POLYZOA—_continued_
CLASSIFICATION—GEOGRAPHICAL DISTRIBUTION—PALAEONTOLOGY—METHODS FOR THE
EXAMINATION OF SPECIFIC CHARACTERS—TERMINOLOGY—KEY FOR THE
DETERMINATION OF THE GENERA OF BRITISH MARINE POLYZOA 515
ADDENDUM TO CHAETOGNATHA 534
INDEX 535
SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK
PLATYHELMINTHES (p. 3)
Family.
TURBELLARIA (p. 3)
|
+— POLYCLADIDA (p. 7)
| |
| +—— ACOTYLEA (p. 16)
| | |
| | +—— Planoceridae (p. 19).
| | +—— Leptoplanidae (p. 19).
| | +—— Cestoplanidae (p. 19).
| | +—— Enantiidae (p. 19).
| |
| +—— COTYLEA (p. 16)
| |
| +—— Anonymidae (p. 19)
| +—— Pseudoceridae (p. 19).
| +—— Euryleptidae (p. 19).
| +—— Prosthiostomatidae (p. 19).
|
+—— TRICLADIDA (p. 30)
| |
| +—— PALUDICOLA (p. 30)
| | |
| | +—— Planariidae (p. 42).
| |
| +—— MARICOLA (pp. 30, 32)
| | |
| | +—— Procerodidae (p. 42). = Gundidae.
| | +—— Bdellouridae (p. 42).
| |
| +—— TERRICOLA (pp. 30, 33)
| |
| +—— Bipaliidae (p. 42).
| +—— Geoplanidae (p. 42).
| +—— Rhynchodemidae (p. 42).
|
+—— RHABDOCOELIDA (p. 42)
|
+—— ACOELA (p. 42)
| |
| +—— Proporidae (p. 49).
| +—— Aphanostomatidae (p. 49).
|
+—— RHABDOCOELA (p. 43)
| |
| +—— Macrostomatidae (p. 49).
| +—— Microstomatidae (p. 49).
| +—— Prorhynchidae (p. 49).
| +—— Mesostomatidae (p. 49).
| +—— Proboscidae (p. 49).
| +—— Vorticidae (p. 50).
| +—— Solenopharyngidae (p. 50).
|
+—— ALLOEOCOELA (p. 43)
|
+—— Plagiostomatidae (p. 50).
+—— Bothrioplanidae (p. 50).
+—— Monotidae (p. 50).
TREMATODA (pp. 3, 51)
|
+—— MONOGENEA (pp. 5, 52) = Heterocotylea + Aspidocotylea (p. 73)
| |
| +—— Temnocephalidae (pp. 53, 73).
| +—— Tristomatidae (pp. 53, 73).
| +—— Polystomatidae (pp. 53, 73).
| +—— Gyrodactylidae (pp. 53, 61, 73).
| +—— Aspidobothridae (p. 73).
|
+—— DIGENEA (pp. 5, 52, 62) = Malacocotylea (p. 73)
|
+—— Holostomatidae (p. 73).
+—— Amphistomatidae (p. 73).
+—— Distomatidae (p. 73).
+—— Gasterostomatidae (p. 73).
+—— Didymozoontidae (p. 73).
+—— Monostomatidae (p. 73).
CESTODA (pp. 3, 74) ——————+
|
+—— Cestodariidae = Monozoa (p. 91).
+—— Bothriocephalidae (p. 91).
+—— Tetrarhynchidae (p. 91).
+—— Tetraphyllidae (p. 91).
+—— Taeniidae (p. 91).
MESOZOA
MESOZOA (pp. 3, 92) ——————+
|
+—— Dicyemidae (p. 93).
+—— Orthonectida (p. 94).
NEMERTINEA (p. 99)
HOPLONEMERTEA (p. 110) = Metanemertini (p. 112).
SCHIZONEMERTEA (p. 111) = Heteronemertini (_ex parte_) (p. 113).
PALAEONEMERTEA (p. 111) = Protonemertini (p. 112).
+ Mesonemertini (p. 112).
+ Heteronemertini (_ex parte_) (p. 113).
NEMATHELMINTHES (p. 123)
NEMATODA (pp. 123, 124) ——+
|
+—— Ascaridae (p. 138).
+—— Strongylidae (p. 142).
+—— Trichotrachelidae (p. 144).
+—— Filariidae (p. 147).
+—— Mermithidae (p. 150).
+—— Anguillulidae (p. 154).
+—— Enoplidae (p. 157).
+—— Chaetosomatidae (p. 158).
+—— Desmoscolecidae (p. 159).
NEMATOMORPHA (pp. 123, 164)
|
+—— Gordiidae (p. 164).
ACANTHOCEPHALA (pp. 123, 174)
|
+—— Echinorhynchidae (p. 182)
+—— Gigantorhynchidae (p. 183).
+—— Neorhynchidae (p. 184).
+—— Arhynchidae (p. 185).
CHAETOGNATHA (p. 186)
ROTIFERA (p. 197)
FLOSCULARIACEAE (p. 220) —+
|
+—— Flosculariidae (p. 221).
+—— Apsilidae (p. 221).
MELICERTACEAE (p. 221) ———+
|
+—— Melicertidae (p. 221).
+—— Trochosphaeridae (p. 221).
BDELLOIDA (p. 222) ———————+
|
+—— Philodinidae (p. 222).
ASPLANCHNACEAE (p. 222) ——+
|
+—— Asplanchnidae (p. 223).
SCIRTOPODA (p. 223) ——————+
|
+—— Pedalionidae (p. 223).
PLOIMA (p. 223)
|
+—— ILLORICATA (p. 223)
| |
| +—— Microcodonidae (p. 224).
| +—— Rhinopidae (p. 224).
| +—— Hydatinidae (p. 224).
| +—— Synchaetidae (p. 224).
| +—— Notommatidae (p. 224).
| +—— Drilophagidae (p. 224).
| +—— Triarthridae (p. 224).
|
+—— LORICATA (p. 224)
|
+—— Rattulidae (p. 225).
+—— Dinocharididae (p. 225).
+—— Salpinidae (p. 225).
+—— Euchlanididae (p. 225).
+—— Cathypnidae (p. 225).
+—— Coluridae (p. 225).
+—— Pterodinidae (p. 225).
+—— Brachionidae (p. 225).
+—— Anuraeidae (p. 225).
SEISONACEAE (p. 225) —————+
|
+—— Seisonidae (p. 226).
GASTROTRICHA
GASTROTRICHA (p. 231).
|
+—— EUICHTHYDINA (p. 235)
+—— APODINA (p. 235)
KINORHYNCHA (p. 236)
CHAETOPODA (p. 241)
ARCHIANNELIDA (p. 241)
POLYCHAETA (pp. 241, 245)
|
+—— PHANEROCEPHALA (p. 303)
| |
| +—— NEREIDIFORMIA (p. 303)
| | |
| | +—— Syllidae (p. 306).
| | +—— Hesionidae (p. 308).
| | +—— Aphroditidae (p. 309).
| | +—— Phyllodocidae (p. 313).
| | +—— Tomopteridae (p. 315).
| | +—— Nereidae (p. 315).
| | +—— Nephthydidae (p. 317).
| | +—— Amphinomidae (p. 318).
| | +—— Eunicidae (p. 318).
| | +—— Glyceridae (p. 320).
| | +—— Sphaerodoridae (p. 320).
| | +—— Ariciidae (p. 321).
| | +—— Typhloscolecidae (p. 321).
| |
| +—— SPIONIFORMIA (p. 304)
| | |
| | +—— Spionidae (p. 321).
| | +—— Polydoridae (p. 323).
| | +—— Chaetopteridae (p. 323).
| | +—— Magelonidae (p. 325.
| | +—— Ammocharidae (p. 325).
| |
| +—— TEREBELLIFORMIA (p. 304)
| | |
| | +—— Cirratulidae (p. 325).
| | +—— Terebellidae (p. 327).
| | +—— Ampharetidae (p. 330).
| | +—— Amphictenidae (p. 330).
| |
| +—— CAPITELLIFORMIA (p. 305)
| | |
| | +—— Capitellidae (p. 331).
| |
| +—— SCOLECIFORMIA (p. 305)
| |
| +—— Opheliidae (p. 331).
| +—— Maldanidae (p. 332).
| +—— Arenicolidae (p. 333).
| +—— Scalibregmidae (p. 334).
| +—— Chlorhaemidae (p. 334).
| +—— Sternaspidae (p. 335).
|
+—— CRYPTOCEPHALA (p. 303)
|
+—— SABELLIFORMIA (p. 305)
| |
| +—— Sabellidae (p. 336).
| +—— Eriographidae (p. 338).
| +—— Amphicorinidae (p. 339).
| +—— Serpulidae (p. 339).
|
+—— HERMELLIFORMIA (p. 306)
|
+—— Hermellidae (p. 341).
MYZOSTOMARIA (pp. 241, 341)
OLIGOCHAETA (pp. 241, 347)
|
+—— MICRODRILI (p. 373)
| |
| +—— Aphaneura (p. 374).
| +—— Enchytraeidae (p. 375).
| +—— Discodrilidae (p. 376).
| +—— Phreoryctidae (p. 376).
| +—— Naidomorpha (p. 377).
| +—— Tubificidae (p. 378).
| +—— Lumbriculidae (p. 379).
| +—— Moniligastridae (p. 380).
|
+—— MEGADRILI (pp. 373, 374).
|
+—— Perichaetidae (p. 380).
+—— Cryptodrilidae (p. 382).
+—— Acanthodrilidae (p. 384).
+—— Eudrilidae (p. 385).
+—— Geoscolicidae (p. 386).
+—— Lumbricidae (p. 388).
HIRUDINEA (p. 392)
RHYNCHOBDELLAE (p. 405) ——+
|
+—— Ichthyobdellidae (p. 406).
+—— Glossiphoniidae (p. 406).
GNATHOBDELLAE (p. 407) ———+
|
+—— Gnathobdellidae (p. 407).
+—— Herpobdellidae (p. 407).
GEPHYREA (p. 411)
SIPUNCULOIDEA (pp. 412, 420).
PRIAPULOIDEA (pp. 412, 430).
ECHIUROIDEA (pp. 412, 434).
EPITHETOSOMATOIDEA (pp. 412, 444).
_PHORONIS_ (p. 450)
POLYZOA (p. 465)
ENTOPROCTA (pp. 475, 487)
ECTOPROCTA (p. 475)
|
+—— GYMNOLAEMATA (p. 476)
| |
| +—— CYCLOSTOMATA (p. 477)
| | |
| | +—— Articulata (p. 517).
| | +—— Inarticulata (p. 517).
| |
| +—— CHEILOSTOMATA (p. 477)
| | |
| | +—— Cellularina (p. 518).
| | +—— Flustrina (p. 518).
| | +—— Escharina (p. 518).
| |
| +—— CTENOSTOMATA (p. 477)
| |
| +—— Alcyonellea (p. 518).
| +—— Vesicularina (p. 518).
|
+—— PHYLACTOLAEMATA (pp. 476, 493)
PLATYHELMINTHES AND MESOZOA
BY
F. W. GAMBLE, M.Sc. (VICT.)
Demonstrator and Assistant-Lecturer in Zoology in the Owens College,
Manchester.
{3}CHAPTER I
TURBELLARIA
INTRODUCTION: DESCRIPTION OF THE POLYCLAD _LEPTOPLANA TREMELLARIS_—
APPEARANCE—HABITS—STRUCTURE: POLYCLADIDA—CLASSIFICATION—HABITS—
ANATOMY—DEVELOPMENT: TRICLADIDA—OCCURRENCE—STRUCTURE—CLASSIFICATION:
RHABDOCOELIDA—OCCURRENCE—HABITS—REPRODUCTION—CLASSIFICATION.
The Platyhelminthes, or Flat Worms, form a natural assemblage of animals,
the members of which, however widely they may differ in appearance, habits,
or life-history, exhibit a fundamental similarity of organisation which
justifies their separation from other classes of worms, and their union
into a distinct phylum. Excluding the leeches (Hirudinea), and the long
sea-worms (Nemertinea)—which, though formerly included, are now treated
independently—the Platyhelminthes may be divided into three branches: (1)
Turbellaria (including the Planarians), (2) Trematoda (including the
liver-flukes), and (3) Cestoda (tape-worms). The Mesozoa will be treated as
an appendix to the Platyhelminthes.
The Turbellaria were so called by Ehrenberg[1] (1831) on account of the
cilia or vibratile processes with which these aquatic animals are covered,
causing by their incessant action, tiny currents ("turbellae,"
disturbances) in the surrounding water. The ciliary covering distinguishes
this free-living group from the parasitic Trematodes and Cestodes, some of
which possess such an investment, but only during their early free
{4}larval stage, for the short period when they have left the parental host
and are seeking another (Figs. 26, 27, 42).
Some Turbellaria (Rhabdocoelida) resemble Infusoria in their minute size,
shape, and movements. Nevertheless they possess an organisation of
considerable complexity. The fresh-water Planarians (Fig. 14), abounding in
ponds and streams, vary from a quarter to half an inch in length, and are
elongated and flattened. Their body is soft, and progresses by a
characteristic, even, gliding motion like a snail. The marine Planarians or
Polyclads (Fig. 8) are usually broad and leaf-like, sometimes attaining a
length of six inches, and swim or creep in a most graceful way. Land
Planarians occur in this country (Fig. 15), but far more abundantly in
tropical and sub-tropical districts, in moist places, venturing abroad at
night in pursuit of prey. They are elongated and cylindrical, in some cases
measuring, when fully extended, a foot or more in length, and are often
ornamented with brilliantly coloured, longitudinal bands.
Turbellaria are carnivorous, overpowering their prey by peculiar cutaneous
offensive weapons, and then sucking out the contents of the victim by the
"pharynx." Land Planarians feed on earthworms, molluscs, and wood-lice;
fresh-water Planarians on Oligochaet worms, water-snails, and
water-beetles; marine forms devour Polychaet worms and molluscs. Some
Turbellaria seem to prefer freshly-killed or weakly examples of animals too
large to be overpowered when fully active. Certain Rhabdocoelida are
messmates of Molluscs and Echinoderms, and a few others are truly
parasitic—a mode of life adopted by all Trematodes save _Temnocephala_.
