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 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 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, 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.
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. 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. |