iii. 
scarcely receive one-third of it. Testacella has a very small shell 
attached to the upper surface of the tail, for the protection of the 
respiratory organs, which extend over a large portion of the body, 
and it is remarkable that the Nudibranch in its embryo state has 
a shell, dispensing with it when the branchial appendages appear. 
The Polyplacophora (Chitons) are furnished with a shell composed 
of aseries of plates jointed to cach other ; the animal is thus able to 
roll itself up like the Woodlouse, although not so completely. It 
has no tentacles, but a large muscular foot, and a well-developed 
lingual ribbon. Cuvier divided the Gastropods into seven 
Orders. Fischer proposes three :—1, Pulmonata, terrestrial, her- 
maphrodite; 2. Opisthobranchiata, marine and hermaphrodite, 
respiration branchial, or cutaneous, the branchie and auricle 
placed behind the ventricle ; 3. Prosobranchiata, sexes distinct, 
respiration branchial, or pulmonary ventricle in rear of the auricle. 
The greater proportion of the marine Gastropods belong to this 
Order. Nearly all the terrestrial Gastropods are vegetable feeders, 
Testacella is an exception ; its food consists principally, if not 
entirely of the earth-worm. The marine Gastropcds with a notched 
peristome (for the extrusion of the siphon) are mostly carnivorous. 
Those with an unnotched are herbivorous. Beyond the depth of 
15 fathoms vegetation is restricted to calcareous alge (Nullipores, 
Corallines, &c.) These were formerly ranked as animals on account 
to their resemblance to Zoophytes. The tissue of their stems is so 
consolidated with carbonate of lime as to obtain a stony hardness, 
only yielding to an application of acid, which restores it 
to the condition of an ordinary alga. The Gastropoda, in contrast 
to the Pelecypoda, whose life results in cephalic atrophy, 
lead an active life, and have not only a well-developed head, but a 
special organ in connexion with the mouth, the odontophore, 
which consists of a lingual ribbon, or radula with its cushion and 
muscles. This radula is set with minute teeth like a rasp and it is 
continuously growing forward as the fiuger-nail does on its bed. 
The radula and jaws are not only used for feeding, but in some 
instances for boring the shells of other Mollusca or for crushing 
