (_a_), situated
terminally at the posterior end of the body. The intestine is thrown out
throughout the greater part of its course into paired lateral pouches.
[Illustration: FIG. 52.—Diagrammatic transverse section of a Nemertine
(Schizonemertea) through the middle region of the body. _b.m_, Basement
membrane; _c.m_, circular muscle layer; _d.b_, dorsal blood-vessel; _ep_,
epidermis; _g_, generative organs; _int_, intestine; _l.b_, lateral
blood-vessel; _l.m_, longitudinal muscle layers; _n.c_, lateral nerve-cord;
_n.l_, nerve plexus; _p_, proboscis; _p.s_, proboscis sheath; _s.t_,
subcutaneous layer.]
The alimentary canal is lined throughout by a ciliated epithelium. The
oesophagus has, in addition to this layer, an outer thick coat of large
granular cells, which probably have a glandular function.
PROBOSCIS.—The most characteristic organ of the Nemertines is the proboscis
(Figs. 50, 53, 54). For many years its disposition and function were
misunderstood, and it was supposed to be a portion of the digestive system.
The proboscis, which lies dorsal to the alimentary canal, opens at the
extreme anterior end of the body by a small pore (Figs. 51, 53, 58). When
retracted it is sometimes considerably folded, and lies in a long pouch or
sheath. To the walls of this sheath it is attached round its anterior
{104}end; and strong muscles unite its posterior extremity to the sheath a
short distance from the posterior end of the latter.
The proboscis seems to be exclusively a tactile and protective and
defensive organ, for which functions it is eminently fitted by the great
ease and rapidity with which it is everted or thrust out from the body. It
consists of two distinct regions (Fig. 54, _g.p_ and _m.p_). In the
retracted state the anterior part is a hollow tube with very thick muscular
walls made up of several layers. At the base of this part in many of the
Nemertines there is situated a sharp-pointed spine projecting forward into
the lumen, and several smaller stylets situated in a pair of vesicles close
to the base of the central spine. The position of the spines in the everted
proboscis is shown in Fig. 57. The posterior part of the proboscis is also
a tube, but instead of being muscular, its walls are glandular. This
posterior glandular part is never everted.
[Illustration: FIG. 53.—Diagrammatic drawing of a Nemertine from the dorsal
surface to show the position of some of the principal organs. _a_, Anus;
_c.s_, cephalic slit; _g_, generative organs; _int_, intestine with its
lateral diverticula; _m_, mouth; _n.c_, lateral nerve-cord; _n.g_, nerve
ganglion; _oes_, oesophagus; _p_, proboscis; _p.p_, proboscis pore; _p.s_,
proboscis sheath.]
The eversion is effected by a turning inside out of the anterior part of
the proboscis (Fig. 54). The process whereby the proboscis is retracted has
been very aptly compared to the effect which would be produced by the
inversion of the finger of a glove, accomplished by pulling a string
attached to its tip on the inside, the anterior muscular part being
comparable to the finger and the glandular part to the string. It is thus
obvious that in the everted condition the stylet will form the anterior tip
of the {105}proboscis, and will there be in a position for offence or
defence (Fig. 57, _s_).
NERVOUS SYSTEM.[131]—The brain is composed of two ganglionic masses (Fig.
53, _n.g_) lying at the anterior end of the body, one on each side of the
proboscis, and united by commissures passing round it (Fig. 55, _d.c_ and
_v.c_). Each ganglionic mass is often partially divided into a dorsal and
ventral lobe (_n.g.d_ and _n.g.v_). From the brain a pair of cords pass off
backwards along the sides of the body (_n.c_); these cords, which have no
ganglionic swellings, in some forms unite with one another above the anus.
Anteriorly nerves are given off from the brain to the eyes and front part
of the head (_a.n_). A nerve to the proboscis is given off from the
commissure which unites the two halves of the brain dorsal to the proboscis
(_d.n_).
