CLASS II. GASTEROPODA: OEDER 3. HETEROPODA. 517 
THE APLYSIAD^. 
These animals, called Sea-Hares, are slug-like in form, the head distinct and furnished with 
tentacles and eyes ; the shell is absent or rudimentary. They are found among sea-weed, from 
which they derive a great part of their subsistence; they feed also on animal substances. 
When alarmed or molested, they emit a violet or reddish fluid from the mantles, long supposed 
to be poisonous, but now knoM'n to be harmless. Forty species are found in the Atlantic, Medi- 
terranean, Indian, and Chinese seas. 
The Jjylijsia depilans of Linnaeus, the type of this genus, early attracted the attention of 
naturalists *, its singular resemblance of form to a crouch- 
ing hare, was observed by the ancient Greeks and Romans, 
who bestoAved upon it the title it still bears. By them it 
was regarded with no little superstition, and its haunts 
along the coasts, supposed to be inhabited by the Nereids, 
were shunned by the fishermen. 
THE BURSATELLIDiE. 
We may place here an animal of which little is known, 
Leach's Bursatblla, B. Leachii, found in the Indian seas. 
The descriptions given of it are very obscure ; it appears 
to be destitute of shell, the body globular, the lower part 
having an oval space, circumscribed by thick lips; the 
animal is furnished with curious swimming and branchial 
appendages. Its place is not very definitely settled by 
naturalists. Rang arranges it with the Aplysiadse. 
THE BULLION. 
These, which are called Bubble- Shells, have a delicate globular shell, more or less inclosed in 
the mantle. The foot is large and often furnished with lateral lobes, used 
in swimming ; the gill is single, placed on the right side of the back and 
concealed by the shell. They are carnivorous, and found generally on 
the Atlantic shores. The Bulla amiJulla is beautifully mottled with white, 
plum-color, and reddish. The B. veluin is very elegant, and of a light- 
brown color ; the apex and base are both white, bordered with dark-brown 
bands. The preceding species are found in the European and Indian seas. 
The B. Ugnaria, noted for having a testaceous gizzard, appears to be com- 
mon on the British shores. There are several species on our coasts, but 
all are small, mostly about one-third of an inch long. Among them is 
the B. triticea ; the B. debilis, an exceedingly minute species ; the B. 
hiemalis, found in the stomach of codfish ; and the B. oryza, found south of Ciipe Cod. 
LEACH S BUESATELLA. 
THE BULLA VELUM. 
ORDER 3. HETEROPODA. 
The animals of this order, the Nudedbranchiata of some authors, are all inhabitants of the 
ocean, where they swim about rapidly, the whole structure of their bodies being adapted par- 
ticularly to this mode of existence. The foot, when present, is converted into a broad, flattened, 
fin-like organ, furnished with a small sucker, by which the animals adhere to floating sea-weed. 
The whole body is usually compressed ; and it is by the fin-like action of the tail that the crea- 
tures swim. The head is distinct, and usually furnished with a pair of tentacles and eyes ; the 
mouth is generally provided with a long proboscis. The animals are usually of a transparent 
gelatinous texture ; they swim with the back downward, and appear to feed upon minute marine 
animals. 
