MOLLUSCA. 



Sub Order II. PTEROPODA. Pelagic forms which in most points of 

 structure agree with the Teetibranchs. The head and usually eyes and 

 tentacles are lacking, while the fins (in reality greatly developed para- 

 podia) are highly characteristic, giving the name ' wing-footed ' to these 

 forms. Like the Teetibranchs they are hermaphroditic, orthoneurous, 

 have a single ctenidium and a posterior auricle. The THECASOMATA 



Fig. 377. — A, C'lione papilionacea ; B, Hyalea trldeniaia. (After Verrill.) 



have shells, those of Limacinid.e (spiral) and Hyaleid^ (pyramidal) being 

 calcareous. The Cymbulid^ have transparent gelatinous pseudo-shells 

 formed by the subepithelial connective tissue. The long nearly cylindrical 

 shells of the Cavolinid.b make up much of the ' pterpod ooze' of the deep 

 seas. GYMNOSOMATA ; shell lacking. Pneumodermon, with suckers like 

 those of cephalopods on the proboscis. Clione,* arctic. 



Sub Order II. NUDIBPvANCHIA. Shell, otenidia, and osphradia 

 hicking ; most possessing accessory gills (or cerata) of varying form and 



w^i/m^ 



Fig. 3TH. 

 Fig. :S7S.— j(I<m ik l„l,n„,n,itn 

 Fig. d7ii.~.-Ei>li<lin i,„i,ill„s, 



(From Liid\\' 



distribution. In tlic Douidiid.e they form a cluster of retractile bushes 

 around the anus (fig. 378). In iIicTuitomid.e they are in two rows, right 

 and left (often branched) u]ion tlio liack. The jE(.)Lin.E have several rows 

 (Dendronotus*), while in the Elysiid.k cerata are lacking. The .iEolidae 

 are noteworthy in having ueraatocysts like those of the ccelenterates 

 (p. 329). 