[Illustration: FIG. 54.—Diagrammatic representation of the proboscis, (A)
in the retracted condition, (B) in the everted condition. _g.p_, Glandular
portion of the proboscis; _m_, muscle attaching the proboscis to its
sheath; _m.p_, muscular portion of the proboscis; _p.p_ in A, proboscis
pore; _p.p_ in B represents the position of the proboscis pore in the
retracted condition of the proboscis; _p.s_, proboscis sheath.]
In two out of the three groups into which the Nemertines are divided, the
lateral nerve-cords are in connexion with a network or plexus of nerves
lying between the muscular layers of the body-wall (Fig. 52, _n.l_), and in
some forms constituting a comparatively thick layer. In these two groups
there are no definite {106}nerve branches except the anterior ones to the
head. In the third group of Nemertines the lateral nerve-cords lie within
the muscular layers of the body-wall, and in this case paired nerve
branches are given off at definite intervals throughout the whole length of
the body. These branches divide up among the organs to which they pass, and
no nerve plexus is present.
The lateral cords vary in position in different cases. Sometimes they lie
laterally, at others the cords tend to approximate to one another in the
median dorsal or in the median ventral line, though in every case they
remain distinctly separated.
SENSE ORGANS.—Sense organs are usually present in the form of eyes arranged
at the sides of the head (Fig. 51, _e_), sometimes as a single pair and
sometimes in one or more groups on each side. The structure of the eyes
varies from a simple pigment spot to an organ which receives a special
nerve-supply from the brain, and possesses a refracting body answering to a
lens, and behind this a pigment layer and a layer of rods. Some forms are
devoid of all traces of eyes.
[Illustration: FIG. 55.—Diagram to show the relations of the nervous
system, circulatory system, and proboscis sheath in the anterior end of the
body in the _Hoplonemertea_, modified from M‘Intosh. _a.n_, Nerves to
anterior part of body and eyes; _d.c_, dorsal commissure; _d.n_, median
dorsal nerve; _d.v_, dorsal vascular trunk; _l.v_, lateral vascular trunk;
_n.c_, lateral nerve-cord; _n.g.d_, dorsal lobe of nerve ganglion; _n.g.v_,
ventral lobe of nerve ganglion; _p.p_, proboscis pore; _p.s_, proboscis
sheath; _v.c_, ventral commissure; _v.s_, vascular ring or collar.]
A pair of simple auditory capsules has been found in some of the
_Hoplonemertea_, where they occur as small vesicles on the brain.
The whole surface of the body appears to be remarkably sensitive. In a few
forms small tufts of tactile hairs are said to be present in the region of
the head, while in others there {107}are a few long hairs scattered
sparsely among the cilia of the epidermis.
_Frontal Organ._—In many Nemertines there is present at the anterior tip of
the head a disc-shaped group of cells bearing long hairs or bristles. On
this disc open the secreting ducts of a number of gland cells lying in the
head. It seems possible that this frontal organ may function as an organ of
taste.
_Side Organs._—In the Carinellidae there is a pair of circular epithelial
patches lying one on each side of the body in the region of the excretory
pore. The cells composing them are richly ciliated and provided with a
plentiful nerve-supply. The function of these epithelial patches is not
known, but it has been suggested that they may be auditory organs.
_Cephalic Slits and Cerebral Organs._—In most Nemertines there is a
peculiar pair of organs (Figs. 50, 53, _c.s_), situated in the head and in
close connexion with the brain. The function of these organs is not known.
Hubrecht has suggested that they may be respiratory, while Bürger[132]
conjectures that they may be organs which are used for discriminating the
condition of the surrounding medium. In an external examination of the
head, the cephalic slits may usually be seen as a pair of lateral furrows
or pits. Their form and direction vary considerably; they may take the form
of shallow circular depressions, or they may lie longitudinally and be
slit-like in shape (Fig. 50), or the slit may lie at right angles to the
long axis of the body and be beset with short transverse furrows. In some
forms these slits are merely superficial depressions, but in others they
are continued into ciliated ducts, which pass inwards and penetrate into
special lobes, consisting of glandular tissue and ganglion cells, in close
connexion with the brain. These lobes are called the cerebral organs.
