HABITS OF POLYCLADIDA 



23 



and 



illy employed- 



swimmmg, are usually employed creeping by the cilia, 

 aided possibly, as in the case of some Gasteropod Mollusca, by 

 the longitudinal muscles of the ventral surface ; and swimming, 

 by undulations of the expanded margins of the body. In the 

 former case the cilia work in a glandular secretion which bathes 

 the body, and enables them to effect their purpose equally well 

 on different substrata. The anterior region is generally lifted 

 up, exploring the surroundings by the aid of the tentacles, which 

 are here usually present. The rest of the body is closely appressed 

 to the ground. 



Swimming is particularly well performed by the Pseudoceridae, 

 certain species of Prostheceraeus, the large Planoceridae, some 

 Stylochoplana, Discocelis, and Leptoplana, and in the same manner 

 as in Leptoplana tremellaris (p. 9). In Cryptocelis, Leptopla na 

 alcinoi, and L. pallida, how- 

 ever, the whole body executes 

 serpentine movements like an 

 active leech (e.g. JVephelis) ; a 

 cross section of the body would 

 thus present the same appear- 

 ance during the whole move- 

 ment. Many Polyclads, notably 

 Anonymus (Lang), if irritated, 

 spread out in all directions, 

 becoming exceeding thin and 

 transparent. 



Discocelis lichenoides, Plan- 

 ocera graffii, and Anonymus 

 virilis have peculiar modes of 

 progression. The first, accord- 

 ing to Mertens, will climb up 

 the sides of a vessel by means 

 of the expanded lobes of the 

 pharynx (Fig. 9,ph), a habit of 

 considerable interest, since we 

 know that certain Ctenophores 

 Lampetia, for instance pro- 

 gress when not swimming on 

 the expanded lobes of their " stomach."' 

 1 Chun, "Ctenophoren," Fauna u. Flora G. v. 



Fig. 9. Discocelis lichenoides Mert. (after 

 Mertens), creeping on the inner side of a 

 glass vessel by means of the lobes of the 

 extended and exceedingly mobile phar- 

 ynx (ph). These lobes also serve to enclose 

 Crustacea (a), and one lobe may then 

 be withdrawn independently of the rest, 

 back into the body (b). The brain (>>,) 

 and shell-gland (s>j) are shown by trans- 

 parency. 



Planocera and Anonym us 

 Neapel, Monogr. I. 1880, p. 180. 



