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.
(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 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.
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.)