The Trematodes[2] may be divided into those living on the outer surface of
various aquatic animals, usually fish (Ectoparasites); and those which
penetrate more or less deeply into the alimentary canal or the associated
organs of the host (Endoparasites). They are oval, flattened
Platyhelminthes ranging from a microscopic size to a length of three feet
(_Nematobothrium_, Fig. 22), and are provided with organs of adhesion by
which they cling to the outer surface, or to the interior, of the animals
they inhabit. Trematodes occur parasitically in all groups of Vertebrates,
but, with the exception of the liver-flukes of the sheep (_Distomum
hepaticum_ and _D. magnum_), and of _Bilharzia haematobia_ found in man (in
the blood-vessels of the urinary bladder) over the greater part {5}of
Africa, their attacks are not usually of a serious nature. Ectoparasitic
Trematodes are _Monogenetic_; that is, their larvae grow up directly into
mature forms. The Endoparasitic species, however, are usually _Digenetic_.
Their larvae enter an Invertebrate and produce a new generation of
different larvae, and these another. The last are immature flukes. They
enter a second host, which is swallowed by the final Vertebrate host in
which they become mature.
The Cestodes or Tape-worms have undergone more profound modifications both
in structure and in mode of development. They are all endoparasitic, and,
with one exception (_Archigetes_), attain maturity solely within the
alimentary canal of Vertebrates. In length they range from a few
millimetres to several metres, but this great size is attained from the
need for the rapid production and accumulation of enormous numbers of eggs.
The "head" or "scolex" is attached to the mucous membrane of the host by
suckers or hooks, but there is no mouth nor any certain trace of a
digestive tract at any stage of the life-history of Cestodes. For
nourishment they absorb, through the skin, the previously-digested food (of
the host) that bathes them. In a few Cestodes the body is simple and not
divided into "proglottides" or generative segments, but in most cases it is
jointed in such a way that the last segment is the oldest, and each
contains a set of reproductive organs. The life-histories of Cestodes are
most remarkable. The proglottides containing the eggs pass out of the final
host along with the faeces and enter the intermediate host with the food.
The larvae hatch, and boring their way into the blood-vessels, are carried
by the circulation to various internal organs. Here they usually become
"bladder-worms," and develop the "head" of the future sexual form. Then,
if, as is usually the case, the intermediate host is preyed upon by the
final host, the larval Cestodes enter the alimentary canal of the latter.
The head of the larva alone survives digestion, and from it the mature worm
is formed.
Of these three branches of the phylum Platyhelminthes, the Turbellaria
possess features of special interest and importance. Not only do they
furnish the explanation of the structure of the two parasitic groups (which
have probably arisen from Turbellarian-like ancestors), but they occupy the
lowest position in the whole group of worms. There are reasons for thinking
that this is the simplest group of bilateral animals which adopt the habit
of {6}creeping. The Turbellaria are most closely allied to that great
extinct group from which they, the Nemertinea, Rotifera, and even the
Annelids, offer increasingly convincing evidence of having been derived.
Many questions relating to the affinities of, or the origin of organs in,
the Annelids, resolve themselves into similar questions about the
Turbellaria. For these reasons, this group is here dealt with at greater
length than the others, the interest of which is of a more special nature.
The history of our knowledge of the Cestodes dates back to ancient times,
as the presence and effects of tape-worms early attracted the attention of
physicians. Trematodes are first distinctly referred to in the sixteenth
century, while Turbellaria first figure in Trembley's memoir on Hydra
(1744).[3] The whole subject of the increase in our knowledge of parasitic
Platyhelminthes is dealt with in the standard work, _The Parasites of Man_,
by Leuckart,[4] and a complete list of references in zoological literature
to Cestodes and Trematodes is to be found in Bronn's _Thierreich_.[5] O. F.
Müller[6] and Ehrenberg founded our knowledge of the Turbellaria, but for a
long time the group remained in a most neglected condition. In this country
Montagu, G. Johnston, and in Ireland, William Thompson, discovered several
marine species, one of which, _Planocera folium_ (from Berwick), has not
again been met with on British shores. Dalyell[7] conducted classical
researches on the habits of Planarians, and Faraday[8] made interesting
experiments on their power of regenerating lost parts. The credit of
assigning the correct interpretation to most of the various organs of
fresh-water Planarians belongs to von Baer[9] and Dugès,[10] while
Mertens[11] effected a similar service for the marine forms, or Polyclads.
The minute Rhabdocoels were first successfully investigated and classified
by Oscar Schmidt.[12] The great work on this group is, however, the
{7}monograph by von Graff.[13] A similarly comprehensive and indispensable
treatise by Lang, on the Polycladida,[14] contains references to all
previous publications on the group, among which the papers by Quatrefages,
Johannes Müller, Keferstein, Minot, and Hallez stand out conspicuously.
Moseley's work[15] on the Land Planarians of Ceylon is undoubtedly the most
revolutionary paper referring to this group, and the best contribution
towards elucidating the structure of the Tricladida at a time when the
subject was very obscure. A monograph on Land Planarians is being prepared
by von Graff.
The Turbellaria are divided into: (1) _Polycladida_, marine forms with
multiple intestinal branches; (2) _Tricladida_, marine, fresh-water, and
terrestrial Planarians with three main intestinal branches; (3) the
_Rhabdocoelida_, as varied in habit as the Triclads, but possessing a
straight and simple or slightly lobed, intestine. A detailed description of
an example of the Polyclads, and then a comparative account of each
division, will now be given.
[Illustration: FIG. 1.—_Leptoplana tremellaris_ O. F. M. Seen from the
dorsal surface. The alimentary canal runs down the middle line and sends
branches to the margin of the body. × 6.]
TURBELLARIA. I. POLYCLADIDA.
_Description of Leptoplana tremellaris._
APPEARANCE AND HABITS.—An account of the Polyclad Turbellaria may be fitly
prefaced by a description of a very common representative, _Leptoplana
tremellaris_, so called on account of the thin, flat body which executes
when disturbed, quivering or tremulous swimming movements.
{8}Like all Polyclads, _Leptoplana_ is marine. It is probably found on all
European shores, northwards to Greenland and southwards to the Red Sea,
while vertically it ranges from the littoral zone down to fifty fathoms.
There is, however, an apparently well-marked difference between the
littoral specimens, which vary from three-quarters to one inch in length,
are brownish in colour and firm in consistency, and the more delicate
examples half an inch long, white with a brown tinge, which occur in deeper
water.
[Illustration: FIG. 2.—_Leptoplana tremellaris._ Three-quarters view from
the ventral surface. The pharynx (_ph_) is widely protruded through the
month (_mo_) as in the act of attacking prey. _br_, Brain with nerves,
close to which are the four groups of eyes; _mg_, stomach; _mgc_, "marginal
groove"; _pe_, penis; _sc_, sucker; _ut_, uterus; _vd_, vasa deferentia; ♀,
female genital aperture surrounded by the shell-gland; ♂, male aperture.
(Semi-diagrammatic, and × 6.)]
At low water _Leptoplana_ may be found buried in mud or on the under
surface of stones, in pools where darkness and dampness may be ensured till
the return of the tide. It is, however, by no means easy to detect and
remove it from the encrusting Polyzoa, Ascidians, or Sponges with which it
is usually associated. The flat, soft, unsegmented body is so closely
appressed to the substratum that its presence is usually only betrayed by
its movement, an even gliding motion of the mobile body, which suggested
the apt name "la pellicule animée" to Dicquemare. The creeping surface is
called ventral, the upper one dorsal, and as the broader end of the body
always goes first, it is anterior as opposed to the more pointed posterior
extremity. With a lens the characters shown in Figs. 1 and 2 may be
observed. The eyes are seen as black dots near the anterior end, and are
placed at the sides of a clear oval space, the brain. Along the transparent
margin of the body, the ends of the intestinal branches may be seen. These
ramify from a lobed stomach or main-gut, and should the specimen be mature,
the "uterus" loaded {9}with eggs forms a dark margin round the latter
(Figs. 1 and 2, _ut_). The ventral surface is whitish, and through it the
"pharynx," a frilled protrusible structure, may be dimly observed. The
"mouth,"[16] through which the pharynx at the time of feeding is thrust out
(Fig. 2, _mo_), is almost in the centre of the ventral surface. Behind
this, a white, V-shaped mark (_vd_) indicates the ducts of the male
reproductive organs, and still further back is the irregular opaque mark of
the "shell-gland," by which the egg-shells are formed (Fig. 2, ♀).
[Illustration: FIG. 3.—_Leptoplana tremellaris_ in the act of swimming. A,
Seen from the right side during the downward stroke (the resemblance to a
skate is striking); B, from above, showing the upward stroke and
longitudinal undulations of the swimming lobes; C, side view during the
upward stroke; D, transverse sections of the body during the strokes. × 5.]
{10}_Leptoplana_ employs two kinds of movement, creeping and swimming.
Creeping is a uniform gliding movement, caused by the cilia of the ventral
surface, aided perhaps by the longitudinal muscular layers of this surface,
and is effected on the under side of the "surface-film" of water almost as
well as on a solid substratum. Swimming is a more rapid and elegant
movement, employed when alarmed or in pursuit of prey. The expanded
fore-parts of the body act as lobes, which are flapped rapidly up over the
body and then down beneath it, undulations running rapidly down them from
before backwards. The action in fact is somewhat similar to that by which a
skate swims, a resemblance pointed out long ago by Dugès[17] (Fig. 3).
We have few direct observations on the nature of the food of _Leptoplana_,
or the exact mode by which it is obtained. Dalyell,[18] who observed this
species very carefully, noticed that it was nocturnal and fed upon a
_Nereis_, becoming greatly distended and of a green colour after the meal,
but pale after a long fast. Keferstein[19] noticed a specimen in the act of
devouring a _Lumbriconereis_ longer than itself, and also found the radulae
of _Chiton_ and Taenioglossate Molluscs in the intestine. That such an
apparently weak and defenceless animal does overpower large and healthy
Annelids and Mollusca, has not hitherto been definitely proved. Weak or
diseased examples may be chiefly selected. The flexible _Leptoplana_
adheres firmly to its prey, and the rapid action of the salivary glands of
its mobile pharynx quickly softens and disintegrates the internal parts of
the victim. The food passes into the stomach (Fig. 2, _mg_), and is there
digested. It is then transferred to the lateral branches of the intestine,
and, after all the nutritious matters have been absorbed, the faeces are
ejected with a sudden contraction of the whole body through the pharynx
into the water.
_Leptoplana_ probably does not live more than a year. In the spring or
summer, batches of eggs are laid and fixed to algae or stones by one
individual, after having been fertilised by another. Young _Leptoplana_
hatch out in two to three weeks, and lead a {11}pelagic existence till they
are three or four millimetres in length. In late summer, numbers of such
immature examples may be found among sea-weeds and _Corallina_ in tide
pools. In the succeeding spring they develop first the male and then the
female reproductive organs.
[Illustration: FIG. 4.—Portion of a transverse section of _Leptoplana
tremellaris_ in the hinder part of the body. × 100. _bm_, Basement
(skeletal) membrane; _cil_, cilia; _d.m_, diagonal muscles; _d.v.m_,
dorso-ventral muscles; _ep_, epidermis; _f.p_, food particles; _l.g_,
lateral intestinal branches cut across; _l.m ext_, external, and _l.m int_,
internal longitudinal muscle layers; _m.c_, glandular (mucous) cells; _md_,
their ducts; _N_, longitudinal nerve; _Nu_, nuclei of the intestinal
epithelium; _ov_, ovary; _ovd_, oviduct; _par_, cells of the parenchyma;
_r.d_, vasa deferentia, with spermatozoa; _rm_, circular musculature; _rh_,
rhabdites; _sh_, cells of the shell-gland; _te_, testes; _ve_, vasa
efferentia; _y.c_, "yellow cells." (After Lang.)]
ANATOMY OF LEPTOPLANA TREMELLARIS.—_Leptoplana_ may be divided into
corresponding halves only by a median vertical longitudinal plane. The body
and all the systems of organs are strictly bilaterally symmetrical.
Excepting the cavities of the organs themselves, the body is solid. A
connective "parenchyma" (Fig. 4, _par_) knits the various internal organs
together, while it allows free play of one part on another. These organs
are enclosed in a muscular body-wall, clothed externally by the ciliated
epidermis, which is separated from the underlying musculature by a strong
membrane (Fig. 4, _bm_), the only skeletal element in the body.
{12}BODY-WALL.—The epidermis (Fig. 4, _ep_) is composed of a single layer
of ciliated cells, containing small, highly refractive, pointed rods or
"rhabdites" (_rh_), and gives rise to deeply-placed mucous cells (_m.c_),
which are glandular and pour out on the surface of the body a fluid in
which the cilia vibrate. The tenacious hold on a stone which _Leptoplana_
exerts if suddenly disturbed, or when grasping its prey, is probably due to
the increased glutinous secretion of these glands, aided perhaps by
rhabdites, which on such occasions are shot out in great numbers. The
basement membrane is an elastic skeletal membrane composed of stellate
cells embedded in a firm matrix. It serves chiefly for the origin and
insertion of the dorso-ventral muscles (_d.v.m_). Under the basement
membrane lies a very thin layer of transverse muscular fibres (Fig. 4,
_rm_), which are, however, apparently absent on the ventral surface. Then
follows a stout layer of longitudinal fibres (_l.m ext_), and beneath this
a diagonal layer (_d.m_), the fibres of which intersect along the median
line in such a way that the inner fibres of one side become the outer
diagonal fibres of the other. Lastly, within this again, on the ventral
surface, is a second stout longitudinal layer (_l.m int_). The sucker
(_sc_, Figs. 2 and 5) is a modification of the body-wall at that point. In
addition to the dorso-ventral muscles, there exists a complex visceral
musculature regulating the movements of the pharynx, intestine, and
copulatory organs.
PARENCHYMA.—The spaces between the main organs of the body are filled by a
tissue containing various kinds of cells, salivary glands, shell-glands,
and prostate glands. Besides these, however, we find a vacuolated,
nucleated, thick-walled network, and to this the word parenchyma is
properly applied. Besides its connective function, the parenchyma confers
that elasticity on the body which _Leptoplana_ possesses in such a high
degree. Pigment cells are found in the parenchyma in many Polyclads.
DIGESTIVE SYSTEM.—The general arrangement of this system may be seen in
Figs. 2, 5, and 7; and may be compared, especially when the pharynx is
protruded, as in Fig. 2, with the gastral system of a Medusa. The "mouth"
(there is no anus) is placed almost in the centre of the ventral surface.
