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 tins, 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, 
CUone (somewhat enlarged), 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- 
larva of Pneumoderma (magnified). 