In many forms the nervous system is charged with haemoglobin, which gives
to it a bright red colour.
CIRCULATORY OR BLOOD-VASCULAR SYSTEM.—The circulatory system consists of
three main longitudinal vessels, a median dorsal and a pair of lateral
ones. These are connected together posteriorly by a transverse trunk, and
also throughout the whole length of their course by branches, which are
given off at regular intervals. Anteriorly the three longitudinal vessels
{108}either all unite and form a collar (Fig. 55, _v.s_) round the
oesophagus, or they break up into a number of lacunar or open spaces in
free communication with one another.
The blood is usually colourless, but in some cases the corpuscles are
coloured red by haemoglobin.
[Illustration: FIG. 56.—Excretory system of Nemertines. A, _Drepanophorus
spectabilis_ Qtrf., part of one of the lateral vessels encircled by
branches of the excretory organ, × 585; _e_, main canal of the excretory
system: B, _D. crassus_ Qtrf., a terminal branch of the excretory system, ×
585; _f_, ciliated flame: C, _Malacobdella grossa_ O. F. Müll., entire
animal, slightly magnified, showing the excretory system (black) and the
vascular system; _e.a_, excretory aperture; _d.v_, dorsal vessel; _l.v_,
lateral vessel. (From Bürger.)]
EXCRETORY SYSTEM.—Max Schultze[133] found in _Tetrastemma obscurum_, on the
outer side of, but near to the lateral blood-vessels, a pair of canals. He
observed ciliary movements in the canals, but could not discover flame
cells. Further contributions to our knowledge of the excretory system were
made by Semper,[134] von Kennel,[135] Hubrecht,[136] and Oudemans.[137] The
latter states that the excretory system consists of a pair of canals
situated laterally near the anterior end of the body. Each canal
communicates with the exterior by one or more ducts having lateral
regularly-arranged apertures. In some cases he was unable to make out any
communication with the vascular system, but in others {109}a direct
communication, by means of open connexions with the lacunar blood spaces,
is said to occur.
Silliman[138] in _Tetrastemma aquarum dulcium_ describes the excretory
vessels as ending in numerous capillary branches, at the blind terminations
of which cilia are present. He states that there is no important difference
between the excretory systems of Rhabdocoeles and Nemertines.
Bürger,[139] as the result of recent investigations on the excretory system
in Nemertines, finds that the minute branches end in flame-cells (Fig. 56,
B) lying on and among the blood-vessels, but having no open connexion with
them.
GENERATIVE SYSTEM.—The Nemertines are for the most part dioecious, only a
few certainly hermaphrodite species having been described, e.g.
_Tetrastemma_ ("_Borlasia_") _kefersteinii_ Mar.[140]
The generative products in both cases are contained in sacs (Figs. 52, 53,
_g_) which lie in the lateral region of the body between the pouches of the
alimentary canal. The ova and spermatozoa are conveyed to the exterior by
short ducts. Most species are oviparous, though a few viviparous species
are known (e.g. _Prosorhochmus claparedii_).
CLASSIFICATION.—Nemertines were divided by M. Schultze[141] into:—
1. _Enopla_, in which the proboscis is armed with stylets.
2. _Anopla_, in which the proboscis is unarmed.
Although this classification was fairly correct as far as it went, since
many other distinctive features were correlated with the presence or
absence of armature in the proboscis, still there are several primitive
forms belonging to the _Anopla_, which possess characters such as render it
necessary to class them together in a separate group.
For this reason Hubrecht divided the Nemertinea into three
Orders—_Hoplonemertea_, _Schizonemertea_, _Palaeonemertea_; the first of
these Orders corresponding with the _Enopla_, and the other two with the
_Anopla_.
{110}ORDER I. HOPLONEMERTEA.
