186 STRUCTURE OF THE MOLLUSCA. 



being generally adapted for receiving water, but sometimes 

 air. In the former ease they somewhat resemble the gills 

 of fishes, and are named branchiae, fig. 9, q; in the latter, 

 they are cavities, on the walls of which the blood vessels 

 are distributed. 



Their digestive apparatus consists of a mouth, fig. 9, e, 

 an oesophagus, a stomach, fig. 9, g, or several stomachs, and 

 an intestine, fig. 9, i. Sometimes the mouth is furnished 

 with jaws, or horny teeth, but frequently has no organs of 

 this nature. In general there are salivary glands, fig. 8, q, 

 and all the species have a very large liver, fig. 9, ji. Several 

 of these animals secrete variously coloured fluids, such, for 

 example, as the ink of the cuttle-fish. 



Almost all the Mollusca have a development of the skin, 

 which covers the body, and may be likened to a mantle, 

 which is the name usually given to it, fig. 1, a b; fig. 7. 

 Those of which the mantle is bare and fleshy are called 

 naked Mollusca. More frequently, however, there is formed 

 in its substance, or at its surface, a deposition of hard cal- 

 careous or horny matter, which constitutes a shell; and the 

 Mollusca which are protected by a covering of this kind are 

 called testaceous. 



Shells increase in size by the deposition of new layers 

 internally upon those already formed. Each new layer 

 extends more or less beyond the margins of the layer to 

 which it is applied, so that as the animal becomes older, its 

 shell becomes larger and thicker. The outer surface is 

 generally covered by a thin layer of membranous or horny 

 matter named the epidermis, and the inner surface is often 

 covered with a layer of a pearly nature. The form of 

 shells varies much : sometimes they resemble a shield, which 

 covers the back of the animal, as in the limpet; at other 

 times they constitute a tube twisted upon itself, as in snails; 

 and frequently also they are composed of two plates, united 

 by a hinge, as in mussels and oysters. 



The Mollusca originate from eggs; but sometimes the 

 eggs are hatched after being laid, and sometimes before, the 

 young in this case being born alive. In all cases, these 



