382 



MOLLUSCA. 



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

 structure agree with the Tectibranchs. 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 Tectibranchs they are hermaphroditic, orthoneurous, 

 have a single ctenidium and a posterior auricle. The THECASOMATA 



FIG. 377. -4, Clione papilionacea ; B, Hyalea tridentata. (After Verrill.) 



have shells, those of LIMACINHWE (spiral) and HYALEID^E (pyramidal) being 

 calcareous. The CYMBULID^B have transparent gelatinous pseudo-shells 

 formed by the subepithelial connective tissue. The long nearly cylindrical 

 shells of the CAVOLINIDTE 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 III. NUDIBRANCHIA. Shell, ctenidia, and osphradia 

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



FIG. 378. FIG. 379. 



FIG. 378. Dor is bilamellata.* 

 FIG. 37$.jEolidia papillosa. (From Ludwig-Leunis.) 



distribution. In the DORIDIID^E they form a cluster of retractile bushes 

 around the anus (fig. 378). In the TRITONIID^: they are in two rows, right 

 and left (often branched) upon the back. The J&OLIVJE have several rows 

 (Dendronotus*), while in the ELYSIID^E cerata are lacking. The ^Eolidae 

 are noteworthy in having nematocysts like those of the coelenterates 

 (p. 229). 