The proboscis is armed. The epidermis rests on a thick layer of connective
tissue plentifully supplied with glands, below which is a prominent
basement membrane. The muscular layers of the body are two in number, an
outer circular and an inner longitudinal. The nerve-trunks lie within the
muscular layers of the body and give off regularly-arranged branches. There
is no nerve plexus. Each of the cephalic slits generally opens by a pore
situated in the centre of a transverse groove, which is beset along one
side by a row of shorter grooves at right angles to it. The apparatus
consists of a ciliated duct surrounded by nerve tissue, and passing into
lobes of tissue which are connected with the brain by thick nerve-cords.
The mouth opens rather far forward in front of the brain. The intestinal
pouches are symmetrically arranged. Auditory organs are said to exist in
some forms, consisting of vesicles containing otoliths. The vascular trunks
are connected anteriorly by closed vessels and not by lacunar spaces.
[Illustration: FIG. 57.—Anterior end of the everted proboscis
(Hoplonemertea). _g.p_, Glandular portion of the proboscis; _l.s_, lateral
sacs containing stylets; _m.p_, muscular portion of the proboscis; _s_,
stylet; _s.b_, granular basal portion of stylet.]
The principal British genera and species[142] are:—
_Amphiporus bioculatus_ M‘Int., _A. dissimulans_ Riches, _A. hastatus_
M‘Int., _A. lactifloreus_ M‘Int., _A. pulcher_ Johnst.
_Drepanophorus rubrostriatus_ Hubr. (= _A. spectabilis_ Qtrf.).
_Tetrastemma ambiguum_ Riches, _T. candidum_ O. F. Müll., _T. dorsale_
Abildg., _T. flavidum_ Ehrenb., _T. immutabile_ Riches, _T.
melanocephalum_ Johnst., _T. nigrum_ Riches, _T. robertianae_ M‘Int., _T.
vermiculatum_ Qtrf.
_Prosorhochmus claparedii_ Keferstein.
_Nemertes carcinophila_ Köll., _N. gracilis_ Johnst., _N. neesii_ Oerst.
_Malacobdella grossa_ O. F. Müll.
{111}ORDER II. SCHIZONEMERTEA.
The proboscis is unarmed. The epidermis is separated from the layer of
connective tissue by a thin basement membrane, hence the glands in the
connective tissue are more deeply situated and have long ducts. The
muscular layers are three in number, an outer and an inner longitudinal
layer between which lies a layer of circular muscles. The lateral
nerve-cords lie between the outer longitudinal and the circular muscle
layers. They are connected throughout the body by a nerve plexus, the only
definite nerve branches given off being those to the brain, oesophagus, and
proboscis. The cephalic slits are a pair of deep longitudinal grooves at
the sides of the head. From each groove a canal passes inwards into a
posterior brain-lobe. The mouth opens behind the brain, and is an elongated
slit bounded by corrugated lips. Auditory organs have not been observed.
The longitudinal vascular trunks are connected anteriorly by lacunar
spaces, and not by closed vessels.
[Illustration: FIG. 58.—Head end of _Cerebratulus marginatus_ Ren., from
the ventral surface. Drawn from a spirit specimen. Naples. × 1. _c.s_,
Cephalic slit; _m_, mouth; _p.p_, proboscis pore.]
Principal British genera and species:—
_Lineus bilineatus_ Ren., _L. lacteus_ Mont., _L. marinus_ Mont. (= _L.
longissimus_ Gunnerus), _L. gesserensis_ O. F. Müll. (= _L. obscurus_
Desor and _L. sanguineus_ M‘Int.).
_Borlasia elizabethae_ M‘Int.
_Cerebratulus angulatus_ O. F. Müll., _C. fuscus_ M‘Int., _C.
pantherinus_ Hubr.
_Micrura aurantiaca_ Grube, _M. candida_ Bürger, _M. fasciolata_ Ehrenb.,
_M. purpurea_ J. Müll.
_Meckelia asulcata_ M‘Int.
ORDER III. PALAEONEMERTEA.
