946 WORMS 



phenomena of its development, several distinct forms being passed 

 through between one sexual generation and another. These have 

 been especially studied in the Distoma, which infests Paludina, 

 the ova of which are not developed into the likeness of their 

 parents, but into minute worm-like bodies, which seem to be little 

 else than masses of cells inclosed in a contractile integument, no 

 formed organs being found in them ; these cells, in their turn, are 

 developed into independent larvae, which escape from their contain- 

 ing cyst in the condition of free ciliated animalcules ; in this con- 

 dition they remain for some time, and then imbed themselves in 

 the mucus that covers the tail of the mollusc, in which they undergo 

 a gradual development into true Distomata ; and having thus ac- 

 quired their perfect form, they penetrate the soft integument, and 

 take up their habitation in the interior of the body. Thus a con- 

 siderable number of Distomata may be produced from a single ovum 

 by a process of cell-multiplication in an early stage of its develop- 

 ment. In some instances the free ciliated larvae are provided with 

 pigment-spots or rudimentary optic organs, although these organs are 

 wanting in the fully developed Distoma, the peculiar ' habitat ' of 

 which would render them useless. 1 



Turbellaria. This group of animals, which is distinguished by 

 the presence of cilia over the entire surface of the body, contains 

 forms which are among the simplest of those in which the Metazoic 

 organisation obtains. It deserves special notice here chiefly on ac- 

 count of the frequency with which the worms of the Planarian 

 tribe present themselves among collections both of marine and of 

 fresh-water animals (particular species inhabiting either locality) 

 and on account of the curious organisation which many of these 

 possess. Most of the members of this tribe have elongated, flattened 

 bodies, and move by a sort of gliding or crawling action over the 

 surfaces of aquatic plants and animals. Some of the smaller kinds 

 are sufficiently transparent to allow of their internal structure being 

 seen by transmitted light, especially when they are slightly com- 

 pressed ; and the opposite figure (fig. 714) displays the general 

 conformation of their principal organs as thus shown. The body 

 has the flattened sole-like shape of the Trematode Entozoa ; its 

 mouth, which is situated at a considerable distance from the anterior 

 extremity of the body, is surrounded by. a circular sucker that is 

 applied to the living surface from which the animal draws its nutri- 

 ment ; and the buccal cavity (b) opens into a short oesophagus (c) 

 which leads at once to the cavity of the stomach. This cavity does 

 not give origin to any intestinal tube, nor is it provided with any 

 second orifice ; but a large number of ramifying canals are prolonged 

 from it, which carry its contents into every part of the body. This 

 seems to render unnecessary any system of vessels for the circulation 

 of nutritive fluid ; and the two principal trunks, with connecting 

 and ramifying branches, which may be observed in them may be 



1 On the development and life-history of the ' Liver-fluke ' see Professor A. P. 

 Thomas, Quart. Journ. Microsc. Sci. xxiii. p. 1 ; and K. Leuckart, Arcliiv fur Natur- 

 gesch. xlviii. p. 80. On its anatomy, see Dr. P. Sommer, Zeitschr. fur wiss. Zool. 

 xxxiv. 



