NATURAL HISTORY. 
148 
[east. zool. 
orifices of generation on the right side of the base of the 
fins. Some have distinct eyes. 
The Thecosomata are so called because their body is in¬ 
closed in a thin shell; their head is indistinct, the mouth 
being placed in the centre of the two large wings, which are 
united into a funnel-shaped expansion; their gills are in¬ 
ternal. They use their fins as oars to their boat-like 
shell, when they swim on the calm ocean. 
The family of Cleodoridce have an elongate or subglobose 
conical glassy shell, and the jins ar§ simple without any 
intermediate foot-like lobe. The body is divided into 
two distinct parts, the head with the two fins, and the 
large and swollen body. The gills are superior and internal. 
In some there are lateral slits in the sides of the shell, 
which are interrupted in front in the globular shells of 
the Hyalczce , and continued to the mouth in the elongate 
Diacrice; in others which have an elongate tapering shell 
there are no lateral slits, as Cleodora , &c. 
The family Limacinidce is probably allied to the 
former, but it has a spiral discoidal shell. 
The family of Cuvieridce have a glassy conical cylin¬ 
drical shell, which becomes truncated in its adult state. 
The animals have a small foot-like fin between the bases of 
the two side fins. 
The family of Cymlndiadce have the intermediate fin 
like the former, but the shell is only of a firm gelatinous 
substance. They have two eyes and tw r o tentacles. The 
shells are variable in shape, but generally somewhat re¬ 
semble a slipper. 
TheGYMNOsoMATA, or naked-bodied Pteropodes, are des¬ 
titute of any shell; their head is distinct, and they have two 
or four distinct fins on the neck, and a central foot-like ap¬ 
pendage between their bases. Their gills are external. 
The family of Pneumodermidce have a fusiform body, 
and the head furnished with two contractile arms, armed 
with peduncled suckers; they have two wings, and the 
gills are posterior; in Pneumodermon they are in the 
form of four leaves; in Spongobranchia they are like a 
prominent spongy ring; and in Trichocyclus in the form 
of three hair-like rings. 
The family of Cymodoceidce have two wings on each side, 
placed in the space that separates the body into two parts. 
