44 PLATYHELMINTHES TURBELLARIA CHAP. 



coinitant 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 Ehabdoc^la, the absence of 

 a digestive tract indeed being probably more apparent than 

 real. 1 



The Bliabdocoela live under varied conditions. One form, Pro- 

 rliynchus sphyrocephalus, has been found among plants far from 

 water in the neighbourhood of Leyden, by De Man. 2 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, fJastrada, 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. Two latter occur in chains formed 

 by fission ; but the sexual individuals (which are of dis- 

 tinct 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 MacTorliynelius, Acrorliynclius, Promeso- 

 stoma, 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." 3 Much, however, 

 still remains to be done before we possess an adequate idea of 

 the occurrence of this group on our coasts. 



Some Ehabdocoels are parasitic. Fecampia erythrocephala, 



1 The development of the Acoela has been worked out recently by Mdlle. Pereyas- 

 lawzewa (Zapiski. Novoross. Obshch. Odessa, 17 Bd. 1892) and Gardiner (Journal 

 of Morphology, xi. No. 1, 1895, p. 155) with conflicting results. The former finds 

 four endoderm cells, which give rise to a larval intestine. The Acoela are for her, 

 J'scudacocla. Gardiner, on the other hand, finds no trace of an eudoderm at any 

 stage of the development of Polychoerus caudatus. 



2 Tijdschr. Ncdcrland. Dicrk. Ver. Deel ii. 1875. 



3 Yon Graff, Monographic d. Turbcllaricn : I. MJuibdococlidcn, 1882. Gamble, 

 Quart. Journ. Microscop. Science, vol. xxxiv. 1893, p. 433. 