It leads (Fig. 7, B, _phs_) into a chamber (the peripharyngeal space)
divided into an upper and a lower division by the insertion of a muscular
collar-fold (the pharynx, _ph_), which may be protruded, its free lips
{13}advancing, through the mouth (Fig. 2), and is then capable of enclosing
by its mobile frilled margin, prey as large as _Leptoplana_ itself. The
upper division of the chamber communicates by a hole in its roof[20] (the
true mouth, Figs. 5 and 7, _g.m_) with the cavity of the main-gut or
stomach (_m.g_), which runs almost the length of the body in the middle
line, forwards over the brain (Fig. 5, _up_). Seven pairs of lateral
gut-branches convey the digested food to the various organs, not directly
however, but only after the food mixed with sea-water has been repeatedly
driven by peristalsis first towards the blind end of the gut-branches and
then back towards the stomach. Respiration is probably largely effected by
this means. The epithelium of the intestine (Fig. 4, _l.g_) of a starving
specimen is composed of separate flagellated cells frequently containing
"yellow cells."[21] After a meal, however, the cell outlines are invisible.
Gregarines, encysted Cercariae, and Orthonectida[22] occur parasitically in
the gut-branches.
An EXCRETORY SYSTEM of "flame-cells" and fine vessels has hitherto been
seen only by Schultze[23] in this species, which will not, however, resist
intact the compression necessary to enable the details to be determined.
They are probably similar to those of _Thysanozoon_ described on p. 25.
NERVOUS SYSTEM.—The brain, which is enclosed in a tough capsule (Fig. 5,
_br_), is placed in front of the pharynx, but some distance behind the
anterior margin of the body. It is of an oval shape, subdivided
superficially into right and left halves by a shallow depression, and is
provided in front with a pair of granular-looking appendages, composed of
ganglion-cells from which numerous sensory nerves arise, supplying the eyes
and anterior region. Posteriorly the brain gives rise to a chiefly motor,
nervous sheath (Fig. 5, _nn_), which invests the body just within the
musculature. This sheath is thickened along two ventral lines (Fig. 5,
_ln_) and two lateral lines (_n.s_), but is very slightly developed on the
dorsal surface. Ganglion-cells occur on the course of the nerves, and are
particularly large at the point of origin of the great motor nerves.
{14}[Illustration: FIG. 5.—Diagrammatic view of the structure of
_Leptoplana tremellaris_ as a type of the Polycladida. The body is cut
across the middle to show the relative position of organs in transverse
section. In the posterior half the alimentary canal has been bisected and
removed from the left side, to exhibit the deeply placed nervous sheath
(_nn_) and the male reproductive organs. _br_, Brain; _dp_, "diaphragm";
_e_, cerebral group of eyes; _et_, tentacular eye-group; _gr_, marginal
groove; _gm_, true mouth; _lg_, lateral gut-branch; _ln_, longitudinal
nerve stem; _m_, external mouth; _mg_, _mg'_, main-gut, whole, and
bisected; _n_, sensory nerve supplying the eyes; _nn_, nervous network
lying on the ventral musculature; _n.s_, lateral nerve; _od_, oviduct;
_ov_, ovary; _pe_, penis (in section); _ph_, pharynx; _pr_, prostate or
"granule gland"; _sc_, sucker; _sg_, shell-gland; _te_, testes; _up_,
anterior unpaired gut-branch; _ut_, uterus; _va_, vagina (in section);
_vd_, vas deferens; _ve_, vasa efferentia; ♂, male genital pore; ♀, female
pore.]
{15}SENSE ORGANS.—_Leptoplana_ possesses eyes, stiff tactile, marginal
cilia, and possibly a sense organ in the "marginal groove." The eyes, which
are easily seen as collections of black dots lying at the sides of the
brain, may be divided into two paired groups: (1) cerebral eyes (Fig. 5,
_e_), and (2) tentacle eyes (_et_), which indicate the position of a pair
of tentacles in allied forms (Fig. 8, A, _t_ and B). Each ocellus consists
of a capsule placed at right angles to the surface of the body in the
parenchyma, below the dorsal muscles, and with its convex face outwards. It
is a single cell in which pigment granules have accumulated. The light,
however, can only reach the refractive rods, which lie within it, obliquely
at their outer ends. These rods are in connexion with the retinal cells,
and thus communicate by the optic nerve with the brain. The cerebral eyes
are really paired, and are directed some upwards, some sideways, some
downwards.
The "marginal groove" is a shallow depression of the epidermis (Fig. 5,
_gr_) lined by cilia, and containing the ducts of very numerous
gland-cells. It runs almost parallel to the anterior margin of the body, a
short distance from it, but we have no observations on its functions.
[Illustration: FIG. 6.—Diagram of an eye of _Leptoplana_ from the tentacle
group. × 600. (After Lang.)]
REPRODUCTIVE ORGANS.—_Leptoplana_ is hermaphrodite, and, as in most
hermaphrodites, the reproductive organs are complicated. The male organs
are the first to ripen, but this does not appear to prevent an overlapping
of the periods of maturity of the male and female products, so that when
the eggs are being laid, the male organs are, apparently, still in a
functional state. The principal parts are seen in Fig. 5. The very numerous
_testes_ (_te_) are placed ventrally, and are connected with fine vasa
_efferentia_ (_ve_), which form a delicate network opening at various
points into the two _vasa deferentia_ (_vd_). These tubes, especially when
distended with spermatozoa, may easily be seen (Fig. 2, _vd_) converging at
the base of the penis, and connected posteriorly by a loop that runs behind
the female genital pore (Fig. 5). The _penis_ (_pe_) is pyriform and
muscular, and is divided into two chambers, a large upper one for the
spermatozoa, and a smaller lower one for the secretion of a special
{16}"prostate" gland. The apex of the penis is eversible and not merely
protrusible, being turned inside out when evaginated. The _ovaries_ (Fig.
5, _ov_) are numerous and somewhat spherical. They are dorsally placed, but
when fully developed extend deeply wherever they can find room to do so,
and they not only furnish the ova, but elaborate food-yolk in the ova, as
there are no special yolk-glands. The slender _oviducts_ (_od_) open at
several points into the "uterus" (_ut_) (a misnomer, as no development
takes place within it), which encircles the pharynx, and opens by a single
duct into the _vagina_ (_va_). Here the ova are probably fertilised, and
one by one invested by the _shell-gland_ (_sg_) with a secretion which
hardens and forms a resistant shell. They are then laid in plate-like
masses which are attached to stones or shells. The development is a direct
one, and the young _Leptoplana_, which hatches in about three weeks, has
the outline of a spherical triangle, and possesses most of the organs of
the adult. After leading a floating life for a few weeks it probably
attains maturity in about nine months.
CLASSIFICATION, HABITS, AND STRUCTURE OF THE POLYCLADIDA.
The Polyclads were so called by Lang on account of the numerous primary
branches of their intestine. They are free-living, purely marine
Platyhelminthes, possessing multiple ovaries, distinct male and female
genital pores (Digonopora), but no yolk-glands. The eggs are small, and in
many cases give rise to a distinct larval form, known as "Müller's larva"
(Fig. 12). The Polyclads, with one exception,[24] fall into two sub-groups,
Acotylea and Cotylea:—
Character. Acotylea. Cotylea.
Sucker A sucker absent.[25] A sucker always present (Figs.
8, D, _s_; 7, A, _sc_).
Mouth In the middle, or behind the In the middle, or in front of
middle, of the ventral the middle, of the ventral
surface. surface.
Pharynx More or less intricately Rarely folded. Usually
folded. cylindrical or trumpet-
shaped.
Tentacles A pair of dorsal tentacles A pair of marginal tentacles
usually present. (except in _Anonymus_).
Development Usually direct. Larva when Müller's larva present.
present, not a typical Metamorphosis, however,
Müller's larva. extremely slight.
{17}Fig. 8 shows that, starting with a member (A, D) of each division, in
which the mouth is almost in the middle of the ventral surface, and the
brain and sense organs somewhat remote from the anterior end, we find in
the Acotylea a series leading to an elongated form (Cestoplanidae), in
which the mouth, pharynx, and genital pores are far back near the hinder
end of the body; while in the Cotylea the series leads similarly to the
elongated Prosthiostomatidae, in which, however, the pharynx and external
apertures are in the front part of the body. This view of the morphology of
the Polyclads is due to Lang, and is based on the assumption that the more
radially-constructed forms (Fig. 8, A, D) are the primitive ones.
[Illustration: FIG. 7.—Diagrammatic vertical longitudinal sections: A, Of
_Prosthiostomum_ (type of Cotylea); B, of _Leptoplana_; C, of _Cestoplana_
(types of Acotylea). (After Lang.) These figures illustrate the changes
which follow the shifting of the mouth from a central position (B) to
either end of the body. _br_, Brain; _dphm_, "diaphragm"; _gm_, true mouth;
_lg_, openings of lateral gut-branches; _m_, mouth; _mg_, main-gut or
stomach; _mgbr_, median gut-branch; _ph_, pharynx; _ph.m_, aperture in
pharyngeal fold; _phs_, peripharyngeal sheath; _sc_, sucker; ♂, male, and
♀, female, genital aperture.]
{18}[Illustration: FIG. 8.—Chief forms of Polycladida: A-C, Acotylea; D-F,
Cotylea. A, _Planocera graffii_ Lang, nat. size; B, _Stylochoplana
maculata_ Stimps, × 7; C, _Cestoplana rubrocincta_ Lang, × 4/3; D,
_Anonymus virilis_ Lang, × 3, ventral surface; E, _Thysanozoon brocchii_
Grube, nat. size; the head is thrown back and the pharynx (_ph_) is
protruded. F, _Prosthiostomum siphunculus_ Lang, × 3. _Br_, Brain; _CG_,
cerebral eye group; _DM_, true mouth; _Ey_, marginal eyes; _m_, mouth;
_MG_, main-gut or stomach; _P_, dorsal papillae; _Ph_, pharynx; _s_, sucker
(ventral); _T_, tentacles; _UP_, dorsal median gut-branch. ♂, male, and ♀,
female, genital aperture, except in D, where ♂ refers to the multiple
penes. (After Lang and Schmidt.)]
{19}CLASSIFICATION OF POLYCLADIDA.
ACOTYLEA.
Family. Genus. British Representatives.
{_Planocera_
{ (Fig. 8, A). {_Planocera folium_
{_Imogine._ { Grube. Berwick-on-
PLANOCERIDAE. {_Conoceros._ { Tweed.
With dorsal tentacles. {_Stylochus._ {_Stylochoplana maculata_
Mouth sub-central. {_Stylochoplana_ { Quatref. Among brown
{ (Fig. 8, B). { weeds in Laminarian
{_Diplonchus._ { zone.
{_Planctoplana._
{_Leptoplana tremellaris_
{_Discocelis._ { O. F. Müll.
LEPTOPLANIDAE. {_Cryptocelis._ {_L. fallax_ Quatref.
Without dorsal tenta- {_Leptoplana._ { Plymouth.
cles. Penis directed {_Trigonoporus._ {_L. droebachensis_ Oe.
backwards. {?_Polypostia_ { Plymouth Sound.
{ (see p. 27). {_L. atomata_ O. F. Müll.
{ Doubtful species.
CESTOPLANIDAE. {_Cestoplana_ (Fig. 8, C).
No tentacles. Body { In Mediterranean and
elongated. Penis { on French side of the
directed forwards. { Channel.
ENANTIIDAE. {
No sucker. No tenta- {
cles. Main-gut very {_Enantia._
short. External { Adriatic Sea.
apertures as {
in _Euryleptidae_. {
COTYLEA.
ANONYMIDAE. {
Mouth central. No {_Anonymus_ (Fig. 8, D).
tentacles. With two { Naples (two specimens).
rows of penes. {
PSEUDOCERIDAE. {_Thysanozoon_ (Fig. 8, E).
Marginal tentacles {_Pseudoceros._
folded. Mouth in {_Yungia._
anterior half. {
{_Prostheceraeus vittatus_
{ Mont. On west coast.
{_P. argus_ Quatref.
{ Guernsey.
EURYLEPTIDAE. {_Prostheceraeus._ {_Cycloporus papillosus_
Tentacles usually {_Cycloporus._ { Lang. On Ascidians in
present and pointed, {_Eurylepta._ { 2-30 fms.
or represented by two {_Oligocladus._ {_Eurylepta cornuta_
groups of eyes. Mouth {_Stylostomum._ { O. F. Müll. On sponges
close to anterior end. {_Aceros._ { and shells, 2-10 fms.
Pharynx cylindrical. { {_Oligocladus sanguinolentus_
{ Quatref.
{_O. auritus_ Clap. Doubtful.
{_Stylostomum variabile_
{ Lang.
PROSTHIOSTOMATIDAE. {
Tentacles absent. Body {
elongated. Pharynx {_Prosthiostomum_ (Fig. 8, F).
long, cylindrical. {
Penis with accessory {
muscular vesicles. {
{20}APPEARANCE AND SIZE OF POLYCLAD TURBELLARIA.—Polyclads are almost
unique amongst animals in possessing a broad and thin, delicate body that
glides like a living pellicle over stones and weeds, moulding itself on to
any inequalities of the surface over which it is travelling, yet so fragile
that a touch of the finger will rend its tissues and often cause its speedy
dissolution. The dorsal surface in a few forms is raised into fine
processes (_Planocera villosa_), or into hollow papillae (_Thysanozoon
brocchii_), and in very rare cases may be armed with spines (_Acanthozoon
armatum_,[26] _Enantia spinifera_); in others, again, nettle-cells
(nematocysts) are found (_Stylochoplana tarda_, _Anonymus virilis_). Some
Polyclads, especially the pelagic forms, are almost transparent; in others,
the colour may be an intense orange or velvety black, and is then due to
peculiar deposits in the epidermal cells. Between these two extremes the
colour is dependent upon the blending of two sources, the pigment of the
body itself and the tint of the food. Thus a starved _Leptoplana_ is almost
or quite white, a specimen fed on vascular tissue reddish. Many forms are
coloured in such a way as to make their detection exceedingly difficult,
but this is probably not merely due, as Dalyell supposed, to the substratum
furnishing them with food and thus colouring them sympathetically, but is
probably a result of natural selection.
The largest Polyclad, the bulkiest Turbellarian, is _Leptoplana gigas_ (6
inches long and 4 in breadth), taken by Schmarda, free-swimming, off the
coast of Ceylon. The largest European form is _Pseudoceros maximus_, 3½
inches in length and stoutly built. A British species, _Prostheceraeus
vittatus_, attains a length of from 2 to 3 inches. These large forms,
especially the Pseudoceridae (pre-eminently the family of big Polyclads),
are brightly coloured, and usually possess good swimming powers, since,
being broad and flat, they are certainly not well adapted for creeping
rapidly, and this is well shown by the way these Polyclads take to swimming
when in pursuit of prey at night. The size of any individual is determined,
amongst other factors, by the period at which maturity sets in, after which
probably no increase takes place. Polyclads apparently live about twelve
months, and mature specimens of the same species vary from ½ inch to 2½
inches in length (_Thysanozoon brocchii_), {21}showing that growth is,
under favourable conditions, very rapid.