The proboscis is unarmed. The epidermis and connective tissue form one
layer, below which is the basement membrane. The muscular layers are three
in number, two circular separated by a longitudinal layer. The nerve-cords
lie altogether external {112}to the muscular layers, and are connected
together throughout by a plexus. No nerve branches are given off. The brain
is not divided into lobes. The cephalic slits are only represented by a
shallow depression on each side of the head, and no canals have been
observed leading from them. The intestine is straight, and the pouches are
usually absent or rudimentary. The circulatory system is largely made up of
lacunar spaces, the closed system being but little developed.
Principal British genera and species:—
_Carinella annulata_ Mont., _C. linearis_ (Mont., MS.) M‘Int., _C.
macintoshi_ Bürger (Fig. 59), _C. polymorpha_ Ren.
_Cephalothrix bioculata_ Oerst., _C. linearis_ Rathke.
_Valencinia lineformis_ M‘Int.
[Illustration: FIG. 59.—_Carinella macintoshi_ Bürger, drawn from the
living specimen, slightly contracted. Plymouth. Considerably magnified.
_a_, Anterior end; _b_, posterior end.]
A most important monograph by Bürger[143] on Nemertines has just been
published, but unfortunately it appeared too late to be adequately
considered here. He gives an elaborate account, illustrated by admirable
figures, of the present state of our knowledge of this group, and his work
will be indispensable to future students of the subject. The older systems
of classification are criticised, and the following scheme is adopted in
their place:—
ORDER I. PROTONEMERTINI (= part of the Palaeonemertea, e.g.
_Carinella_).—The brain and lateral nerve-cords lie outside the muscle
layers in the epithelium or below the basement membrane. The body-wall
consists of the following layers: epidermis, basement membrane, circular
muscles, and longitudinal muscles. The mouth lies behind the brain. The
proboscis is unarmed.
ORDER II. MESONEMERTINI (= part of the Palaeonemertea, e.g.
_Cephalothrix_).—The characters of this Order are similar to those of the
Protonemertini except that the brain and lateral nerve-cords lie in the
muscle layers.
ORDER III. METANEMERTINI (= Hoplonemertea).—The brain and lateral
nerve-cords lie in the parenchyma of the body internal to the muscle
layers. The layers of the body-wall are {113}similar to those of the
Protonemertini. The mouth lies in front of the brain. The proboscis is
armed. At the junction of the fore- and mid-gut a diverticulum is given off
which projects forwards beneath the fore-gut and ends blindly in front.
ORDER IV. HETERONEMERTINI (= Schizonemertea, and the genera _Eupolia_ and
_Valencinia_, placed provisionally by Hubrecht in the Palaeonemertea).—The
body-wall consists of the following layers: epidermis, thick cutis, and an
outer and an inner longitudinal muscle layer separated from one another by
a circular muscle layer. The brain and lateral nerve-cords lie between the
outer longitudinal and the circular muscle layers. The mouth lies behind
the brain. The proboscis is unarmed.
DEVELOPMENT OF THE NEMERTINEA.—The development of the Palaeonemertea is at
present not known: in the Schizonemertea a larval stage occurs; while in
the Hoplonemertea the egg develops directly without undergoing any
metamorphosis.
There are two forms of larva characteristic of the Schizonemertea, known
respectively as Pilidium and the Type of Desor. The Pilidium is hatched
early and leads a free-swimming existence, whereas the Type of Desor,
though in many respects resembling it, never passes through the
free-swimming phase.
[Illustration: FIG. 60.—Diagram of a Pilidium larva. (After Salensky.) _c_,
Tuft of cilia; _m_, muscle-fibres; _mo_, mouth, seen through one of the
lateral lobes; _n_, nerve-fibres; _n.r_, nerve-ring; _n.g_, nerve ganglion;
_oes_, oesophagus; _st_, stomach.]
