Page:Popular Science Monthly Volume 10.djvu/39

Rh though ganglia exist around the œsophagus which must receive afferent impressions of some kind, and from which branches proceed to the various muscles and viscera of the body.

Such low sensory endowments as are presented by the Brachiopoda would be wholly incompatible with that degree of visceral complexity of organization which they possess, had it not been for the fact that they lead such a passive existence in respect to quest of food. They do not go in search of it at all—they remain securely anchored while food is brought to the entrance of their alimentary canal by means of cilia. The absence of sense-organs and of a brain is, indeed, only compatible with a quasi-vegetative existence such as this.

The lamellibranchs, or ordinary headless bivalve Mollusca, also include some representatives—such as the oyster and its allies—which lead a sedentary life after the fashion of the Mollusca already mentioned. The valves of the shell in these lamellibranchs are lateral, instead of being dorsal and ventral as among the branchiopods. The shell is, however, closed by a single adductor muscle, and it is opened, when this relaxes, by means of an elastic hinge.

The mouth of the oyster is surrounded by four labial appendages, whose functions are not very definitely known. It presents no other appendages of any kind in the neighborhood of the mouth, and, as in the two types of Mollusca already described, the food which it swallows is brought to the entrance of its œsophagus by means of ciliary currents. This well-known animal has a large and important nervous ganglion (Fig. 8, b) situated posteriorly, and close to the great adductor muscle. It gives off' branches to this muscle, to each half of the mantle, to the gills (c, c), and it sends forward two long parallel branches (d, d), which serve to connect it with a much smaller anterior ganglion (a, a) situated on each side of the mouth. These anterior or labial ganglia are joined by a commissure arching over the mouth, and also by a more slender thread beneath the mouth, from which filaments (e) are given off to the stomach. These latter filaments may be considered to have a function similar to that of the stomatogastric nerves in insects. The anterior ganglia receive nerves (f) from the labial processes, probably for the most part afferent in function. At all events, these processes have no distinct muscular structure.

Other lamellibranchs possess a remarkable muscular appendage known as the foot, which is in relation with an additional single or double nervous ganglion, and is used in various ways as an organ of locomotion. The animals possessing this organ are also provided with a second adductor muscle for closing their shells. Speaking of the various uses of the foot among bivalves, Prof. Owen says: "To some which rise to the surface of the water it acts, by its expansion, as a float; to others it serves by its bent form as an instrument to drag them along the sands; to a third family it is a burrowing organ; to many it aids in the execution of short leaps."