HABITS OF POLYCLAD TURBELLARIA.—Polyclads are exclusively marine, and for
the most part littoral, animals. Moreover, there is no evidence of their
occurrence in those inland seas where certain marine animals (including one
or two species of otherwise characteristically marine Rhabdocoelida, p. 46)
have persisted under changed conditions. From half-tide mark down to 50
fathoms, some Polyclads probably occur on all coasts, but as to their
relative abundance in different seas we have very little accurate
information. The southern seas of Europe possess more individuals and
species than the northern, and probably the maximum development of the
group takes place on the coasts and coral islands of the tropics.[27] No
Polyclads have been taken below 60 fathoms; but their delicacy and
inconspicuousness render this negative evidence of little value. Six truly
pelagic forms, however, are known,[28] and these are interesting on account
of their wide distribution (three occurring in the Atlantic, Pacific, and
Indian oceans), and also from the distinct modifications they have
undergone in relation to their pelagic existence.
Whatever may be the interpretations of the fact, Polyclads are notoriously
difficult to detect, and this fact doubtless explains the scanty references
to them by the older naturalists who collected even in tropical seas. Lang,
who worked seven years at Naples, added to the Mediterranean fauna as many
Polyclads as were previously known for all Europe, in spite of the
assiduous labours of his predecessors, Delle Chiaje and Quatrefages. Again
Hallez, collecting at Wimereux at low-water, obtained some twenty specimens
of _Leptoplana tremellaris_ in an hour, while some other collectors working
by his side could only find two or three. Yet, even making allowance for
the difficulty of finding Polyclads, few of them appear to be abundant.
_Leptoplana tremellaris_ is frequently associated with colonies of
_Botryllus_, and if separated soon perishes, whereas the free-living
individuals are distinctly hardy (Hallez). A closely allied but possibly
distinct form lives upon the surface of the Polyzoon {22}_Schizoporella_,
on the French side of the Channel, and cannot long endure separation from
its natural habitat, to which it is adaptively coloured. A striking case of
protective mimicry is exhibited by _Cycloporus papillosus_, on the British
coasts. This species, eminently variable in colour and in the presence or
absence of dorsal papillae, is usually a quarter of an inch in length and
of a firm consistency. Fixed by its sucker to Polyclinid and other
Ascidians, _Cycloporus_ appears part and parcel of the substratum, an
interesting parallel to _Lamellaria perspicua_,[29] though we are not
justified in calling the Polyclad parasitic. Indeed, though a few cases of
association between Polyclads and large Gasteropods, Holothurians, and
Echinids are known,[30] there is only one case, that of _Planocera
inquilina_,[31] in the branchial chamber of the Gasteropod _Sycotypus
canaliculatus_, which would seem to bear the interpretation of parasitism.
The jet-black _Pseudoceros velutinus_ and the orange _Yungia aurantiaca_ of
the Mediterranean, are large conspicuous forms with no attempt at
concealment, but their taste, which is not known, may protect them. Other
habits, curiously analogous with devices employed by Nudibranch Mollusca
(compare _Thysanozoon brocchii_ with _Aeolis papillosa_), emphasise the
conclusion that the struggle for existence in the littoral zone has adapted
almost each Polyclad to its particular habitat.
As regards the vertical distribution of this group on the British coasts,
_Leptoplana tremellaris_ has an extensive range, and appears to come from
deeper to shallower water to breed.[32] In the upper part of the Laminarian
zone, _Cycloporus papillosus_, and, among brown weeds, _Stylochoplana
maculata_ are found. At and below lowest water-mark _Prostheceraeus
vittatus_, _P. argus_, and _Eurylepta cornuta_ occur. _Stylostomum
variabile_ and _Oligocladus sanguinolentus_, though occasionally found
between tide-marks, especially in the Channel Islands, are characteristic,
along with _Leptoplana droebachensis_ and _L. fallax_, of dredge material
from 10 to 20 fathoms.
LOCOMOTION.—Locomotion is generally performed by Polyclads at night when in
search of food, and two methods, creeping {23}and swimming, are usually
employed—creeping by the cilia, aided possibly, as in the case of some
Gasteropod Mollusca, by the longitudinal muscles of the ventral surface;
and swimming, by undulations of the expanded margins of the body. In the
former case the cilia work in a glandular secretion which bathes the body,
and enables them to effect their purpose equally well on different
substrata. The anterior region is generally lifted up, exploring the
surroundings by the aid of the tentacles, which are here usually present.
The rest of the body is closely appressed to the ground.
Swimming is particularly well performed by the Pseudoceridae, certain
species of _Prostheceraeus_, the large Planoceridae, some _Stylochoplana_,
_Discocelis_, and _Leptoplana_, and in the same manner as in _Leptoplana
tremellaris_ (p. 9). In _Cryptocelis_, _Leptoplana alcinoi_, and _L.
pallida_, however, the whole body executes serpentine movements like an
active leech (e.g. _Nephelis_); a cross section of the body would thus
present the same appearance during the whole movement. Many Polyclads,
notably _Anonymus_ (Lang), if irritated, spread out in all directions,
becoming exceeding thin and transparent.
[Illustration: FIG. 9.—_Discocelis lichenoides_ Mert. (after Mertens),
creeping on the inner side of a glass vessel by means of the lobes of the
extended and exceedingly mobile pharynx (_ph_). These lobes also serve to
enclose _Crustacea_ (_a_), and one lobe may then be withdrawn independently
of the rest, back into the body (_b_). The brain (_br_) and shell-gland
(_sg_) are shown by transparency.]
_Discocelis lichenoides_, _Planocera graffii_, and _Anonymus virilis_ have
peculiar modes of progression. The first, according to Mertens, will climb
up the sides of a vessel by means of the expanded lobes of the pharynx
(Fig. 9, _ph_), a habit of considerable interest, since we know that
certain Ctenophores—_Lampetia_, for instance—progress when not swimming on
the expanded lobes of their "stomach."[33] _Planocera_ and _Anonymus_
{24}creep by extending parts of the anterior margin and dragging the rest
of the body behind. In consequence, the brain and dorsal tentacles may come
to lie actually behind the middle of the body, and thus no definite
anterior end or "head" advances first. Along with this curious habit it may
be noticed (Lang) that the radial symmetry of the body is well marked; but
even without accepting this author's suggestion of the concurrent
development of a "head" with locomotion in a definite direction, the facts,
whether these two forms are primitive or not, are highly interesting.
FOOD.—Though we are probably right in calling Polyclads a carnivorous
group, the food of very few forms has been ascertained. Those which possess
a large frilled pharynx (most Acotylea) probably enclose and digest large,
and, it may be, powerful prey, as appears to be the case in _Leptoplana
tremellaris_. _Cryptocelis alba_ has been seen by Lang with the pharynx so
distended, owing to a large _Drepanophorus_ (Nemertine) which it contained,
as to resemble a yolk-sac projecting from the under surface of an embryo.
The Cotylea such as _Thysanozoon_, with a bell- or trumpet-shaped pharynx,
are fond of fixing this to the side of the aquarium, but whether they thus
obtain minute organisms is not clear. _Prosthiostomum_ shoots out its long
pharynx with great vehemence (Fig. 8, F) and snaps up small Annelids by its
aid (Lang). Those Polyclads which, as _Cycloporus_ and others, are
definitely associated with other organisms are not certainly known to feed
upon the latter, though "_Planaria velellae_" has been seen by Lesson[34]
devouring the fleshy parts of its host. The salivary glands which open on
the lips and the inner surface of the pharynx powerfully disintegrate the
flesh of the prey. Digestion takes place in the main-gut, and the
circulation of the food is accomplished by the sphinctral musculature of
the intestinal branches (conf. _Leptoplana_, p. 13).
[Illustration: FIG. 10.—Diagram of the musculature, causing peristaltic
movements of the intestinal branches of Polyclads. (After Lang.)]
A distinct vent or anus is always absent. After a meal the {25}faecal
matter collects in the main-gut, and is discharged violently by the pharynx
into the water. In a few species, however, the intestinal branches open to
the exterior (Lang). _Yungia aurantiaca_, a large and abundant Neapolitan
form, possesses such openings over the greater part of the dorsal surface;
_Cycloporus papillosus_ has marginal pores; _Oligocladus sanguinolentus_
apparently possesses an opening at the posterior end of the main-gut; and
_Thysanozoon brocchii_ frequently rends at this point, in consequence of
the accumulation of food.
RESPIRATION.—The oxygen of the atmosphere dissolved in the sea-water is, in
default of a special circulatory fluid, brought to the tissues of Polyclads
in two ways. The ciliated epidermis provides a constant change of the
surrounding water, by which the superficial organs may obtain their supply;
and the peristaltic movements of the digestive system, aided by the cilia
of the endoderm cells, ensure a rough circulation of the sea-water, which
enters along with the food, to the internal organs. The papillae of
_Thysanozoon brocchii_, containing outgrowths of the intestinal branches,
are possibly so much additional respiratory surface, although still larger
forms (other Pseudoceridae) are devoid of such outgrowths.
EXCRETION.—The excretory system of only one Polyclad (_Thysanozoon
brocchii_) is accurately known. Lang, by compressing light-coloured
specimens, found the three parts of the system known to occur in many
Platyhelminthes: (1) the larger longitudinal canals, and (2) the capillary
vessels, which commence with (3) the flame-cells in the parenchyma of the
body. The mode of distribution of these parts is not, however, ascertained.
The canals are delicate, sinuous, apparently intracellular tubes, coursing
close to the margin of the body and sending offsets which suspend the
canals to the dorsal surface, where possibly openings may occur. In
dilatations of these vessels bunches of cilia, and occasionally
flame-cells, are found. Usually, however, flame-cells occur at the
commencement or during the course of the capillaries, which are straight,
rarely branching, tubes of exceeding tenuity, and appear (Lang) to be
outgrowths of the flame-cells, just as the duct is an outgrowth of a
gland-cell. In fact there is little doubt that the stellate flame-cells are
modified parenchymatous gland-cells, containing a lumen filled with a fluid
into which a number of cilia project and vibrate synchronously. The cells
excrete {26}nitrogeneous waste substances, which are then discharged into
the capillaries, whence the cilia of the main vessels drive them presumably
to the exterior, though external openings of the excretory system are not
known. Traces of this system have been observed in young _Leptoplana_
(first by Schultze in 1854) and also in _Cestoplana_.
SENSATION.—A nervous sheath, with scattered ganglion cells, everywhere
underlies the musculature. It is exceedingly faintly marked on the dorsal
surface, but laterally and ventrally forms a dense network with polygonal
meshes. Thickenings of this sheath give rise to lateral nerves, and also to
a pair of stout longitudinal nerves from which the internal organs are
probably innervated. The brain, hardly distinct in pelagic Polyclads, in
most forms does not differ greatly from that of _Leptoplana_ (p. 13).
The sense organs of Polyclads have the form of tentacles, eyes, otocysts
(in _Leptoplana otophora_), and stiff tactile cilia. The solid dorsal
tentacles of Planoceridae contrast strongly with the folded or pointed
hollow processes of the Cotylea. The former (Fig. 8, A, _T_) are muscular
and very contractile, and are placed near the brain some distance from the
anterior end. The latter are outgrowths of the front margin of the body,
and are sometimes (_Yungia_) provided superficially with olfactory pits and
internally with eyes and intestinal coeca.
The eyes which occur in Polyclads may be divided into (_a_) a pair of
cerebral groups overlying the brain; (_b_) those embedded in the tentacles
(tentacular group); and (_c_) the marginal eyes, which in _Anonymus_ occur
all round the margin. A complex form is sometimes assumed by the cerebral
eyes of Pseudoceridae, resulting probably from incomplete fission (Fig.
11). _Leptoplana otophora_ was obtained by Schmarda on the south coast of
Ceylon. On each side of the brain is a capsule containing two otoliths.
This is the only known case of the occurrence of these organs in Polyclads.
REPRODUCTION.—Although Polyclads are able to repair the result of injuries
to a very considerable extent, they are not known to multiply asexually.
The two processes are intimately associated, but, though probably all
Turbellaria can regenerate certain lost parts, asexual reproduction only
occurs sporadically.
{27}[Illustration: FIG. 11.—Double eye from the cerebral group of
_Pseudoceros maximus_. (After Lang.)]
All known Polyclads are hermaphrodite. The male organs, scattered, like the
testes of _Leptoplana_, over the ventral surface, develop earlier than the
ovaries, though the periods of maturation overlap; hence the possibility of
self-fertilisation, though remote, is still worth consideration. The
genital apertures, through which, in the male, spermatozoa, and in the
female, ova, are emitted, are usually situated as in _Leptoplana_ (Figs. 2
and 5, ♂ and ♀). In _Trigonoporus_, a genus once found at Naples, a
secondary female aperture has been discovered leading into the female
genital canal[35]; and in _Anonymus_, _Polypostia_, and _Thysanozoon_ (Fig.
7, E, ♂) two or more male pores and penes have been found. _Anonymus_ has
several penes (Fig. 7, D, ♂) arranged radially round the body.
_Polypostia_, a remarkable form described by Bergendal,[36] belonging to
the Acotylea, possesses about twenty such structures ranged round the
female genital aperture. Lang, whose attention was attracted by these
singular facts, made the interesting discovery that _Thysanozoon_ uses its
penes as weapons of offence rather than as copulating organs, burying them
in the skin of another Polyclad (_Yungia_) that happened to cross its path,
spermatozoa being of course left in the wound. Lang further found that
_Prostheceraeus albocinctus_ and _Cryptocelis alba_ in this way implanted a
spermatophore in the skin of another individual of the same species, and he
suggested that from this point the spermatozoa wandered through the tissues
till they met with and fertilised the eggs. It is now known that a similar
process of "hypodermic impregnation" occurs sporadically in several groups
of animals.[37] {28}Nevertheless, in some Polyclads it is probable, and in
_Stylochus neapolitanus_ it is certain, that normal copulation takes place.
