Chapter VII: Introduction: Description of the Polyclad Leptoplana Tremellaris (2)

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}

{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.

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}

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."

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.

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).

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.

The reproductive organs of the Polystomatidae may be understood from Figs. 24, 27, and 28. At the point of union of the oviduct (Fig. 28, _ovd_), the vitelline ducts (_yd_), and the commencement of the uterus (_ut_), a slender duct is given off which opens into the intestine, and is known as the "vitello-intestinal canal" (Fig. 24, _dvi_; Fig. 28, _gic_). This duct has apparently the same relations as the "canal of Laurer" of Digenea,[73] except only that the latter opens to the exterior directly. In connexion with this vitello-intestinal canal a "vagina" is present, which in _Polystomum_ and most Monogenea is paired (Fig. 24, _vag_), in _Diplozoon_ and in one {58}or two other forms, however, unpaired. The vagina receives the penis of another individual during copulation (Fig. 26), and does not appear to have an homologue in the liver-fluke or other Digenea.

LIFE-HISTORIES OF THE POLYSTOMATIDAE.[74]—_Polystomum integerrimum._ After the mutual fertilisation of two individuals, the eggs are laid in the water by the protrusion of the body of the parent through the urinary aperture of the Frog. About 1000 eggs are laid in the spring at the rate of 100 a day for ten days. After about six weeks, the larva (.3 mm. long) hatches out, and swims about freely by means of bands of large ciliated cells (Fig. 26, A); but if it does not meet with a tadpole within twenty-four hours, it dies. Should it, however, encounter one, the larva creeps along it in a looping fashion until it approaches the opercular spout, or opening of the branchial chamber, on the left side; into this it darts suddenly, fixes itself, and throws off its cilia. Here it remains eight or ten weeks, feeding, increasing in size, and forming the suckers from behind forwards. {59}At the time of the tadpole's metamorphosis, the young _Polystomum_ works its way down the pharynx into the oesophagus and along the intestine, till it reaches and enters the opening of the bladder. Three years afterwards it becomes mature.

Sometimes, however, _Polystomum_ experiences another fate. The larvae settling down on the external gills of a young, recently-hatched tadpole, and obtaining a richer supply of blood than in the previous case, grow far more rapidly, so that in five weeks they are mature, although still in the branchial chamber of the tadpole. They do not then wander into the alimentary canal, but usually, having discharged their eggs, die at the time of the tadpole's metamorphosis. Still more interesting, however, is the difference between the genitalia in these and in the normal _Polystomum_. In contrast with the latter, these possess (1) one testis and a rudimentary penis; and their spermatozoa differ in structure and shape from those of the normal _Polystomum_. (2) The vaginae are absent, a fact connected with the absence of a functional copulatory organ. (3) In compensation for the loss of these, a duct connects the single testis and the point of union of oviduct and yolk-ducts, and by this self-fertilisation occurs. (4) The uterus is absent; the "ootype" or duct into which the shell-gland opens, communicating directly with the exterior. In (1) and (4) these aberrant _Polystomum_ resemble _P. ocellatum_, from the Tortoise _Emys europaea_.

{60}

{61}_Diplozoon paradoxum._—The life-history of _Diplozoon_ is unique. For whereas the larvae of most animals grow up, each into a single adult, in _Diplozoon_, of the few larvae that survive the dangers of their free-swimming existence, only those become mature which conjugate permanently with another individual. But although there are thus only half as many adult _Diplozoon_ as there were conjugating larvae (or _Diporpa_, as they were called when they were considered distinct forms), yet the total number of eggs produced is probably as great as if each larva became individually mature.

_Diplozoon paradoxum_ lays its eggs on the gills of the Minnow, which it frequently infests in great numbers. The ovum divides rapidly at the expense of the yolk-cells, and in a fortnight a larva (.2 mm. long) of the shape and complexity shown in Fig. 27, B, hatches out, which, however, succumbs if it does not meet with a Minnow in five or six hours. Should it survive, a dorsal papilla, a median ventral sucker, and a second pair of posterior suckers develop. Thus the _Diporpa_ stage is attained. These _Diporpa_ may acquire a third and even a fourth pair of suckers, and continue to live three months, but they only develop and mature their reproductive organs, if each conjugates with another _Diporpa_ (Fig. 27, C, D), and this only occurs in a small percentage of instances. Each grasps the dorsal papilla of the other by its own ventral sucker, thus undergoing a certain amount of torsion. Where the two bodies touch, complete fusion occurs, and, as shown in Fig. 28, the united _Diporpa_ (or _Diplozoon_, as the product is now called) decussate, each forming one limb of the X-shaped _Diplozoon_, within which the two sets of complex genitalia develop (Fig. 28).

