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, Li. nigra, Pl. XITI., Fig. 22. 
Synonyms, Chalidis, Qu. Pontolimax, Cr. 
Animal minute, leech-like; head truncated in front, with 
arched lateral ridges on which are the eyes; foot linear. 
Distribution, Norway, England, and France, between half- 
tide and high-water, feeding on Conferve, 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. 
RHODOPE, Kolliker, 1847. 
Example, R. Veranii. 
Animal minute, similar to Limapontia ? worm-shaped, rather 
conyex apove, flat beneath; without mantle, gills, or tentacles. 
Upon algze, Messina. 
ORDER IV.—NUCLEOBRANCHIATA. Bu.* 
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 
jirolas and 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 nucleobranchs swim rapidly by the vigorous movements 
of their fin-lke 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-podiwm) of the snail and whelk; 
the yentral fin is homologous with the anterior division of the 
* So called because the respiratory and digestive organs form a sort of nucieus on 
tie posterior part of the back. See Fig. 141, s, 0, and Pl. XIV., Fig. 24. 