The Pilidium (Fig. 60) is a helmet-shaped larva bearing a tuft or spike
dorsally, and prolonged downwards laterally into a pair of lobes. The whole
larva is covered with cilia, there being a specially strong band round its
ventral surface. The dorsal spike is composed of a bunch of strongly
developed cilia or of a long flagellum. The alimentary canal consists of a
sac constricted into {114}oesophageal and gastric regions (Fig. 60, _oes_
and _st_). In this condition the larva swims about freely in the water. The
helmet-shaped Pilidium-skin forms no part of the future Nemertine, the skin
of which is developed as ingrowths from it; these meet one another and
unite to form a complete covering round the alimentary canal; the larval
skin is then cast off, and by a series of gradual steps the embryo develops
into the adult.
HABITS.—Nemertines are often found under stones between high- and low-water
marks, lying on sandy or muddy bottoms. They are usually in the form of
coiled masses, and are generally in a state of quiescence. Hence it is
probable that their period of activity is during high-water, and that when
left by the receding tide they subside into a resting condition.
The large kinds, such as _Lineus marinus_, seem to be always found living
alone, but some of the smaller kinds, notably _Tetrastemma dorsale_ and
_Prosorhochmus claparedii_, have gregarious habits and live in masses, the
coils of the different individuals being inextricably mixed.
Some species, such as _Micrura purpurea_, _Amphiporus pulcher_, and
_Cerebratulus angulatus_, frequent empty bivalve shells, while Nemertines
are often found in empty limpet shells adhering to rocks in tidal pools.
Other smaller forms resort to no such definite protection, but live among
seaweeds; some of these remain naked, while others secrete for themselves
tubes of a membranous or gelatinous consistency. _Borlasia elizabethae_
lives in a burrow of clay.
Nemertines are commonly dredged from a depth of six or eight fathoms. They
may sometimes be found floating on the surface of the water, and some
possess the power of swimming rapidly, propelling themselves by a lateral
motion of the tail, the sides of which are in such cases prolonged into a
thin fin-like edge. This mode of progression is usually adopted by those
which frequent deep water. A pelagic Nemertine (_Pelagonemertes_) was
discovered by Moseley near the southern verge of the South Australian
current, being found in a trawl with deep-sea forms from a depth of 1800
fathoms. This animal was leaf-like in shape, bluntly pointed behind and
rather square in front.
The power possessed by Nemertines of secreting mucus is very great, their
course being often traceable by the tracks which they leave behind them.
Many of them glide along with great rapidity, a mode of progression which
is probably due to the {115}cilia covering the whole outer skin, and to the
extreme contractility of the muscles of the body-wall. In some locomotion
is effected by the proboscis; this is protruded and attaches itself by
means of its spines to some foreign body, after which the body is drawn up
after it. This has been specially observed in a land form, _Tetrastemma
agricola_, discovered by Willemoes-Suhm in the Bermudas. On solid bodies
the movement is a kind of crawling action, the head and mouth acting as
suckers in much the same way as in many Leeches.
Most Nemertines can be very readily kept in confinement. The chief apparent
effect of such a life is a loss of colour, the animal gradually becoming
pallid in hue. Owing also to the absence of proper food they diminish very
much in size, though even when all food is kept away an animal will
sometimes continue to live as long as eighteen months.
FOOD.—Nemertines are carnivorous in their habits and are very voracious,
devouring any prey which comes in their way, whether it be living or dead.
No animal food seems to come amiss to them, and they will devour creatures
of considerable size. When in contact with its prey, the Nemertine dilates
its mouth to a large extent, and the anterior end of the oesophagus is
thrust out and engulfs the animal. Chaetopods form a favourite food
material, the whole animal being swallowed quite regardless of the hard
chitinous bristles and spines with which it is beset. The soft parts are
gradually digested, the bristles and other indigestible portions being
extruded by the anus. The larger spines often pass out by perforating
passages through the wall of the intestine and through the body-wall. The
aperture thus formed appears speedily to heal after the foreign body has
been extruded.
The carnivorous habits of Nemertines even extend to cannibalism, and when
kept in confinement they frequently devour one another. For this reason it
is unsafe to keep large and small kinds together, as the small ones
speedily disappear, being used as food material by the large. If one be
divided into several pieces, the pieces are very rapidly demolished by
other individuals.
