TURBELLARIA. 583 



here to enter : it is remarkable, however, for the branching form 

 of its digestive cavity, which extends throughout almost the 

 entire body, very much as in the Planarise (Fig. 273) ; and also 

 for the curious 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 the Lymnceus ; 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 enclosed 

 in a contractile integument, no formed organs being found in 

 them ; these cells, in their turn, are developed into independent 

 "zooids," which escape from their containing cyst in the con- 

 dition of free ciliated animalcules ; in this condition they remain 

 for some time, and then imbed themselves in the mucus that 

 covers the tail of the Mollusk, in which they undergo a gradual 

 development into true Distomata; and having thus acquired 

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

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

 able number of Distomata may be produced from a single ovum, 

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

 lopment. In some instances, the free ciliated larva possesses 

 distinct eyes ; although they are wanting in the fully developed 

 Distoma, the peculiar "habitat" of which would render them 

 useless. 



361. Turlellaria. This group of animals, which is distin- 

 guished by the presence of cilia over the entire surface of the 

 body, seems intermediate in some respects between the "trema- 

 tode" Entozoa and the Leech-tribe among Annelida. It de- 

 serves special notice here, chiefly on account of the frequency 

 with which the worms of the Planarian tribe present themselves 

 among collections both of marine and fresh-water animals (par- 

 ticular species inhabiting either locality), and on account of 

 the curious organization which many of these present. 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 accompanying figure (Fig. 273) displays the 

 general conformation of their principal organs, as thus seen. 

 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 cir- 

 cular sucker, that is applied to the living surface from which the 

 animal draws its nutriment; and the buccal cavity (b) opens into 

 a short oesophagus (<?), which leads at once to the cavity of the 

 stomach. In the true Planarice, the mouth is furnished with a 

 sort of long funnel-shaped proboscis; and this, even when de- 

 tached from the body, continues to swallow anything presented 