IV. _Gyrodactylidae._—_Gyrodactylus_ (Fig. 29), the structure of which is in many ways peculiar, produces one large egg at a time. An embryo, in which the large and smaller hooks of the adhesive disc can be seen (_emb_), develops from this egg while still within the body of the parent, and may give rise to yet another generation within itself. The details of the process have not, however, been well ascertained.

{62}TREMATODA DIGENEA (_endoparasitica_).

OCCURRENCE AND HABITS OF DIGENEA.—Endoparasitic Trematodes have been found in almost all the organs of Vertebrate hosts excepting in the nervous, skeletal, and reproductive systems. The alimentary canal, however, is the most usual habitat. From the buccal cavity to the large intestine, or even to the cloaca, its different regions are the resorts of various Trematodes. No Digenea have been found in the mouth, pharynx, or oesophagus of Mammals; but in Birds, Reptiles, Amphibia, and especially in Fishes, these parts are largely affected. It is a striking fact that Trematodes should occur in the stomach of (chiefly) large predaceous fishes, such as the Pike, Sharks, the Angler-fish, and others, considering the powerful digestive action of the gastric juice of these carnivores. The peculiar nature of the defence which must be employed by the parasites against this digestive action, becomes still more marked when it is considered that if a Trematode normally living in the stomach of one host be transferred to that of another, it is usually speedily digested, as is shown (p. 65) in the case of _Distomum macrostomum_. From these considerations the suggestion has been made that the cutaneous secretions of these Trematodes must act, not only as a protection against digestive or other ferments, but that the action in each case must be a specific one (Frenzel, Braun).

It is, however, in the small intestine that most Trematodes occur, as the examination of the common Frog[75] will readily demonstrate. Both this and the edible Frog are attacked by a dozen Distomatidae, only a few of which, however, are common {63}to both hosts, and a number of Holostomatidae also pass a stage of their development within these Amphibia. Some idea of the extent to which animals, whose habits lead to infection, may be attacked by Trematodes (to say nothing of Cestodes and Nematodes, which often occur also) may be gathered from the fact that in dissecting a black stork, Nathusius found several hundred _Holostomum excavatum_ and about a hundred _Distomum ferox_ in the small intestine, twenty-two _D. hians_ in the oesophagus, five others in the stomach, and one _D. echinatum_ in the intestine. Snipe, Woodcock, Sandpipers, Dunlin, Gulls, Bittern, Geese, and Wild Ducks are, to mention a few cases, greatly infested by members of this group.

The following Trematodes have occurred in man[76]:—

_Distomum hepaticum_ Abild.
" _lanceolatum_ Mehlis.
" _conjunctum_ Cobbold.
" _spathulatum_ Leuckart (= _D. sinense_ Cobb.,
_D. japonicum_ R. Blanch.).
" _rathouisi_ Poir. (probably = _D. crassum_ Busk,
_D. buskii_ Lank.).
" _heterophyes_ v. Sieb.
" _pulmonale_ Bälz (= _D. ringeri_ Cobb., _D. westermanni_ Kerb.).
" _oculi humani_ Ammon (= _D. ophthalmobium_ Dies.).
_Monostomum lentis_ v. Nord.
_Amphistomum hominis_ Lewis and M‘Connell.
_Bilharzia haematobia_ Cobb.

