110 



COMPARATIVE ANATOMY 



CHAP. 



ing. There is no division into parts, and the adult rarely (Adccon) carries an 

 operculum. 



The epipodium is wanting. 



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

 of the sole, are highly developed in many Opisthobranchia (e.g. the Elysiadce among the 

 Ascoglossa, and very many T<'<-tiln-<in<-hift, such as the Scaphandridce, Bullidce, Aplus- 

 tridce, Gastropteridw (Fig. 14, p. 10), Philinidce, Doridiidw, Aplysiidce (Fig. 75, 



p. 78), Oxynocidce). The parapodia 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 

 (Gastroptcridcc, Philinidce, Doridiidce, Aplysiido') 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 /'///////> and Doridimn) 

 by the parapodia (Fig. 100). 



The parapodia may fuse posteriorly along their 

 upturned edges (Aplys-iidcc, Oxynoe). In Lobiger each 

 parapodium is transversely slit, so that two long 

 win-j-like processes are formed on each side. Many 

 Opisthobranchia (Aplysiidce, n.r ; /n,ii^ Gaslropteridcc) 

 can propel themselves through the water by means of 

 the waving motion of their parapodia. Phyllirhoe 

 is a Nudibrancli 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. p. 12). The foot has disappeared. 



Pteropoda. The Pteropoda, which are Tecti- 

 branchiate Opisthobranchs, have, like the Proso- 

 branchiate Heteropoda, become pelagic animals adapted 

 for swimming. 



While in the Heteropoda the propodium becomes 



Fn;. 100. Diagrammatic trans- 

 verse sections of Gastropods, to 

 illustrate the arrangement of the . 

 shell (black, 1), visceral dome and transformed into a medio-ventral vertical rowing fin, 

 mantle (dotted, 2), and foot (streak- in the Pteropoda the paired Tectibranchiate parapodia 



which, as we have already seen, can be used for swim- 

 ming, develop into the paired fins or wings of these 

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



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

 be derived from Cephalaspidce (Bulloidea), in which 

 the parapodia lie on each side as direct prolongations 

 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 Cephalaspidoe, 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 



-d, 3). A, Prosobranchiate with 

 outer shell and epipodium (4). B, 

 Tectibranchiate with lobes (6) of 

 the mantle turned back over the 

 outer surface of the shell. Dorsally 

 the shell is still uncovered ; 5, para- 

 podia ; 7, ctenidium. C, Tecti- 

 branchiate with internal shell, i.e. 

 completely overgrown by the lobes 

 of the mantle. 



