122 NATURAL HISTORY. [NEW BUILDING. 
The animals have a small foot-like fin between the bases of 
the two side fins. 
The family of Cymbuliadce 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 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 Pneumoderviidce 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. 
The family of Clionidce have only two wings, which are 
said to be covered with a vascular net-vrork, and serve 
the purpose of gills ; their head is formed of two rounded 
lobes, with small conical tentacles and two fleshy lips; 
their body is oblong depressed; they and the Lhnacinoe are 
the chief food of the whales, though they are v^rv small. 
The fifth class of Headwalking Mollusca, or Cepha¬ 
lopoda (Case 38), have a large head armed with strong 
jaws, like a parrot’s beak, furnished with two large eyes, 
and crowned with fleshy tentacles. The head is separated 
from the oblong body by a distinct neck, which has an 
excretory tube in the front of its lower part. Their gills 
are inclosed in the bag-like body. They are all marine, 
changing the colour of their skin with great rapidity; 
they live on marine animals, are voracious and cruel, and 
some are themselves esteemed as food. They are divided 
into two orders. 
The SepiophoPvA have a naked oblong or conical 
body, often furnished with two longitudinal fins, and they 
have eight or ten fleshy conical tentacles on the head, 
furnished with sucking disks. Their eyes are sessile, 
and they have have only two gills. They swim tail fore- 