LIFE-HISTORIES OF THE DIGENEA.—The classification of Trematodes according to their life-histories, expressed in the divisions Monogenea and Digenea, though a very useful one, breaks down entirely in the case of certain forms. Thus the life-history of _Gyrodactylus_ is probably digenetic rather than monogenetic. _Aspidogaster conchicola_,[77] which lives in the pericardial cavity of the fresh-water mussel (possibly the only case of a Trematode becoming normally mature in an Invertebrate host, since other species of _Aspidogaster_ live in Chelonia), produces larvae which enter another _Anodonta_ and develop directly into the sexual form. In other words, _Aspidogaster_, though structurally a digenetic form, possesses a life-history which is direct and simple, _i.e._ monogenetic.

The Holostomatidae, which live in birds of prey and aquatic birds, give rise to eggs from which a minute larva escapes. The fate of this aquatic larva is not directly known, {64}but in all probability after entering a host (Fish, Amphibian, Mollusc), it undergoes a gradual change into what has long been known as a _Tetracotyle_, from the frequent presence of four (sometimes only three) adhering organs. Fig. 31 exhibits a species which is abundant in the lens and vitreous humour of the eye of the Perch. Its further history is not known, but presumably the Perch is presently devoured by the final host in which the _Diplostomum_ attains maturity. Thus the Holostomatidae are "metastatic" (Leuckart), their (probably) direct development requiring the presence of two hosts.[78]

The other Digenea, the life-histories of which are known, belong to the Distomatidae and Amphistomatidae, and we may distinguish the steps by which the complex life-history of the liver-fluke (_Distomum hepaticum_) has been brought about, by a consideration of that of _Distomum macrostomum_.

_Distomum macrostomum._—This form occurs in the intestine of several common Passerine birds. It is remarkable not only for the large oral sucker, but also on account of the position of the common genital pore at the hinder, and not as usual, at the anterior, end of the body (Fig. 32, A). The eggs pass out through this pore, and are discharged with the bird's excrement. Should a certain snail (_Succinea putris_) happen to rasp off the epidermis of a leaf upon which the faeces have fallen, the eggs are swallowed and a minute active larva is set free (Fig. 32, B). This penetrates through the thin wall of the digestive tract of the snail, and passing into the connective tissue, throws off its cilia and assumes the shape of Fig. 32, C. This _sporocyst_, as the larva is now termed, grows rapidly in all directions (Fig. 32, D) at the expense of the snail's tissues, until it becomes impossible to separate parasite and host completely.

{65}

Those branches which lie superficially in the cephalic region of the snail become greatly swollen, cylindrical, and contractile. They are banded with green and white, ornamented with red terminal spots, and pulsate rapidly. Hence these fertile branches of the sporocyst (which in this condition was known as _Leucochloridium paradoxum_, Fig. 33, B) naturally attract the attention of insectivorous birds, which peck off the tentacles of the snail, and with it the swollen sporocyst-branch. A sphincter muscle closes the cut end of the fertile sac when the bird's bill nips it off. The sac contains large numbers of young _D. macrostomum_ (Fig. 32, A), produced by the division of embryonic cells of the larva (Fig. 32, B), which are apparently blastomeres of the egg reserved for this future use. It is a remarkable circumstance that the old bird itself is immune from infection, and if it swallows these young Distomes, they are digested. Should, however, the snail's tentacle and its contents be offered as food to the nestlings, their weaker digestive powers merely set the Distomes free from the protective membranes (Fig. 32, A), and thus the Blackcaps, Sparrows, and other birds infested by _D. macrostomum_ have acquired the parasite when they were {66}nestlings by the unintentional agency of their parents.[79] The snail regenerates its lost tentacles only for the sporocyst to again bud off fertile branches into them.

The egg of this Distome thus gives rise to a larva which enters the tissues of one particular Mollusc. Here it becomes a branched sporocyst within which the sexual worms are formed, apparently each from a single embryonic blastomere ("Keimzelle"), by a process comparable with the development of a parthenogenetic ovum, and the whole cycle has been termed _Alloiogenesis_, _i.e._ alternation of sexual and parthenogenetic generations (Grobben).[80] Leuckart[81] and Looss,[82] however, consider that what was once a metamorphosis of an individual (as in the {67}Holostomatidae) has now become, by maturation of the Cercaria in the comparatively modern warm-blooded bird, a metamorphosis extending over two or more generations.