The sperm-masses are transferred to a coecal diverticulum of the female
genital canal, and then by a delicate mechanism, of which we know only the
effects, one spermatozoon obtains entrance into one matured ovum, which
differs from the ova of most Turbellaria in that it contains in its own
protoplasm the yolk necessary for the nutrition of the embryo. In other
words, there are no special yolk-glands. After fertilisation, the ovum in
all Polyclads is coated with a shell formed by the shell-gland, which also
secretes a substance uniting the eggs together. They are deposited on
stones and shells, either in plate-like masses or in spirals (like those of
Nudibranchs). _Cryptocelis alba_ lays masses of an annular shape, with two
ova in each shell, and buries them in sand.
DEVELOPMENT.[38]—The first stages in the embryology of Polyclads appear to
be very uniform. They result, in all Cotylea and in certain Planoceridae,
in the formation of a Müller's larva (Fig. 12) about a couple of weeks
after the eggs are laid. This larva (1-1.8 mm. long), which is modified in
the Planoceridae, is distinguished by the presence of a ciliated band,
running somewhat transversely round the body, and usually produced into a
dorsal, a ventral, and three pairs of lateral processes. When swimming the
body is placed as in Fig. 12, and twists round rapidly about its
longitudinal axis by means of the strong locomotor cilia placed in
transverse rows upon the processes. The cilia of each row vibrate
synchronously, and recall the action of the swimming plates of a
Ctenophore. It is noteworthy that whereas _Stylochus pilidium_ passes
through a modified or, according to some authors, a primitive larval stage,
its near ally, _S. neapolitanus_, develops directly. Most {29}Acotylea
indeed develop directly, and their free-swimming young differ from Müller's
larva merely in the absence of the ciliated band and in the mode of
swimming.
[Illustration: FIG. 12.—Section through Müller's larva of _Thysanozoon
brocchii_ (modified from Lang). The right half is seen from inside. × 150.
Semi-diagrammatic. _br_, Brain; _dl_, dorsal ciliated lobe; _dr_, salivary
gland-cells of pharynx; _e_, eye; _ep_, ciliated epidermis containing
rhabdites; _mg_, stomach or main-gut; _mg_{1}_, unpaired gut branch over
the brain; _mo_, "mouth" of larva; _n_, _n_{1}_, section of nerves; _oe_,
ectodermic pit forming oesophagus of larva; _par_, parenchyma filling the
space between the alimentary tract and the body wall; _ph_, pharynx lying
in the cavity of the peripharyngeal sheath, the nuclei of which are
visible; _sl_{1}_, _sl_{2}_, _sl_{3}_, lateral ciliated lobes of the right
side; _vl_, ventral ciliated lobe.]
[Illustration: FIG. 13.—Diagrammatic transverse sections of a larval
Polyclad at different stages, to illustrate the development of the pharynx.
(After Lang.) A, Larva of the eighth day still within the shell. The
main-gut (_mg_) is still solid, the epidermis is slightly invaginated, and
a pair of muscular mesodermic thickenings (_ms_) are present. B, Young
pelagic larva. The epidermic invagination has deepened and developed
laterally. C, The lateral pouches have formed the wall of the
peripharyngeal sheath, enclosing the mesodermic, muscular, thickening or
pharyngeal fold (_ph_). (Compare Fig. 12.) Towards the end of larval life,
when the ciliated processes (_sl_, Fig. 12) have aborted, the stage D is
reached. By the opening outwards of the pharyngeal sheath (_ph.sh_) the two
apertures _gm_, or true mouth, and _m_, or external mouth, are formed,
which together correspond with the oesophageal opening of the younger
larva. (Compare the transverse section in Fig. 5.)]
Polyclads possess an undoubted mesoderm, which gives rise to the muscles,
the pharyngeal fold, and the parenchyma. The ectoderm forms the epidermis,
in the cells of which the {30}rhabdites (Fig. 12) arise, apparently as so
many condensed secretions. From the ectoderm the brain arises as two pairs
of ingrowths, which fuse together, and from these the peripheral nervous
system grows out. Three pigmented ectoderm cells give rise, by division, to
the eyes—an unpaired cell (Fig. 12, _e_) to the cerebral group of eyes, and
the other two to the marginal and tentacular groups. The copulatory organs
apparently arise to a large extent as ingrowths from the ectoderm, from
which the accessory glands (prostates, shell-glands) are also formed. The
endoderm forms the lining of the main-gut and its branches. The pharynx is
developed as in Fig. 13, which shows that the "mouth" of the young larva
(C) does not correspond exactly with that of the adult (D). The salivary
glands arise from ectoderm cells, which sink deeply into the parenchyma.
The reproductive organs (ovaries and testes) possibly arise by
proliferation from the gut-cells (Lang, v. Graff). The change from the
larva to the adult is gradual, the ciliary band being absorbed and the
creeping mode of life adopted.
TURBELLARIA. II. TRICLADIDA.
The Triclads are most conveniently divided into three groups[39]: (i.)
_Paludicola_, the Planarians of ponds and streams; (ii.) the _Maricola_,
the Triclads of the sea; and (iii.) _Terricola_ or Land Planarians. From
the Polyclads they differ in their mode of occurrence; in the elongated
form of their body and almost constant, mid-ventral position of the mouth;
in possessing a single external genital pore (Monogopora); and in the
production of a few, large, hard-shelled eggs provided with food-yolk.
OCCURRENCE OF THE PALUDICOLA.—The Planarians of our ponds and streams are
the most familiar and accessible Turbellaria. Their elongated, flattened
bodies, and gliding movements, render them conspicuous objects on the under
surface of stones and on the leaves of aquatic plants, where they live
gregariously. The variable _Polycelis nigra_ (Fig. 14, H) is very abundant
in stagnant water and slowly-moving streams, whereas its ally, _P. cornuta_
(Fig. 14, G), distinguished by a pair of tentacles, is more local.
_Planaria_ (_Dendrocoelum_) _lactea_ (A), _P. polychroa_ (I), _P. torva_,
and _P. punctata_ are not infrequently found together, but the last is at
once the largest and rarest.
{31}[Illustration: FIG. 14.—Forms of Triclads, with the distinguishing
specific characters of certain British forms. A, _Planaria lactea_ O. F.
M., × 2; B, _Planaria alpina_ Dana, × 4 (after Kennel); C, _Phagocata
gracilis_ Leidy (after Woodworth), × 6; C', the same with the pharynges
(_ph_) extruded; D, _Gunda ulvae_ Oer., × 4; E, _Planaria gonocephala_ Dug.
(after Schmidt), × 4; F, genitalia of _Gunda ulvae_ (after Wendt); G, head
of _Polycelis cornuta_ Schm.; H, head of _Polycelis nigra_ Ehr.; I, head of
_Planaria polychroa_ Schm. K to N show the distinctive characters of the
genital ducts in K, _Polycelis nigra_; L, _Planaria polychroa_; M,
_Planaria alpina_; N, _Planaria torva_ Schultze (after Iijima and
v. Kennel). _ga_, Genital atrium; _go_, common genital opening; _mgr_,
"musculo-glandular organ"; _mo_, "mouth"; _ovd_, oviduct; _pe_, penis;
_ph_, pharynx; _pyr_, pyriform organs of unknown significance; _sc_,
sucker; _sp_, spermatophore lying in (_ut_) uterus; _vd_, vesicula
seminalis. (All except C and E are found in England.)]
_Planaria alpina_ (Fig. 14, B) is characteristic of cold mountain streams,
but occurs down to {32}sea-level in England, the Isle of Man, and Ireland,
and from its abundance in spring water, probably enjoys a wide distribution
underground. In the Swiss Alps it has been found at altitudes of over 6000
feet, at lower levels in the Rhone, and also in the Lake of Geneva. This
wide distribution may perhaps be accounted for, partly, by its faculty for
asexual reproduction in summer, and also, by the production, later in the
year, of hard-shelled eggs which are laid loosely, not attached to stones
or plants.[40] But we have no really direct evidence of the means of
dispersal of this or of any of the foregoing species, although they all
have a wide distribution in Europe. Of extra-European forms the accounts
that exist are very fragmentary. The only indubitable diagnostic character
of a Triclad is the structure of its genital ducts, and this is accurately
known in only a few cases. Several species such as _Dicotylus pulvinar_
(Fig. 16, B), at present known only from Lake Baikal,[41] and others
(_Planaria mrazekii_, _P. albissima_) from Bohemia,[42] will doubtless be
found elsewhere when they are carefully looked for. _Phagocata gracilis_ is
a remarkable North American form, possessing several pharynges (Fig. 14, C
and C'), recalling the independent movement of the pharyngeal lobes of
_Discocelis lichenoides_ (Fig. 9).[43]
OCCURRENCE OF THE MARICOLA.—Little as we accurately know of the
distribution of the fresh-water Planariae, our knowledge of the occurrence
of the marine forms is still more limited. _Gunda_ (_Procerodes_) _ulvae_
(Fig. 14, D) is the commonest European form, occurring abundantly in the
upper part of the littoral zone, on the shores of the Baltic. _G.
segmentata_ from Messina has been carefully described by Lang,[44] but
these are almost the only species of Maricola which can be accurately
determined. They differ from the Paludicola in the position of the "uterus"
behind the genital pore and in the absence of a "musculo-glandular organ"
(Fig. 14, F). A special interest attaches to the Bdellouridae, a family
containing three species, all parasitic on _Limulus_ from the east coast of
America. These remarkable Triclads usually have a sucker at the hinder end
of the body, by which they attach themselves firmly to the cephalo-thoracic
appendages and to the {33}gill-plates, upon which the eggs may be found in
considerable numbers. One species, _Syncoelidium pellucidum_, possesses a
pair of problematical organs in the hinder part of the body, opening to the
exterior ventro-laterally by a couple of chitinous mouth-pieces, but having
no connexion with the genital ducts.[45]
OCCURRENCE AND DISTRIBUTION OF LAND PLANARIANS.—The terricolous Triclads or
Land Planarians are the most interesting division of the group. Some forms,
such as _Bipalium kewense_, attain large dimensions, being usually 6 to 9
inches in length, and specimens fully extended have measured 18 inches.
Their bodies are frequently banded or striped with brilliant colours.
_Geoplana coerulea_ Mos. has a blue ventral surface and is olive green or
dark Prussian blue above. _G. splendens_ Dendy, is marked dorsally by three
stripes of emerald green alternating with four dark brown longitudinal
bands. The mode of coloration, though somewhat variable, is an important
specific character. Its significance, however, is not clearly understood.
The colours may be a warning signal, as some _Geoplana_ at least are
disagreeable to the taste of man and some birds[46]; but since Land
Planarians are largely nocturnal animals, living by day under logs, banana
leaves, and in other moist and dark situations, this explanation is clearly
insufficient. Two _Geoplana_ have been noticed by Mr. Dendy which seem to
be protectively coloured. _G. triangulata_ var. _australis_ occurs
abundantly in the beech forest in the South Island of New Zealand, and its
brown back and yellow or orange ventral surface match the leaves around its
haunts. _G. gelatinosa_ again looks like a mere slimy patch on the rotten
bark where it is found. In arid districts, during the dry season, Land
Planarians burrow in the soil and form a cyst, in which they lie coiled up,
after the manner of earthworms.[47] The glutinous investment of their
delicate bodies forms a moist medium in which the cilia covering the body
(and especially the ventral surface) may constantly and evenly vibrate, and
by which they adhere firmly to their prey. In some tropical Planarians, in
addition to possessing offensive properties, the mucus is so copious in
amount and hardens with such rapidity, that {34}these Triclads may creep
over bridges of it, and may even be blown from one stem or branch of a
plant to another, hanging at the ends of their threads.[48]
[Illustration: FIG. 15.—Some Land Planarians found in Europe. A, _Bipalium
kewense_ Mos. × ⅓ (after Bergendal); B, _Rhynchodemus terrestris_ O. F. M.,
× 2; C, _Geodesmus bilineatus_ Metsch., × 2½ (after Metschnikoff). _mr_,
Region of mouth; _gp_, region of genital pore.]
In Europe there are only two or three indigenous Land Planarians, of which
_Rhynchodemus terrestris_ O. F. M. (Fig. 15, B) is the most widely
distributed, and has been found in moist situations for the most part
wherever it has been carefully looked for. It measures about ¾ inch in
length, and is dark grey above, whitish below, and bears a pair of eyes
near the anterior extremity (Fig. 15, B). _Bipalium kewense_ (Fig. 15, A),
which has been found in the forests of Upolu, Samoa, by Mr. J. J. Lister,
has been accidentally imported, from the (unknown) districts where it is
indigenous, with plants and soil to various parts of the world—England,
Germany, the Cape, and also to Sydney, where it appears to have established
itself. In these Bipalia living in hothouses, the genitalia never appear to
attain maturity, and apparently multiple fission and subsequent reparation
of the missing parts is the only mode of reproduction. _Geodesmus
bilineatus_ (Fig. 15, C), which has occurred at Giessen, Würzburg, and
Dresden, has, in all probability, been introduced with ferns from the West
or East Indies. _Microplana humicola_, described by Vejdovsky from
dunghills in Bohemia, is doubtfully indigenous.
In marked contrast with the poverty of the temperate zones in Land
Planarians, is the abundance and great variety of this group in Southern
Asia, South America, and especially in Australasia, where the rich Land
Planarian fauna has been carefully investigated by Spencer, Dendy,
Fletcher, and others, in {35}certain parts of Victoria, New South Wales,
and New Zealand.[49] About forty species of Planarians have been discovered
on the Australian continent, thirty-five of which belong to the predominant
genus _Geoplana_, distinguished by the presence of numerous eyes along the
border of the simple anterior extremity. Of the remaining five, four belong
to the genus _Rhynchodemus_, with, lastly, the introduced _Bipalium
kewense_. The distribution of any one species, however, is so limited that
only three forms are common to the two former colonies; and although some
of the twenty known New Zealand Planarians (chiefly species of _Geoplana_),
are identical with Australian species, yet only one, or possibly two,
varieties of these species are Australian also. In addition to their
prevalence in Australasia, the _Geoplanidae_ also occur in South America,
South Africa, Japan, and the East Indies. The _Bipaliidae_ are
characteristic of the Oriental region, being found in China, Borneo,
Bengal, and Ceylon. The _Rhynchodemidae_ are a cosmopolitan family,
occurring in Europe, North and South America, the Cape of Good Hope,
Ceylon, the East Indies, Australia (particularly Lord Howe Island), and
Samoa.[50]
HABITS AND STRUCTURE OF TRICLADS.—The common _Planaria_ (_Dendrocoelum_)
_lactea_, which usually progresses by ciliary action, aided, it is said, by
muscular contractions of the ventral surface, performs, if alarmed, a
series of rapid "looping" movements, by affixing a sucker (Fig. 14, A,
_sc_), placed on the under side of the head, to the substratum, and pulling
the posterior end close to this. The sucker, discovered by Leydig, is even
better developed in _P. punctata_ (Fig. 16, A), _P. mrazekii_, and _P.
cavatica_, and is an efficient adhering-organ which has probably been
developed from a similar but simpler structure found in a considerable
number of both fresh-water and marine Triclads (_P. alpina_, Fig. 16, E).
