LECTURES ON MOLLUSCA. 245 



yet equally perfect after its kind. The student of vertebratecl animals 

 and of the various insect tribes, as well as of the Cephalopod and 

 Gasteropod mollusks, naturally looks upon the head as the most im- 

 portant part of every living creature. We are now going to he intro- 

 duced to animals in which not only the head becomes sometimes obso- 

 lete, as in the shell-cased Pteropods, but the whole plan of the organi- 

 zation makes the existence of a head useless, and therefore impossible. 

 The special work appointed for the bivalve and cloaked mollusks in 

 the economy of nature, is to filter the water at the sea bottom from its 

 infusorial particles. They never prey, either upon living creatures or 

 sea plants; hence eyes, jaws, snout, and curiously-armed tongue, 

 which are the characteristics of ordinary mollusks, would be entirely 

 useless. To go about looking for food, when the very air they breathe 

 comes burdened with dainty meat, would be a waste of energies ; so 

 that a swimming or crawling foot is not a requisite of their life. Their 

 special functions are to digest and breathe, in a quiet but uninterrupted 

 manner. All the locomotion they require is to settle themselves in a 

 snug place; and then they simply suck-in the water, and let it bring 

 food to their mouth and air to their blood. When at rest, they are 

 entirely encased in their shelly covering, like the Turbo and Nerite; 

 but when in action, instead of crawling out of their shell, they open 

 the shell itself to let in water. The shell is therefore made of two 

 plates ; which in the ordinary bivalves interlock by means of a toothed 

 hinge, and are fastened together by a ligament. 



The headless tribes of mollusks naturally divide themselves into 

 three great divisions. In the clams, oysters, mussels, and cockles, the 

 animal breathes by means of large plate-shaped gills ; and the valves 

 are, as it were, great wings on each side of the body. But in the 

 lamp-shells, there are no gills, the breathing being performed by the 

 skin, and by the action of very delicate hairs arranged on twisted 

 feelers; and the shelly valves, instead of being side-wings, are shields 

 on the front and back of the animal. In the third division, instead 

 of a shell, the animal is wrapped up in a leathery coat. The ordinary 

 bivalves are often called Acephala (Headless creatures ;) a name which 

 is equally applicable to all three divisions, and to part of the Ptero- 

 pods. Their common name is Conchifera (Conch-bearers ;) but as 

 conchs are univalve shells, and as the name was given to include both 

 the clams and the lamp-shells, it appears best to distinguish them by 

 their leading characteristics as Plate-gilled, Manile-gilled, Cloaked 

 mollusks. 



The oyster tribe lie on one side ; and have neither foot nor breathing 

 pipes. But ordinary bivalves do not lie as their shells are seen in 

 cabinets. They stand upright, like a crawling Cuttle Fish. Their 

 foot, or digger, is at the bottom ; their nose and vent pipes close 

 together at the top. At the back are the digestive organs : in front, 

 a large water chamber, with the gills above, and the mouth below, 

 behind the foot. The mantle enfolds the whole body, and secretes the 

 two shelly plates. These assume an approach to a spiral form, from 

 the growth being in front, the ligament remaining fixed. The breath- 

 ing pipe is not a mere gutter, as in the predacious univalves, but a 

 fleshy tube, armed with muscles to suck in the water, and often ele- 