_Distomum (Fasciola) hepaticum._—The liver-fluke of the Sheep, which produces the disastrous disease, liver-rot, has a distribution as wide as that of a small water-snail, _Limnaea truncatula_, the connexion between the two being, as Thomas[83] and Leuckart discovered, that this snail is the intermediate host in which the earlier larval, sporocyst, and redia stages are passed through, and a vast number of immature flukes (Cercariae) are developed. These leave the snail and encyst upon grass, where they are eaten by the sheep. Over the whole of Europe, Northern Asia, Abyssinia, and North Africa, the Canaries, and the Faroes, the fluke and the snail are known to occur, and recently the former has been found in Australia and the Sandwich Islands, where a snail, apparently a variety of _Limnaea truncatula_, is also found.[84] Over these vast areas, however, the disease usually only occurs in certain marshy districts and at certain times of the year. Meadows of a clayey soil, liable to be flooded (as in certain parts of Oxfordshire), are the places where this _Limnaea_ occurs most abundantly, and these are consequently the most dangerous feeding-grounds for sheep. The wet years 1816, 1817, 1830, 1853, and 1854—memorable for the occurrence of acute liver-rot in England, Germany, and France—showed that the weather also plays a considerable part in extending the suitable ground for _Limnaea_ over wide areas, which in dry years may be safe pastures. In 1830 England lost from this cause,[85] one and a half million sheep, representing some four millions of money, while in 1879-80 three millions died. In 1862 Ireland lost 60 per cent of the flocks, and in 1882 vast numbers of sheep perished in Buenos Ayres from this cause. In the United Kingdom the annual loss was formerly estimated at a million animals, but is now probably considerably less. After infection during a wet autumn, it is usually in the succeeding winter that the disease reaches its height.

{68}The symptoms of "rot" appear about a month after infection, more acutely in lambs than in sheep, and again, less in oxen than in sheep. At first, death may result from cerebral apoplexy, but if the first few weeks are passed through, a pernicious anaemia sets in, the sheep are less lively and fall at a slight touch, the appetite diminishes, and rumination becomes irregular. The conjunctiva is of a whitish-yellow colour, the dry, brittle wool falls off, and there is sometimes fever and quickened respiration. In January, about three months after infection, the wasting, or fatal, period sets in. Oedemas or swellings, usually visible before, become larger at the dependent parts of the body, a large one in the submaxillary region being especially well marked, and this is considered one of the most characteristic symptoms ("watery poke"). Through this period few of the infected sheep survive, but should they do so, the flukes begin to migrate, though some remain much longer within the liver. Migration is effected through the bile-duct into the duodenum and outwith the faeces, in which the altered remains of the _Distomum_ are sometimes scarcely recognisable. Under these circumstances (or owing to death of the fluke _in situ_) the sheep recover more or less fully.

The preventive measures seem to be: (1) Destruction of the eggs and of the manure of rotten sheep; (2) slaughter of badly fluked sheep; (3) adequate drainage of pastures; (4) an allowance of salt and a little dry food to the sheep; and (5) dressings of lime or salt on the ground to destroy the embryos.[86]

_Distomum hepaticum_, contrary to most Trematodes, enjoys a wide range of hosts. Man himself occasionally falls a victim; thus in Dalmatia, in the Narenta Valley, the disease is endemic but slight in its effects. The horse, deer, camel, antelopes, goat, pig, rabbit, kangaroo, beaver, and squirrel have all been known to harbour this fluke occasionally. In the Italian deer-parks at Mandria a large species, _D. magnum_, decimated the herds some years ago; and this species, probably imported from Italy, is now almost as dangerous a parasite on the western plains of the United States as _D. hepaticum_.

_Bilharzia haematobia._[87]—This formidable parasite was discovered by Bilharz in 1853 in the veins of the bladder of patients {69}at the Cairo Hospital, and is remarkable from its abundance on the east coast and inland countries of Africa from Egypt to the Cape, as well as in the districts bordering Lake Nyassa and the Zambesi river, while westwards it occurs on the Gold Coast. Mecca is a source of infection whence Mohammedans carry the disease to distant places. In Egypt about 30 per cent of the native population is affected by the serious disease known as Haematuria, resulting from the attacks of _Bilharzia_, so that, of the many scourges from which in Africa man suffers, this one is perhaps the most severe.