Probably the sucker of the Land Planarian _Cotyloplana_ (D) is the same
structure, but the two suckers of _Dicotylus_ (B) are at present unique.
_Planaria dioica_, found by Claparède on the coast of Normandy,[51] is
covered with minute adhesive papillae, {36}similar to those of certain
Rhabdocoelida (e.g. _Monotus_, Fig. 19, D), enabling it to cling tightly to
the _Zostera_, and so to resist the loosening action of the waves.
[Illustration: FIG. 16.—Suckers of Triclads. A, _Planaria punctata_ Pall.;
_a_, dorsal surface of head; _b_, ventral surface (freely moving) showing
the sucker; _c_, sucker contracted (after Hallez): B, ventral surface of
head of _Dicotylus pulvinar_ Gr., from Lake Baikal (after Grube): C, dorsal
surface of _Procotylea fluviatilis_ Gir. (after Girard): D, sucker of
_Cotyloplana whiteleggei_ Sp. (after Spencer): E, ventral view of head of
_Planaria alpina_ Dana (preserved specimen); _hg_, adhering groove; _m_,
thickened musculature forming the margin of the sucker; _sc_, sucker; _t_,
tentacles.]
The movements of Land Planarians are somewhat peculiar. The ventral surface
of _Bipalium_ has a median groove, into which the ducts of numerous
mucus-glands open. This is bordered by two ridges clothed with long and
powerful cilia, which perform the chief part in propelling the animal,
aided, according to Lehnert,[52] by muscular waves which pass from the
head, backwards, _i.e._ opposite in direction to those by which a snail
slides along. This observation, however, needs confirmation. The whole body
executes sinuous movements, during which the crescentic head, lifted
slightly above the ground (Fig. 15, A), is constantly altering and
regaining its normal shape, somewhat as a _Planaria lactea_ uses the lobes
of its head. Further examination shows that the margin of the head of
_Bipalium_ is not only provided with eyes, but in addition, with ciliated,
(probably) olfactory pits. Such depressions, innervated directly from the
cerebral ganglia, have been found in sixteen species of _Geoplana_, {37}and
in one or two species of _Rhynchodemus_.[53] Some Land Planarians (a
species of _Rhynchodemus_ from Ceylon, and a _Dolichoplana_ from the
Philippines) wriggle out of a box or the hand with great speed (Moseley).
The skin of Triclads is full of minute rods or rhabdites, which are shot
out in great numbers when the animal is irritated, and doubtless serve an
offensive purpose. The Terricola possess two kinds of these: (1)
needle-like rods; and (2) in _Bipalium kewense_, flagellated structures,
bent into a V-form and with a slender thread attached to one end (Shipley).
In _Geoplana coerulea_ these bent rods furnish the blue colour of the
ventral surface. The rhabdites arise in all Triclads in cells below the
basement-membrane, which they are said to traverse in order to reach the
epidermis, thus differing in origin, and also in structure, from the rods
of Polyclads.
FOOD.—Triclads are largely if not wholly carnivorous animals, feeding upon
Annelids, Crustacea, Insects, Insect-larvae, and Molluscs. The mouth is
usually mid-ventral or behind the middle of the body, but in the anomalous
_Leimacopsis terricola_ Schm. from the Andes[54] and in _Dolichoplana_ it
is near the anterior end. The pharynx (Figs. 17, 18, _ph_) is cylindrical
or bell-shaped, exceedingly dilatable and abundantly supplied with glands
and nervous tissue. It opens into the three main intestinal branches, one
of which runs in the median plane forwards, the others backwards right and
left, enclosing a space in which the genital ducts lie (Figs. 14, A, 17).
The fresh-water Planarians prey upon Oligochaeta, Hydrophilidae (aquatic
beetles), and the commoner pond-snails. _Bipalium kewense_ pursues
earthworms, seizes the upper surface of the anterior end by the glutinous
secretion of its ventral surface, and then proceeds to envelop part or the
whole of the worm within its pharynx, which is stretched as a thin skin
over the body of its struggling prey (Lehnert). The tissues of the latter
pass into the intestine of the Planarian, and distend it greatly. After
such a meal, which lasts from one to five hours, a _Bipalium_ may remain
for three months without seeking food. _Geobia subterranea_, a white
eyeless form from Brazil, pursues earthworms (_Lumbricus corethrurus_) in
their burrows, and has been seen by Fritz Müller sucking the blood out of a
young {38}worm.[55] _Geoplana typhlops_, a Tasmanian species, is also
blind, and pursues worms, as does _G. triangulata_ (Dendy). In Trinidad,
von Kennel[56] observed that land-snails (Subulinae) were the food of
certain Land Planarians, the name of which, however, he does not state. The
pharynx was employed to suck out the soft parts of the snail even from the
upper whorls of the shell.
REPRODUCTION.—In _Planaria lactea_ the numerous testes (Fig. 17, _te_) are
placed both above and below the alimentary canal throughout the greater
part of its course. The membrane of each gonad is continued into a minute
vas efferens, which unites with those of neighbouring testes. Two vasa
deferentia (_v.d_) arise thus on each side, one from the posterior, the
other from the anterior testes of the body, and open into the vesiculae
seminales (_v.s_), which may be seen in the living animal as tortuous
whitish tubes at the sides of the pharynx (Fig. 14, A). These open into the
penis (Figs. 14, A; 17, _pe_), a large pyriform organ, the apex of which,
when retracted, points forwards, projecting into the penial cavity. When
this apical portion is evaginated and turned inside out, it is of
considerable length, and is able to pass into the long slender duct of the
uterus (_ut_) of another individual. The penial sheath (_ps_) is part of
the genital atrium (_gs_), which is developed as a pit from the skin, and
invests the end of the genital ducts, the mouth of the pit forming the
common genital pore (_go_), through which both male and female genital
products are emitted.
There are two ovaries (_ov_) placed far forwards, between the third and
fourth pairs of intestinal coeca. The oviducts (_ovd_) lie just over the
lateral nerves, and have a slightly tortuous course, at each outward bend
receiving the duct (_yo_) of a yolk-gland (_yg_), so that ova and yolk are
already associated when the oviducts open by a short unpaired tube into the
genital atrium. The yolk-glands develop rapidly,[57] and when fully formed
are massive glands occupying the spaces between the intestinal branches and
the testes which are then aborting. The so-called _uterus_ (_ut_),
apparently at first a diverticulum of the genital atrium, expands behind
the pharynx into a receptacle lined by long glandular columnar cells,
which, however, are not all of the same kind. The uterine duct opens into
the atrium just above the aperture of a problematical, eversible,
"musculo-glandular organ" (_mgr_).
{39}[Illustration: FIG. 17.—Diagrammatic view of the structure of
_Planaria_ (_Dendrocoelum_) _lactea_. × 7. The body has been cut across and
a portion removed. In the posterior half the alimentary tract of the left
side is removed and the uterus, penis, and muscular organ sliced open
horizontally. The nervous system is represented by black, and the
yolk-glands by dotted lines. _br_, Brain; _ey_, eye with lens and optic
nerve; _go_, external genital aperture for both male and female products;
_gs_, genital atrium; _lg_, paired lateral intestinal branch; _ln_,
longitudinal nerve; _mg_, unpaired anterior intestine, the branches of
which are cut off close to the main stem; _mgr_, eversible
"musculo-glandular organ"; _nc_, nerve-cells in the pharynx; _nn_, lateral
nerve-twigs; _ns_, nerve-sheath; _ov_, ovary; _ovd_, oviduct; _pe_, the
eversible penis, the corrugated inner white portion of which is the apex;
_ph_, pharynx; _phs_, pharyngeal sheath; _pr_, "prostate" or granule-gland
(represented by dotted lines opening into the penis); _ps_, penial sheath;
_te_, testes; _to_, tactile lobe of the head; _ut_, "uterus" opening into
the genital atrium just above _mgr_; _vd_, vasa deferentia; _vs_, vesicula
seminalis; _yg_, yolk-glands; _yo_, openings of the yolk-ducts into the
oviducts.]
{40}Fertilisation appears to occur in the uterus, where ova, yolk, and
spermatozoa, or (in _P. torva_) spermatophores (Fig. 14, N, _sp_), are
found. The formation of the cocoon in _Planaria lactea_ is probably begun
in the "uterus," but is undoubtedly completed in the genital atrium. In _P.
polychroa_, however, the stalked cocoon is formed wholly in the "uterus."
Thus we find two types of cocoons in different species of the genus
_Planaria_ associated with two types of reproductive organs (Hallez):—
I. Planariae in which the two oviducts open separately into the posterior
part of the duct of the uterus. A musculo-glandular organ is absent. The
cocoons are spherical and stalked. Examples—_Planaria polychroa_ (Fig. 14,
L), _P. albissima_, _P. gonocephala_.
II. Planariae in which the two oviducts open by means of an unpaired duct
into the genital atrium. A musculo-glandular organ present (_Planaria
torva_ (Fig. 14, N), _P. mrazekii_, _P. lactea_, _P. cavatica_), or absent
(_P. alpina_, Fig. 14, M). The cocoons are sessile.
The genitalia of the Maricola (Fig. 14, F) and Terricola do not differ very
much from those of _Planaria_. The uterus (greatly reduced in the Land
Planarians) lies behind the genital pore, and several ova, together with
much milky yolk, are enclosed in a capsule which is formed in the genital
atrium.
ASEXUAL REPRODUCTION.[58]—It has long been known that fresh-water
Planarians have not only great powers of repairing injuries, but that they
use this faculty in order to multiply by transverse fission. _Planaria
alpina_ and _Polycelis cornuta_, in summer, separate off the posterior part
of the body, and this ultimately becomes an entire individual. _P.
albissima_, and especially _P. subtentaculata_, anticipate matters so far,
that before fission is complete, the new individual has a head nearly fully
formed. The new organs are largely regenerated in both parent and young,
{41}apparently by the division and specialisation of scattered embryonic
cells in the parenchyma. The asexual reproduction of Land Planarians is not
fully proved, though it is known that they repair injuries to the body
completely, and that _Bipalium kewense_ is often found in hothouses,
divided into fragments which regenerate all the organs of the parent, but
like the latter, do not mature their sexual organs.
[Illustration: FIG. 18.—Semi-diagrammatic view of the excretory system of
_Planaria lactea_. (Partly after Chickoff.) _can_, Capillary network on
both dorsal and ventral surfaces; _g.br_, branches of the intestine; _lg_,
lateral branches of the digestive system; _ln_, longitudinal nerve; _ph_,
pharynx, with intermuscular capillary excretory network arising from the
point marked _pht_; _tp_, principal vessels of the excretory system, the
external opening of which is not certainly known; _vs_, vesicula
seminalis.]
EXCRETION.—The excretory organs of Triclads consist of flame-cells,
canaliculi, and a pair of longitudinal canals, the external openings of
which, have not been satisfactorily ascertained. The flame-cells are
difficult to detect in _Planaria lactea_, and the latest observer,
Chickoff,[59] was unable to see them, although to him we are indebted for
figures of this system in _P. lactea_ (Fig. 18) and _P. alpina_ (_P.
montana_). In the latter, the flame-cells are distinct, and may open
directly into the two main canals or indirectly through unbranched
canaliculi. The pharynx possesses a special supply of excretory tubules
communicating with the main canals. A similar system has been described and
figured in _Gunda segmentata_ by Lang.[60]
{42}CLASSIFICATION OF TRICLADIDA.
PALUDICOLA.
Family. Genus and British Species.
PLANARIIDAE _Planaria lactea_ O. F. M., _P. punctata_ Pall.,
_P. polychroa_ Schm., _P. torva_ M. Sch.,
_P. alpina_ Dana.
_Polycelis nigra_ Ehr., _P. cornuta_ Schm.
_Anocelis._
_Oligocelis._ } Doubtful
_Procotyla._ } genera.
_Sorocelis._
_Dicotylus._
MARICOLA.
PROCERODIDAE _Procerodes_ (= _Gunda_) _ulvae_ Oersted,
_P. littoralis_ van Beneden.
(= Gundidae). _Cercyra._
_Uteriporus._
BDELLOURIDAE _Bdelloura._
_Syncoelidium._
TERRICOLA.
BIPALIIDAE _Bipalium kewense_ Moseley (introduced).
GEOPLANIDAE _Geoplana._
_Geodesmus._
RHYNCHODEMIDAE _Rhynchodemus terrestris_ O. F. M.
Belonging to { _Dolichoplana._
undetermined { _Polycladus._
Families { _Microplana._
{ _Leimacopsis._
TURBELLARIA. III. RHABDOCOELIDA.
The Rhabdocoelida include a very heterogeneous assemblage of usually minute
Turbellaria, distinguished collectively from the Polyclads and Triclads by
the form of the digestive tract. This is a simple or slightly lobed sac,
except in the Bothrioplanidae, which in this and many other points closely
resemble the Triclads. It is to the straight, rod-like nature of the
alimentary canal that the name of the group refers. The size and form of
the body, and the structure of the pharynx and genitalia, vary within wide
limits.
The Rhabdocoelida are subdivided into three tribes:—
(1) _Acoela_, in which a sub-central mouth and pharynx are present, but
lead into the parenchyma of the body, not into an intestine with proper
walls. An excretory system has not hitherto been seen. Yolk-glands are
absent. An otolith underlies the brain. The _Acoela_ are marine.
{43}(2) _Rhabdocoela_, which possess a complete alimentary tract separated
from the body-wall (except for a few suspensory strands) by a space or
body-cavity, filled with fluid. This space is sometimes (_Vortex viridis_)
lined by an endothelium of flattened parenchymatous cells. There are two
compact testes, which are enclosed (as are the ovaries and yolk-glands) in
a distinct membrane. An otolith is present in some genera and species.
Terrestrial, fresh-water, marine.
(3) _Alloeocoela_, in which the body-cavity is greatly reduced. Except in
the Bothrioplanidae, the gonads have no distinct membrane. Testes numerous;
yolk-glands present. Marine with a few exceptions.