REGENERATION.[144]—This power is, no doubt, of great service to these
animals, since injury, or even violent local irritation, often causes
complete rupture at the point affected. It seems that the {116}chief power
of regeneration is situated in the head, as, if a very short piece be
broken off the anterior end of the body, it very rapidly reproduces itself
into a new individual. The hind end of the original body often lives for a
considerable time, but it does not in most cases appear to possess the
power of reproducing a head, and after existing for a time it dies. For a
while, however, it so far retains its vital powers that the generative
products continue to grow, and actually attain to perfection. Severe wounds
also heal very quickly and completely, and all local injuries are speedily
repaired.
Owing to the force with which it is shot out, the proboscis is often
completely severed from the body, and in such a case the animal grows a new
one in an extremely short space of time. The proboscis thus broken off
retains its power of movement and contractility for a considerable time,
and has been more than once mistaken for a worm. This great vital power is
probably due to the great development of nervous tissue, the proboscis
being usually richly supplied with nerve plexuses.
One large form, _Lineus sanguineus_, seems to possess great recuperative
powers. It shows a marked tendency to break up into pieces, when not only
the head end, but also the other portions develop into perfect animals,
each one growing a head and all the organs belonging to it. Thus in this
case an animal may multiply by a simple process of transverse fission, and
form numerous complete individuals.
BREEDING.—The breeding season only appears to cease in the extreme of
winter. Different genera and species seem to mature their generative
products at different times.
In the armed Nemertines the eggs are deposited separately, and are not
connected together except by such accidental mucus as the animal deposits
normally; but in the unarmed a special mucous secretion forms a thick
investment for the eggs.
M‘Intosh[145] has observed the process of the deposition of the male and
female products in _Nemertes gracilis_. He put into a glass vessel a male
and female of this species in which the products were apparently ripe. Soon
spermatozoa began to issue in wreath-like jets from the body of the male,
at first from the middle region of the body, and afterwards anteriorly and
posteriorly, until the animal was enveloped in a dense cloud of
{117}spermatozoa. The whole process only lasted a few minutes. When all the
spermatozoa had apparently been given out, the female was seen to protrude
her head from the sand; she then passed to the side of the vessel and
deposited a group of eggs about three inches distant from the spermatozoa.
With only a few exceptions Nemertines are oviparous. _Prosorhochmus
claparedii_, _Tetrastemma obscurum_, and _Monopora vivipara_ have been
observed to contain embryos at certain times of the year. In other forms
the eggs are laid when ripe, and development takes place subsequently to
their deposition.
GEOGRAPHICAL DISTRIBUTION.—Nemertines have been found in all seas from the
arctic to the equatorial regions. Many forms are found in the British Isles
both between tide-marks and also at greater depths around our coasts. Some
genera seem to be confined to warm climates and others to cold; while
others appear to be indifferent to climate, and to subsist equally well
under very various degrees of temperature. So far as is known, the land
forms are all indigenous to warm countries.
LAND FORMS.—Land forms, which occur on or in moist earth under stones or
decaying vegetable matter, have been discovered and described by
Semper,[146] Willemoes-Suhm,[146] and von Graff.[146]
The species found by Semper, and called by him _Geonemertes palaensis_,
lives under damp leaves and the roots of trees on Pelew Island in the North
Pacific. It is about 2 inches long, of a reddish-white colour, with narrow,
brownish-black, longitudinal stripes on its dorsal surface. It possesses
six eyes and very small cephalic slits and cerebral organs. The proboscis
is armed, and opens by the mouth instead of by a special pore.
The same peculiarity as to the opening of the proboscis is found in
_Geonemertes chalicophora_, discovered by von Graff in pots of _Corypha
australis_ in the palm-house at Frankfurt-on-Main. He found specimens on
and beneath the surface of the earth. As it was only found in pots in which
this Australian plant was growing, von Graff thought it almost certain that
it was a native of Australia. Those found below the surface of the earth
were surrounded by a transparent tube in which particles of earth were
embedded. The animal is small, only about two-fifths of an inch in length.
