340 MANUAL OF THE MOLLUSCA. 



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. 



Synonyms, Chalidis, Qu. Pontolimax, Cr. 



Animal minute, leech-like ; head truncated in front, with 

 arched lateral ridges on which are the eyec ; foot linear. 



Distrihiitwn, Norway, England, and Prance, 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. 



Ehodope, Kolliker, 1847. 



Example, E. Veranii. 



Animal minute, similar to Limapontia ? worm-shaped, rather 

 convex above, flat beneath ; without mantle, gills, or tentacles. 

 Upon algse, Messina. 



OEDEE lY.— NUCLEOBEANCHIATA. Bl.* 



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 affinities entitle them to the first place in the 

 class; but their extremely aberrant form, and unusual mode of 

 progression, have caused us to postpone their description till 

 after that of the ordinary and typical gasteropoda. 



There are two families of nucleobranchiate molluscs ; the 

 firolas Sind carinarias, with large bodies and small or no shells, 

 and the Atlantas, which can retire into their shells and close 

 them with an operculum. Both animal and shell are sym- 

 metrical, or nearly so * the nucleus of the shell is minute and 

 dextrally spiral. 



The nucleohranclis swim rapidly by the vigorous movements 

 of their fin-like tails, or by a fan-shaped ventral fin ; and 

 adhere to sea- weed by a small sucker placed on the margin of 

 the latter. Mr. Huxley has shown that these organs repre- 

 sent the three essential parts of the foot in the most highly- 

 developed sea- snails. The sucker represents the central part of 

 the foot, or creeping disk [meso-podium,) of the snail and whelk; 

 the ventral fin is homologous with the anterior division of the 



* So called because the respiratory and digestive orcrans foi-m a sort of nudaa, on 

 U-e posterior part of the back. See Fig. 141, 5, b, aiid PI. XIV., Fig. 24. 



