no 



GOMPABA TIVE ANA TOMY 



CHAP. 



A 



iiig. Tlicie is no division into parts, and the adult laiely (Ackeon) carries an 

 operculum. 



The epipodium is wanting. 



The parapodia, on the contrary, i.e. lateral loljes or fold-like extensions of the edges 

 of the sole, are highly developed in many Opisthobranchia {e.g. the Elyslacloc among the 

 Ascoglossa, and very many Tcctihrnnchia, such as the Scaphandridcc, Bullidce, Aplus- 

 tridce, Gastroptcridie (Fig. 14, ji. 10), Philiuidcc, Doridiidw, Aplysiidcv (Fig. 75, 

 ji. 78), Oxynoeidm). The jjarapodia are often bent 

 back over the shell, their edges sometimes touching, 

 so that the shell may be entirely roofed over by them. 

 In many forms which are provided with parapodia 

 {Gustroptcridm, Philinidcc, Dorldiidce, Aplysiidcc) the 

 mantle also bends, back over the shell, more or less 

 completely covering it. In these cases the shell is 

 to some extent doubly internal, being covered first 

 by the mantle and then (not in Philiiir and Boridivm) 

 by the parapodia (Fig. 100). 



The parapodia may fuse posteriorly along their 

 upturned edges {Aplysiidcc, Oxynoe). In Lobiger each 

 Iiarapodinni is transversely slit, so that two long 

 wing-like processes are formed on each side. Many 

 0}>isthobranchia {Aplysiidcv, Oxynoe, Gastropteridoi) 

 can propel themselves through the water by means of 

 the waving motion of their parapodia. Phyllirhoe 

 is a Nudibraneh which appears to have become 

 adapted to a pelagic swimming manner of life by the 

 compression of its body into the shape of a long 

 narrow leaf with sharp dorsal and ventral edges ; it 

 travels through the water with an undulating motion 

 (Fig. 19. JI. 12). The foot has disappeared. 



Pteropoda. — The Pteropoda, which are Tecti- 

 brauchiate Opisthobranchs, have, like the Proso- 

 branchiatc Hcteropoda, become pelagic animals adapted 

 for swimming. 



While in the Hcteropoda the propodimn becomes 

 transformed into a medio-ventral vertical rowing fin, 

 in the Pteropoda the paired Tectibranchiate parapodia 



F 1 1 : . 1 0(». —Diagrammatic trans- 

 verse sections of Gastropods, t(i 

 illustrate tlie anaiiyciiieiit of the 

 shell (black, 1), visceral dome and 

 mantle (dotted, 2), and foot (streak- 

 ed, 3). A, Prosobranchiate with which, as we have already seen, can be used for swim- 

 pilMHiiuiH (4). B, niing, develop into the ^laired fins or wings of these 

 animals (Figs. 16 and 17, p. 11 ; 87, p. 91). 



In the Thecosomata (Fig. 87, p. 91), which must 

 be derived from Cephalaspidce {Bulloidca), in which 

 the parapodia lie on each side as direct })rolongations 

 of the reptant surface of the foot, this organ, i.e. the 

 foot, has become confined to the anterior end of the 

 body, and consists of three portions — the median un- 

 paired mesopodium and the two lateral parapodia or fins. The mesopodium is small, 

 and the ventral side of it (which corresponds with the sole of the Cephalaspidce, but 

 can no longer be used for creeping) is strongly ciliated. The ciliary movement is from 

 behind forward, i.e. towards the oral aperture which lies anteriorly on the foot, 

 and no doubt serves for conveying to it the minute marine animals on which the 

 creature feeds. On the dorsal side of the mesopodium, which projects freely back- 

 wards, the Limacinidcc carry a delicate transparent operculum, which often becomes 



outer .shell and 

 Tectibranchiate with lubes (6) of 

 the mantle turned back over the 

 outer surface of the shell. Dor.sally 

 the shell is still uncovered ; 5, ijara- 

 podia ; 7, ctenidium. C, Tecti- 

 branchiate with internal shell, i.e. 

 LMjnipli'tely overgrown by the lobes 

 of the mantle. 



