GASTROPODS. 



367 



Thecosomata. 



The forms belonging to this section of Pteropoda are characterised 

 by the presence of a delicate external shell, by the foot being repre- 

 sented only by the two anterior symmetrical fins, by the existence of a mantle, 



by the absence of eyes, and (except in a 



few cases) of a gill. The head is indistinct 



and furnished with a single pair of 



tentacles. The mouth is unprovided with 



hook-sacs, as in the Gymnosomata. The 



radula has only a single lateral tooth on 



each side, with a small basal piece. The 



Thecosomata are divided into the three 



families, Limacinidce, Cymbuliidce, and 



Cavoliniidce. 



The members of the family Lima- 



cinidce possess a small sinistrally coiled 



shell, provided with a delicate glassy 



operculum, which is attached to the 



posterior lobe of the foot. The animal 



can withdraw completely within the 



shell, which, when the animal is swim- 

 ming, has a lateral position, or possibly 



rests with the spire inclining somewhat 



downwards. Two genera are included 



in this family, namely, Limacina and 



Peraclis. In Limacina, of which ten 



species are known, the shell is rather 



globose, umbilicated, with a short spire, 



and the aperture somewhat prolonged at the base. The lip of the aperture is 



simple, but the columella is reflexed. The fins of L. helicina are broad and 



squarish at the ends and notched on the inner edge. The operculum is oblong, 

 transparent, paucispiral, the nucleus being lateral. This 

 mollusc lives in immense shoals in the Greenland seas, and 

 it is one of those fed upon by various whalebone whales. 

 Two species only are known of Peraclis. The shells are 

 excessively fragile, sinistral, rather like a fresh- water Physa 

 in shape, but having the aperture produced below into an 

 acute rostrum. The animal has a minute glassy operculum, 

 and differs from Limacina in having a distinct head, pro- 

 longed into a proboscis, and other anatomical characters. 

 Both forms occur in the Atlantic, but P. reticulata is also 

 known from the Pacific and the Mediterranean. 



The molluscs of the family Cavoliniidce have external, 

 semi-transparent shells, which are non-operculate, bilaterally 

 symmetrical, and not spirally coiled. They are variable in 



form in the different genera. The animals are completely retractile within the 



shell, and the form of the fins and of the hinder lobe of the foot varies consider- 



LAEVA OF Pneumoderma (magnified). 



Clione (somewhat enlarged). 