OCCURRENCE AND HABITS OF THE RHABDOCOELIDA.—The _Acoela_ are usually
minute, active Turbellaria abounding amongst weeds throughout the lower
half of the Littoral, and the whole of the Laminarian zone, but are most
plentiful in the pools exposed during spring-tides on our coasts,
especially on the shores of Devonshire. The species of _Haplodiscus_,
however, and _Convoluta henseni_ are modified pelagic forms found in the
Atlantic Ocean.[61] _Convoluta paradoxa_ (Fig. 19, B) is the commonest
British species. It is from 1 to 9 mm. in length, and of a brown colour,
marked above by one or more transverse white bars. The brown colour is due
to a symbiotic alga, the nature of which has not been thoroughly
investigated. In an allied species, however (_C. roscoffensis_), from the
coast of Brittany, the alga, which is here green, has been carefully
examined by Professor Haberlandt,[62] and it appears from his researches
that the algae form a special assimilating tissue, enabling the _Convoluta_
to live after the fashion of a green plant. At Roscoff, these elongated
green _Convoluta_ live gregariously in the sandy tide-pools, fully exposed
to the sun's rays, and have the appearance of a mass of weed floating at
the surface of the water. Access to the atmosphere and to sunlight are
necessary in order to enable the assimilating tissue to form the
carbohydrates, upon which this form lives exclusively. Not only has the
alga itself undergone such profound changes (loss of membrane, inability to
live independently after the death of the host) as to disguise its true
nature (a tissue-cell derived from algal ancestors), but the _Convoluta_
has also undergone {44}concomitant changes, in form, in the loss of a
carnivorous habit, and in the development of marked heliotropic movements,
thus adapting itself to an holophytic or plant-like mode of nutrition.
Nevertheless the Acoela, as a group, are carnivorous, feeding upon Diatoms,
Copepoda, and small Rhabdocoela, the absence of a digestive tract indeed
being probably more apparent than real.[63]
The _Rhabdocoela_ live under varied conditions. One form, _Prorhynchus
sphyrocephalus_, has been found among plants far from water in the
neighbourhood of Leyden, by De Man.[64] With this exception the group is
purely aquatic, and though a few genera and even individuals of the same
species occur both in salt and fresh water, whole sub-families and genera
are either marine or paludicolous. Among the latter, _Mesostoma_,
_Castrada_, _Vortex_, and _Derostoma_ are common in brooks and ponds,
especially at certain times, often only for one month (May or June) in the
year. Species of _Macrostoma_, _Stenostoma_, and _Microstoma_ are also
abundant in similar places. The two latter occur in chains formed by
fission; but the sexual individuals (which are of distinct sexes, contrary
to the usual hermaphrodite condition of Flat Worms) only appear at stated
times and are not well known. A large number of genera are purely marine,
and one family, the Proboscidae (distinguished by having the anterior end
invaginated by special muscles and converted into a sensory organ), is
entirely so. The most cursory examination of littoral weeds reveals species
of _Macrorhynchus_, _Acrorhynchus_, _Promesostoma_, _Byrsophlebs_, and
_Proxenetes_, the character of which may be gathered from von Graffs great
monograph, or from Gamble's paper on the "British Marine Turbellaria."[65]
Much, however, still remains to be done before we possess an adequate idea
of the occurrence of this group on our coasts.
{45}[Illustration: FIG. 19.—Forms of Rhabdocoelida. A, _Mesostoma
tetragonum_ O. F. M. (Rhabdocoela), × 10; B, _Convoluta paradoxa_ Oe.
(Acoela), × 10; C, _Vorticeros auriculatum_ O. F. M., × 6; D, _Monotus
fuscus_ Oe. (Alloeocoela), × 4. _ap_, Adhesive papillae; _d_, intestine;
_m_, pharynx; _ot_, otolith; _rh_, rhabdites; _te_, testes; _ut_, uterus
with eggs; _yg_, yolk-glands; ♂, male, ♀, female genital pores. (A after
Braun.)]
Some Rhabdocoels are parasitic. _Fecampia erythrocephala_, which occurs in
the lacunar spaces and alimentary canal of young shore crabs (_Carcinus
maenas_), is a white cylindrical animal ¼ inch long, with a red snout.
After attaining maturity it works its way out of the crab and encysts under
stones, forming a pyriform mass in shape like a "Prince Rupert's drop."
Within this case the eggs develop, and the young probably emerge through
the open narrow end of the hard white tube, but how they reach the crab is
not known. _Graffilla muricicola_ is found in the kidney of _Murex
brandaris_ and _M. trunculus_, at Naples and Trieste; _G. tethydicola_ in
the foot of _Tethys_. _Anoplodium parasiticum_ occurs among the muscles
which attach the cloaca of _Holothuria tubulosa_ to the body-wall; and _A.
schneideri_ occurs in the sea-cucumber, _Stichopus variegatus_. These are
truly parasitic forms, constituting a special sub-family. They have no
rhabdites in the skin; the nervous system and sense-organs are only
slightly developed; and the pharynx has undergone a notable reduction in
relation to the simpler mode of obtaining nourishment. Other cases of
association between certain Rhabdocoels (closely allied to, if not
identical with, certain free-living species) and Lamellibranchs or
Sea-urchins, are, however, of another kind. Thus on the gills or in the
mantle cavity of species of _Mytilus_, _Cyprina_, _Tellina_, and upon the
test of _Clypeaster_, such forms as _Enterostoma mytili_, _Acmostoma
cyprinae_, and _Provortex tellinae_ have been found. But it is probable
that these Turbellaria here obtain merely a temporary {46}shelter and
possibly a supply of the food of the mussel or sea-urchin.
The _Alloeocoela_ afford a well-established case of association. _Monotus
fuscus_ (Fig. 19, D), an abundant, active, elongated animal, lives on our
coasts in the upper part of the littoral zone among _Patella_, _Balanus_,
and sometimes _Chiton_. When the tide is low, the _Monotus_, to obtain
moisture and darkness, creeps between the mantle-folds of these animals,
where it may readily be found. Upon the return of the tide it leaves its
retreat and creeps or swims about freely. Other Alloeocoela collect in
great numbers in tufts of red-seaweeds (Florideae). By placing such tufts
in vessels, the sea-water, especially as darkness sets in, begins to swarm
with _Cylindrostoma 4-oculatum_, species of _Enterostoma_ and
_Plagiostoma_; _P. vittatum_, with three violet bands across the white
body, being a particularly obvious form. _Vorticeros auriculatum_ (Fig. 19,
C), another abundant species, is remarkable for the long tentacles which
can be completely withdrawn, and in this condition it completely resembles
a _Plagiostoma_.
The presence of a species (_P. lemani_) of the characteristically marine
genus _Plagiostoma_, in the Lake of Geneva, and in one or two other Swiss
lakes, at depths varying from 1 to 150 fathoms, is very interesting, and is
perhaps the only well-established case of the survival of a once marine
Rhabdocoelid under changed conditions. _Plagiostoma lemani_ is by far the
biggest of the group to which it belongs, being over half an inch in
length. It is usually found in fine mud, sometimes among _Chara hispida_,
and has the general appearance of an inactive white slug. We are indebted
to Forel and Duplessis for the discovery of this species, and also of
_Otomesostoma morgiense_, a _Mesostoma_ with an otolith, dredged in 10 to
50 fathoms in the Lake of Geneva, the Lake of Zürich, and found recently
also by Zacharias in the Riesengebirge. The genus _Bothrioplana_, first
found by Braun in the water-pipes of Dorpat, has been carefully
investigated by Vejdovsky,[66] who places it in a special family,
Bothrioplanidae, among the Alloeocoela. One species has recently been found
near Manchester.
A comprehensive survey of the Rhabdocoelida shows that, with the chief
exception of the Proboscidae, the more lowly organised forms, the Acoela
and Alloeocoela, are marine, whereas the fresh-water forms are in most
cases the most highly organised {47}genera (_Mesostoma_, _Vortex_). But
_Macrorhynchus helgolandicus_, though minute (1.5-2 mm. long), has a more
complex structure[67] than any other species of the specialised marine
genus to which it belongs, and is a remarkable instance of great complexity
being associated with small size.
REPRODUCTION.—The Rhabdocoelida present the greatest diversity in the
development of the reproductive system. The Acoela and Alloeocoela have the
simplest arrangement. Scattered testes, often without a distinct membrane,
form the spermatozoa, which in most cases wander into parenchymatous
spaces, but in _Monoporus rubropunctatus_ and _Bothrioplana_, into distinct
vasa deferentia. In both groups a protrusible penis opens independently to
the exterior, and may be simply muscular or provided with a chitinous
armature. Two ovaries are present, and the oviducts, if distinct, are
continuations of the ovarian membrane. In most forms a "bursa seminalis,"
which receives the spermatozoa of another individual, is appended to the
female genital canal. In many of the Alloeocoela, however, a portion of the
ovary is sterile, and its cells, forming a yolk-gland, feed the fertile
portion, the whole structure being then spoken of as a germ-yolk-gland. In
many others (Monotidae) this sterile part has become an independent
yolk-gland, which communicates by yolk-ducts with the oviducts. The Acoela
form no egg-case, the body of the parent becoming a bag for the ova, which
elaborate their own food-yolk. The Alloeocoela lay hard-shelled eggs, which
are produced in _Bothrioplana_ and _Automolos_ by the activity and
interaction of reproductive organs, resembling closely those of certain
Triclads.[68]
The Rhabdocoela exhibit every stage in the development of a complex
reproductive system, from the simple ovaries and testes of a _Microstoma_
or _Macrostoma_, to the intricate system of ducts and glands of a
_Macrorhynchus_ (Proboscidae), in which there is still much to be made out.
The complications of the copulatory organs chiefly arise from the way in
which the spermatozoa are brought into contact with a nutritive prostatic
fluid, or are formed into spermatophores; and also from the penial
armature, {48}which is often very complex, and may consist of a curved
chitinoid hook or a coiled loop (_Promesostoma_), of hooks (Proboscidae),
or of an intricate arrangement of plates (_Proxenetes_); or the penis may
take on a complex corkscrew-like form (_Pseudorhynchus_). The (frequently
armed) female genital canal usually possesses a bursa seminalis for the
fertilisation of the eggs, but a receptaculum seminis or spermatheca may
serve for the reception, the bursa, for the lodgment of the spermatozoa of
another individual. The fertilised ovum is provided with a supply of
food-yolk and with a shell, which may be formed in a special diverticulum,
the "uterus." The development of these organs strains the resources of the
animal to the utmost, and in some Proboscidae the alimentary canal is
squeezed out and disintegrates, in order to make room for them.
A few _Mesostoma_ (_M. ehrenbergii_, _M. productum_, _M. lingua_) produce
two kinds of eggs—thin- and thick-shelled. The latter are laid throughout
the summer, and lie dormant through winter. The young which hatch in spring
out of these "winter" eggs develop rapidly, and when only 7 to 8 mm. long
(_i.e._ one-third the size of the parent) already possess functional
genital organs; the penis, however, is rudimentary, and incapable of being
used for copulation. Hence it is probable that this stunted progeny
self-fertilise their thin-shelled or "summer" eggs. After the formation of
these eggs the same parent is said (Schneider[69]) to produce thick-shelled
or winter eggs, but however that may be, the first young which hatch from
the thin-shelled ova are produced in great numbers at a time (April to May)
when food is abundant. These grow rapidly to the full size, and then having
attained maturity, cross-fertilise one another's ova, which become encased
in a thick brown shell; and it is these numerous "winter" eggs that lie
dormant throughout the autumn and winter. Many _Mesostoma_, and practically
all other Rhabdocoela, however, produce only thick-shelled eggs, and in all
cases it is probable that to these many species owe their wide
distribution, the exact range of which is, however, unknown, as is also the
means of dispersal.
{49}CLASSIFICATION OF RHABDOCOELIDA.
ACOELA.
Family. Genus and British species.
PROPORIDAE _Proporus venenosus_ O. Sch. Plymouth.
_Monoporus rubropunctatus_ O. Sch. Plymouth.
_Haplodiscus._
APHANOSTOMATIDAE _Aphanostoma diversicolor_ Oe. Common.
_A. elegans_ Jen. Plymouth.
_Convoluta saliens_ Grff. Plymouth, Millport.
_C. paradoxa_ Oe. (Fig. 19, B). Common.
_C. flavibacillum_ Jen. Plymouth, Port Erin,
Millport.
_Amphicoerus._
_Polychoerus._
RHABDOCOELA.
MACROSTOMATIDAE _Mecynostoma._
_Macrostoma hystrix_ Oe. Stagnant water.
_Omalostoma._
MICROSTOMATIDAE _Microstoma lineare_ Oe. Fresh water.
_M. groelandicum_ Lev. Plymouth, among _Ulva_.
_Stenostoma_ (_Catenula_) _lemnae_ Dug. Near Cork.
_S. leucops_ O. Sch. Common in fresh water.
_Alaurina claparedii_ Grff. Skye.
PRORHYNCHIDAE _Prorhynchus stagnalis_ M. Sch. In Devonshire rivers.
_Promesostoma marmoratum_ M. Sch. Common.
_P. ovoideum_ O. Sch., _P. agile_ Lev. Plymouth.
_P. solea_ O. Sch. Plymouth, Port Erin.
_P. lenticulatum_ O. Sch. Port Erin.
MESOSTOMATIDAE _Byrsophlebs graffii_ Jen. Plymouth, Millport.
_B. intermedia_ Grff. Millport, Port Erin.
_Proxenetes flabellifer_ Jen. Millport, Plymouth,
Port Erin.
_P. cochlear_ Grff. Millport.
_Otomesostoma._
_Mesostoma productum_ Leuck., _M. lingua_ O. Sch.,
_M. ehrenbergii_ O. Sch., _M. tetragonum_ O. F. M.
(Fig. 19, A). All at Cambridge.
_M. rostratum_ Ehr. Widely distributed.
_M. viridatum_ M. Sch. Manchester.
_M. robertsonii_ Grff., _M. flavidum_ Grff. Both
at Millport.
_Bothromesostoma personatum_ O. Sch. Preston.
_Castrada._
PROBOSCIDAE _Pseudorhynchus bifidus_ M‘Int. Millport, St. Andrews,
Port Erin.
_Acrorhynchus caledonicus_ Clap. Generally distributed.
_Macrorhynchus naegelii_ Köll., _M. croceus_ Fabr.
Plymouth, Millport.
_M. helgolandicus_ Metsch. West coast.
_Gyrator hermaphroditus_ Ehrbg. St. Andrews. Also
common in fresh water.
_Hyporhynchus armatus_ Jen. Plymouth, Port Erin.
