GASTEROPODA. 197 



right eye ; female orifice in the middle of the right side ; heart with an 

 auricle behind, and traces of an arterial and venous system, eyes sessile on 

 the sides of the head, tentacles simple or obsolete.* 



ELYSIA, Risso. 



Type, E. viridis, PL XIII. fig. 19. Syn. Action, Oken. 



Animal elliptical, depressed, with wing-like lateral expansions ; tentacles 

 simple, with sessile eyes behind them ; foot narrow. 



Distr. Brit. Medit. On Zostera and sea-weed, in the laminarian zone. 

 Placo-branchus (ocellatus, Rang.) Hasselt, Java; described as 2 inches 

 long, with four small tentacles ; the lateral expansions much developed and 

 meeting behind, the upper surface longitudinally plaited, and forming, when 

 the side-lobes are rolled together, a sort of branchial chamber. 

 ACTEONIA, _Quatrefages. 



Ex. A. corrugata, PL XIII. fig. 20. British channel. 



Animal minute, leach-like; head obtuse, with lateral crests proceeding 

 from two short conical tentacles, behind which are the eyes. 

 CENIA, Alder and Hancock. 



Type, C. Cocksii, PL XIII. fig. 21. Etym. Cenia, Falmouth. 



Syn. ? Fucola (rubra) (Quoy). 



Animal limaciform, back elevated, head slightly angulated, bearing two 

 linear dorsal tentacles, with eyes at their outer bases behind. 

 LIMAPONTIA, Johnston. 



Type, L. nigra, PL XIII. fig. 22. Syn. Chalidis, Qu. Pontolimax, Cr. 



Animal minute, leach-like ; head truncated in front, with arched lateral 

 ridges on which are the eyes ; foot linear. 



Distr. Norway, England and France, between half-tide and high-water, 

 feeding on Conferva, in the spring and summer ; spawn in small pear-shaped 

 masses, each with 50-150 eggs; fry with a transparent nautiloid shell, closed 

 by an operculum. 



ORDER IV. NUCLEOBRANCHIATA. Bl.f 



The present order consists entirely of pelagic animals, which swim at 

 the surface, instead of creeping on the bed of the sea. Their rank and ain- 



* Order Dermi-branchiata, Quatref. (Pelli-branchiata, A. and H.) M. Quatre- 

 fages erroneously described the Elysiadce as wanting both heart and blood vessels, 

 like the Ascidian zoophytes ; with them he associated the family JEolidce, which he 

 described as having a heart and arteries, but no veins, their office being performed by 

 lacunae of the areolar tissue. In both families the product of digestion (chyle) was 

 supposed to be aerated in the gastric ramifications, by the direct influence of the sur- 

 rounding water. To this group, which has been since abandoned, he applied the 

 name Phlebenterata, (phlebs, a vein, entera, the intestines). 



t So called because the respiratory and digestive organs form a sort of nucleus on 

 the posterior part of the back. See fig. 105, s. b., and PI. XIV. fig. 24. 



