304 



Mollusca. 



2. Among the Nudibranohiata, the D o r i d i d se (genus, Doris, etc.) 

 ai-e peculiar in that the anus is situated posteriorly on the dorsal surface, and 



surrounded by a circle of plumose gills. The 

 j3E o 1 i d se (genus, Molis, etc.) have unbranched 

 gills dorsally ; a process extends into each from 

 the liver, which is much branched and not sharply 

 marked off from the gut. Many Nudibranchs are 

 f [ without gUls {e.g., Elysia, Limapontia), and often 



have a striking supei-ficial resemblance to the 

 riatwonns. All the forms mentioned are met with 

 in Northern seas. 



To the Opisthobranchiata belong two aberrant 

 groups of pelagic animals, which have usually, but 

 not quite correctly, been placed together under the 

 name, Pteropoda. The first of these divisions, 

 the Thecosomata [Ewpteropoda), is distinguished 

 among other things by the breadth of the anterior 

 muscular portion of the foot, which forms a pair 

 of fin-like locomotor organs ; the posterior region 

 (f. Fig. 249) is covered on its under surface with long close-set cilia, 

 by which microscopic organisms are driven into the mouth. The mouth lies 

 anteriorly between the fins, and is surroimded by a pair of lip-folds, which 



Fig. 249. A Euptero- 

 pod (C'leodoi'a). m mouth, 

 / posterior portion of foot, 

 V fin, s shell. — Orig. 



Fig. 250. 



Fig. 251. 



Pig. 250. One of the Eupteropoda (Cleodora) with simple tubular shell. — After 

 Souleyet. 



Fig. 251. One of the Pterota (Pneumodermon). In B the arms are furnished with 

 suckers and two hooked processes are projecting ; in A these are all withdrawn, a anus, 

 an .tentacles, b and 6' gills, / — ■/' foot, v fin. — After Souleyet. 



unite in part, and thus prevent the escape of the animalculEe secured by the 

 current of the cUia. The visceral mass is well developed, and enclosed in a shell, 

 which, in some forms, is spirally twisted (and then an opercidum is usually 

 present), but in the majority, is straight, or slightly curved and symmetrical. 