_H. penicillatus_ O. Sch. Plymouth.
VORTICIDAE _Schultzia._ {50}
_Provortex balticus_ M. Sch. Generally distributed.
_P. affinis_ Jen., _P. rubrobacillus_ Gamb. Plymouth.
_Vortex truncatus_ Ehrbg. Abundant in fresh water.
_V. armiger_ O. Sch. Millport (fresh water). _V.
schmidtii_ Grff., _V. millportianus_ Grff.
Millport. _V. viridis_ M. Sch. Generally
distributed.
_Jensenia._
_Opistoma._
_Derostoma unipunctatum_ Oe. Edinburgh.
_Graffilla._
_Anoplodium._
_Fecampia erythrocephala_ Giard. Plymouth, Port Erin.
SOLENOPHARYNGIDAE _Solenopharynx._
ALLOEOCOELA.
PLAGIOSTOMATIDAE _Acmostoma._
_Plagiostoma dioicum_ Metsch., _P. elongatum_ Gamb.,
_P. pseudomaculatum_ Gamb., _P. sagitta_ Ulj.,
_P. caudatum_ Lev., _P. siphonophorum_ O. Sch.,
_P. ochroleucum_ Grff. All at Plymouth.
_P. sulphureum_ Grff. Port Erin. _P. vittatum_ F. and
Leuck. Millport, Plymouth, Port Erin. _P. koreni_
Jen. Plymouth, Millport. _P. girardi_ O. Sch.
Plymouth, Port Erin, Valencia.
_Vorticeros auriculatum_ O. F. M. (Fig. 19, C).
Port Erin, Plymouth. _V. luteum_ Grff. Plymouth.
_Enterostoma austriacum_ Grff. Plymouth, Port Erin.
_E. fingalianum_ Clap. Skye, Plymouth. _E. coecum_
Grff. Millport.
_Allostoma pallidum_ van Ben. Millport.
_Cylindrostoma 4-oculatum_ Leuck. Skye, Millport,
Plymouth.
_C. inerme_ Hall, _C. elongatum_ Lev. Plymouth.
_Monoophorum striatum_ Grff. Plymouth.
BOTHRIOPLANIDAE _Bothrioplana._
_Bothrioplana_ sp.? Manchester.
_Otoplana._
MONOTIDAE _Monotus lineatus_ O. F. M., _M. fuscus_ Oe.
(Fig. 19, D). Both common littoral forms.
_M. albus_ Lev. Plymouth.
_Automolos unipunctatus_ Oe. Skye, St. Andrews,
Plymouth.
_A. horridus_ Gamb., _A. ophiocephalus_ O. Sch.
Plymouth.
{51}CHAPTER II
TREMATODA
CHARACTERS OF TREMATODES—HABITS AND STRUCTURE OF TREMATODA ECTOPARASITICA
(MONOGENEA)—LIFE-HISTORIES OF _POLYSTOMUM INTEGERRIMUM_, _DIPLOZOON
PARADOXUM_, AND _GYRODACTYLUS ELEGANS_—TREMATODA ENDOPARASITICA
(DIGENEA)—OCCURRENCE AND HABITS OF DIGENEA—LIFE-HISTORY OF _DISTOMUM
MACROSTOMUM_—_DISTOMUM HEPATICUM_ AND ITS EFFECTS—_BILHARZIA
HAEMATOBIA_—BISEXUAL TREMATODES—TABLE OF HOSTS—CLASSIFICATION.
From the Turbellaria we now pass on to a consideration of the second great
subdivision of the Platyhelminthes, the Trematodes or "flukes," of which
the "liver-fluke" is the best known, since it is one of the most dangerous
parasites that infest domestic animals.
It has been pointed out that the Polyclads, Triclads, and Rhabdocoels are
carnivorous, and that in each of these groups sporadic cases of parasitism
occur. In other words, when the prey is much larger than the Turbellarian,
the latter tends to become a parasite, and we can trace the development of
the parasitic habit from the gradual association of Turbellaria with
Ascidians, Crustacea, Molluscs, and Polyzoa merely for protective purposes,
through the adoption, not only of the body of the host for shelter, but of
its flesh for food; though it is only in some Rhabdocoels (_Graffilla_,
etc.) that there exists a degeneration corresponding to the easier mode of
nutrition and simpler life. The Trematodes,[70] however, are wholly
parasitic, either on the outer surface, the gills, or internal organs of
their host, which is almost always a {52}Vertebrate. Some Trematodes lodge
in the mouth; others wander down the oesophagus into the stomach or
intestine, where they fix themselves to the mucous membrane. Again, others
work their way into the digestive glands by the ducts, and thus become
further and further removed from the external world, and more adapted to
live in the particular organs of that host in which they best flourish. The
most important result of the adoption of this internal habitat by
endoparasitic Trematodes is, however, seen in their life-history. If a
liver-fluke were to deposit its million or so of eggs in the bile-ducts of
the sheep, and these were to develop _in situ_, the host could not
withstand the increased drain upon its vital resources, and host and
parasites would perish together. Hence it is clear that the infection of a
second host by Trematodes is highly necessary, whether they be
_ectoparasitic_, in which case the infection is easily effected, when two
hosts are in contact, by the adult worms, as well as when they are apart,
by free-swimming larvae. In _endoparasitic_ Trematodes it is brought about
by the migration of the young to the outer world, their entrance into a,
usually, Invertebrate host and their asexual multiplication within it, and
the capture and deglutition of this "intermediate host" by the final
Vertebrate one. Within the latter the immature parasites find out the organ
in which their parents flourished, and here they too grow and attain
maturity. The chances of any one egg of an endoparasitic Trematode
producing eventually an adult are, therefore, far less favourable than in
the case of an ectoparasitic form. In other words, while the former must
lay a great number of small eggs, the latter need only deposit a
(comparatively) few large ones, and this fact has a corresponding influence
on the structure of the genitalia in the two cases. The Digenea, which
employ two hosts in a lifetime, have accordingly a different generative
mechanism from that of the Monogenea. The great need of the latter is a
powerful apparatus for adhering to the surface of the body of its host;
while the adaptations which the endoparasite requires are, in addition, (1)
protection against the solvent action of the glands of its host, (2) the
power of firm adhesion to a smooth internal surface, and (3) the ability
not only to produce a large quantity of spermatozoa and ova, but in the
absence of a fellow-parasite, to fertilise its own ova; and we find these
conditions abundantly satisfied.
{53}TREMATODA MONOGENEA (_ectoparasitica_).
There are four subdivisions of the Monogenea:—
I. _Temnocephalidae_, with four to twelve tentacles, and one sucker
posteriorly (Fig. 20).
II. _Tristomatidae_, with two lateral, anteriorly-placed suckers. Oral
suckers are absent, a large posterior sucker is constant, and is often
armed with hooks (Fig. 22, C).
III. _Polystomatidae_, with, usually, two oral suckers and a
posteriorly-placed adhesive disc armed with suckers and hooks (Figs. 23 and
24).
IV. _Gyrodactylidae_ (Fig. 29).
HABITS AND STRUCTURE OF ECTOPARASITIC TREMATODES.
I. _Temnocephalidae._—These interesting forms, of which a good account has
lately been written by Haswell,[71] occur on the surface (rarely in the
branchial chamber) of fresh-water crayfish and crabs in Australasia, the
Malay Archipelago, Madagascar, and Chili. Others have been found on the
carapace of a fresh-water tortoise, and in the branchial chamber of the
mollusc _Ampullaria_ from Brazil. Wood-Mason discovered others, again, in
bottles containing spirit-specimens of Indian fish. _Temnocephala_ is
rarely more than a quarter of an inch long, and looks like a minute
Cephalopod or a broad flattened _Hydra_. By the ventral sucker each species
adheres to its own particular host, the tentacles being used as an anterior
sucker for "looping" movements. The food, consisting of Entomostraca,
Rotifera, and Diatoms, is first swallowed whole by the large pharynx (Fig.
20, _ph_), which can be protruded through the ventrally-placed mouth, and
is then received into a simple lobed intestine (_d_). The skin, especially
on the surface of the tentacles, is provided here and there with patches of
cilia borne by the cellular epidermis,—the only undoubted case of external
cilia occurring in an adult Trematode. Minute rhabdites formed in special
gland-cells, occur plentifully on the tentacles, and are another distinctly
Turbellarian feature. The excretory system is peculiar (Fig. 21). Fine
ducts proceed from the various organs of the body, and open to the exterior
by means of a pair of contractile sacs {54}placed on the dorsal surface.
Each sac is a single cell, and within it not one merely, but several
"flames," or bunches of rhythmically contractile cilia, are present. These
are placed on the course of excessively fine canals, which perforate the
protoplasm of this cell. The terminal branches of the excretory canals end
in branched cells, apparently devoid of "flames."
[Illustration: FIG. 20.—_Temnocephala novae-zealandiae_ Has. × 10. Ventral
view to show the digestive and reproductive systems. (After Haswell.)
FIG. 21.—The same from the dorsal surface, to show the excretory system
(double line), and the nervous system (black and shaded). (After Haswell.)
_d_, Intestine; _dln_, dorso-lateral nerve; _dn_, dorsal nerve; _ex.o_,
excretory aperture on dorsal surface; _ex.s_, terminal excretory sac; _m_,
mouth; _ov_, ovary; _ovd_, oviduct; _ph_, pharynx; _rh_, rhabdites; _rh.c_,
cells in which the rhabdites are formed; _rv_, yolk receptacle; _sc_,
sucker; _sh_, shell-gland; _te_, testes; _ut_, uterus; _vg_, vagina; _vn_,
ventral nerve; _vs_, vesicula seminalis; _yd_, yolk-duct; _yg_, yolk-gland.
♀, ♂, common genital pore.]
The reproductive system is very similar to that of certain Rhabdocoels. An
armed penis and the female genital duct open into a genital atrium, and
this by a single aperture (♀, ♂, Fig. 20) to the exterior. The fertilised
ovum and yolk are enclosed in a stalked shell formed in the uterus.
The interest and importance of the Temnocephalidae lies in the fact that
they are almost as much Turbellaria as Trematodes. {55}In habits, in the
character of the skin, the muscular, digestive, and reproductive systems,
they find their nearest allies in Rhabdocoels (Vorticidae). But in the
excretory and nervous systems, the latter composed of two dorsal, two
lateral, and two ventral trunks all connected together (Fig. 21), they are
Tristomid Trematodes. Thus they may fitly connect an account of the two
great groups.
[Illustration: FIG. 22.—A, _Nematobothrium filarina_ van Bened. Nat. size.
Two individuals (_a_ and _b_) are found together, encysted on the branchial
chamber of the Tunny. B, _Udonella caligorum_ Johns. A Tristomid, several
of which are attached to the ovary of a Copepod (_Caligus_), itself a
parasite on the gills of the Hake. × 8. C, _Epibdella hippoglossi_ O. F. M.
A Tristomid found on the body of the Halibut. Nat. size. _m_, Mouth; _ms_,
lateral suckers; _ov_, ovary; _ps_, posterior sucker; _te_, testes. (All
after P. J. van Beneden.)]
II. _Tristomatidae and_ III. _Polystomatidae_.[72]—The members of these
families are found on the body, or attached to the gills, of fresh-water
and marine fishes. The edible and inedible fish of our coasts have each
their particular ectoparasitic Trematodes; while the Minnows, Sticklebacks,
and Miller's Thumbs of streams and ponds are attacked by _Diplozoon_,
_Gyrodactylus_, and other forms. The aquatic Amphibia also harbour a
number. _Polystomum integerrimum_ is common in the bladder of Frogs, where
it leads a practically aquatic life. Other species of _Polystomum_ inhabit
the buccal and nasal cavities of certain Chelonia, but naturally no
terrestrial Vertebrates are infested externally by these {56}Trematodes.
The blood and epithelia of the host are sucked, and to this end the pharynx
has frequently a chitinous armature to aid in the abrasion or inflammation
of the tissues upon which the parasite feeds. In the case of a Sturgeon
attacked by _Nitzschia elongata_, a Tristomid, the mouth of the host
appeared to be highly inflamed by these attacks (v. Baer).
[Illustration: FIG. 23.—_Octobothrium merlangi_ Kuhn, from the gills of the
whiting, × 8. _int_, Intestine; _ms_, mouth; _sc_ suckers with chitinoid
armature; _yk_ yolk-glands. (After v. Nordmann.)]
The suckers, in the two families under consideration, vary in number and
complexity. There is always a powerful apparatus at the hinder end of the
body securing the Trematode firmly to the slimy body or gills of its host,
and, usually in the Polystomatidae, a pair of suckers at the sides of the
mouth accessory to the pumping action of the pharynx. In _Axine_, and to a
less extent in _Octobothrium_ (Fig. 23), the suckers are strengthened by a
complex hingework of chitinoid bars or hooks, which serve as insertions for
the muscles of the suckers, and thus increase their efficiency.
The mouth is invariably present just beneath the anterior end of the body.
It leads into a muscular, pumping pharynx (Fig. 24, _ph_), and this into a
bifurcated intestine which ends blindly. The two openings of the excretory
system lie on the dorsal surface (as in _Temnocephala_), and the excretory
canals branch through the substance of the body, ending usually in
"flame-cells." The nervous system is highly developed, and resembles that
of _Temnocephala_ (Fig. 21) in detail. Upon the brain one or even two pairs
of eye-spots are present in the larvae, and may persist throughout life.
Tactile setae occur in _Sphyranura_, a parasite of the North American
Amphibian _Necturus_, but a cellular epidermis is apparently rendered
impossible, perhaps from the nature of {57}the mucus in which the body is
bathed, or to the attempts of the host to free itself from these parasites;
and hence an investing membrane is present, which morphologically is either
a modified epithelium, or a cuticle formed by the glandular secretion of
the parenchyma.
[Illustration: FIG. 24.—_Polystomum integerrimum_ Fröh., from the bladder
of the Frog, and seen from the ventral surface. The alimentary canal is
black, the white dots upon it being the yolk-glands, _dvi_, Ductus
vitello-intestinalis (probably homologous with the Laurer's canal or
"vagina" of Digenea); _eh_, hooks of sucking disc; _int_, intestine; _m_,
mouth; _ov_, ovary; _pe_, penis; _ph_, pharynx; _sc_, suckers with an
embryonic hook persisting in each; _te_, testes; _ut_, uterus with eggs;
_vag_, left vagina; _vd_, vas deferens; _yd_, yolk-duct; _yg_, yolk-glands;
♂ ♀, common genital aperture. (Modified from Zeller.) × 8.]