The colour is milk-white, with a small quantity of red pigment anteriorly:
there are four eyes, and the cephalic slits are absent.
{118}The species which was discovered by Willemoes-Suhm, and named by him
_Tetrastemma agricola_, lives under stones in damp earth in the Bermudas.
It differs from the other two in that the proboscis opens by a special
terminal aperture. It measures nearly an inch and a half in length, and,
like _G. chalicophora_, is milk-white in colour. It resembles it also in
possessing four eyes, and in the absence of cerebral organs and cephalic
slits.
FRESH-WATER FORMS.—In most cases the descriptions of fresh-water forms are
so vague and incomplete that it is difficult to determine whether or not
they are different species.
They are probably more numerous than is at present known, and are certainly
scattered widely over the face of the earth, since they have been found in
Nicaragua, at Tashkend in Turkestan, and at Philadelphia and Monroe in the
United States.
A form of which we have a full description is _Tetrastemma aquarum
dulcium_, found by Silliman[147] at Monroe, under stones in brooks in
company with Planarians. It is a small worm of a red or pink colour, about
half an inch in length, and it possesses usually three pairs of eyes. The
proboscis is armed, and opens by a separate aperture. The excretory system
consists of a vessel on each side of the body, each opening externally by a
pore, and internally dividing into numerous branches which end in ciliated
expansions. An individual of the same species was found by Beddard in one
of the tanks in the Botanical Gardens in Regent's Park, but as the tank is
one in which tropical plants are grown, it had almost certainly been
introduced among the roots of the plants, and cannot be considered as a
British species.
A fresh-water Nemertine belonging to the genus _Tetrastemma_ was, however,
found by Benham[148] on the roots of some water plants in the Cherwell at
Oxford. The specimen was of a bright orange colour and measured half an
inch in length.
Du Plessis[149] found another fresh-water form on the lower surface of
stones in shallow pools on the shores of the Lake of Geneva, and named it
_Tetrastemma lacustre_. It is a small animal, the largest specimens being
rather over an inch in length.
Another European genus was found in 1893 by F. E. Schulze in Berlin. It has
been fully described by T. H. Montgomery,[150] who has given it the name of
_Stichostemma eilhardii_.
{119}PARASITIC FORMS.—The genus _Malacobdella_ was found by von Kennel[151]
in large numbers living on _Cyprina islandica_, a Lamellibranch Mollusc, in
the harbour at Kiel; and it has also been described by Riches[152] as a
British form. It is attached to its host by means of a large round sucker
situated at the posterior end of the ventral surface, while the rest of the
body waves about freely in the mantle-cavity. It is perhaps hardly correct
to describe this animal as parasitic, since it does not appear to obtain
its nutriment at the expense of the host by preying on its juices. The
advantage of its position is, however, obvious, since a perpetual current
of water is kept up in the mantle-cavity of the Mollusc, and from the
stream the Nemertine is able to pick out and take for itself any food
material which it considers suitable. At the same time it is not subjected
to the influence of the winds and waves, as the shell of the mollusc acts
as a barrier to prevent the entrance of disturbing elements.
_Malacobdella_ is short and broad, somewhat flattened dorso-ventrally. The
anterior end is bluntly rounded. The mouth opens into a wide pharynx, which
is constricted behind and then passes into the intestine; this after a few
coils opens by the anus situated dorsally immediately above the sucker. The
proboscis opens into the pharynx.
[Illustration: FIG. 61.—_Malacobdella grossa_ O. F. Müll., a large female
specimen. Kiel. × 1. (From von Kennel.) A, From the dorsal surface; B, from
the ventral surface.]
PALAEONTOLOGY.—Nemertines are unknown in a fossil state; this is probably
owing to the softness of their bodies, which would render their
preservation extremely improbable.