86 NATURAL HISTORY. [EAST. ZOOL. 
towards the break of day. They are all hermaphrodite, with vent and 
the orifices of generation on the right side of the base of the fins. 
Some have distinct eyes. 
The Thecosomata, or Shell-bearing Pteropodes , are so called 
because their body is inclosed 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 internal. They 
use their fins as oars to their boat-like shell, when they swim on the 
calm ocean. 
The family of Cleodorhle have an elongate or subglobose conical 
glassy shell, and the fins are 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 Hyalcece, and con¬ 
tinued to the mouth in the elongate Diacrioe; in others which have an 
elongate tapering shell there are no lateral slits, as Cleodora , &c. The 
Cleodorce have a triangular shell. The Balantice differ in the sides of 
the triangle being ventricose and grooved across. The Pleuropi have 
a cylindrical tapering shell, and Vaginella have a ventricose rather 
depressed shell with a transverse linear mouth. The Brochi are 
cylindrical, curved, transversely ringed shells. Psyche and Euribia have 
a slipper shaped or nearly globular shell. 
The family of Limacinidje is probably allied to the former, but it 
has a spiral discoidal shell. 
The family of Cuvierid^e have a glassy conical cylindrical 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 Cymbuliade have the intermediate fin like the former, 
but the shell is only of a firm gelatinous substance. They have two 
eyes and two tentacles. The shells are variable in shape, but generally 
somewhat resemble a slipper. 
The Gymnosomata, or naked-bodied Pterovodes , are destitute of 
any shell; their head is distinct, and they have two or four distindt fins 
on the neck, and a central foot-like appendage between their bases. 
Their gills are external. 
The family of Pneumodermide 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 Cymodoceide have two wings on each side, placed in 
the space that separates the body into two parts. 
The family of Clionide have only two wings, which are said to be 
covered with a vascular net-work, and serve the purpose of gills ; their 
head is formed of two rounded lobes, w 7 ith small conical tentacles and 
two fleshy lips ; their body is oblong depressed; they and the Limacinc r 
are the chief food of the whales, though they are very small. 
The fifth class of Headwalking Mollusca, or Cephalopoda 
(Case 38), have a large head armed with strong jaws, like a parrot’s 
beak, furnished with two large eyes, and crowded with fleshy tentacles. 
