24 PLATYHELMINTHES TURBELLARIA CHAP. 



creep by extending parts of the anterior margin and dragging 

 the rest of the body behind. In consequence, the brain and 

 dorsal tentacles may come to lie actually behind the middle of 

 the body, and thus no definite anterior end or " head " advances 

 first. Along with this curious habit it may be noticed (Lang) 

 that the radial symmetry of the body is well marked ; hut even 

 without accepting this author's suggestion of the concurrent 

 development of a " head " with locomotion in a definite direction, 

 the facts, whether these two forms are primitive or not, are 

 highly interesting. 



Food. Though we are probably right in calling Polyclads a 

 carnivorous group, the food of very few forms has been ascer- 

 tained. Those which possess a large frilled pharynx (most 

 Acotylea) probably enclose and digest large, and, it may be, 

 powerful prey, as appears to be the case in Leptoplana tremel- 

 laris. Cryptocelis alba has been seen by Lang with the pharynx 

 so distended, owing to a large Drepanophorus (Nemertine) which 

 it contained, as to resemble a yolk-sac projecting from the under 

 surface of an embryo. The Cotylea such as Thysanozoon, with a 

 bell- or trumpet-shaped pharynx, are fond of fixing this to the side 

 of the aquarium, but whether they thus obtain minute organisms 

 is not clear. Prosthiostomum shoots out its long pharynx with 

 great vehemence (Fig. 8, F) and snaps up small Annelids by its 

 aid (Lang). Those Polyclads which, as Cycloporus and others, 

 are definitely associated with other organisms are not certainly 



known to feed upon the latter, 

 though " Planaria velellae " has 

 been seen by Lesson * devouring 

 the fleshy parts of its host. The 

 salivary glands which open on 

 the lips and the inner sur- 

 face of the pharynx powerfully 

 disintegrate the flesh of the prey. 

 FIG. 10. Diagram of the musculature, Digestion takes place in the 



causing peristaltic movements of the ma in-gut, and the circulation of 

 intestinal branches of Polyclads. , ,. ,. i , 



(After Lang.) the food is accomplished by the 



sphinctral musculature of the 

 intestinal branches (conf. Leptoplana, p. 13). 



A distinct vent or anus is always absent. After a meal the 



1 See Lang, " Polycladcn, v \>. 607. 



