Page:Popular Science Monthly Volume 10.djvu/41

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the cockle, Venus, and other bivalves possessing those prolongations of the mantle known as siphon-tubes, the eyes are situated either at the base or on the tips of the numerous small tentacles distributed around the orifices of these tubes, which in those of them living in the sand are often the only parts appearing above the surface. The margins of the mantle are also garnished by a number of short though apparently very sensitive tentacles, in which the creature's most specialized sense of touch seems to reside. Some of these tactile appendages, as well as some of the ocelli, send their nerves to the posterior or parieto-splanchnic ganglia, while those situated on the anterior borders of the mantle communicate with the anterior or oral ganglia. The latter ganglia also receive filaments from the so-called labial appendages, whose function is uncertain, though it has been suggested that they may be organs of taste or smell. Lastly, in close relation with the pedal ganglia or ganglion, there are two minute saccules (Fig. 9, s), to which an auditory function is usually ascribed.

Thus we find among these headless mollusks a distribution of specially impressible parts or sensory organs, such as cannot be paralleled among any other animals. The sense of touch and the sense of sight seem to be more especially in relation with the great posterior ganglia. These sensory functions are, however, to a minor extent shared by the oral ganglia, which are also in relation with parts that may possibly be organs of taste or smell. On the other hand, auditory impressions are invariably brought into relation with the inferior or pedal ganglia. In these headless mollusks, therefore, the functions pertaining to the brain in other animals are distributed in a very remarkable manner, and the anterior ganglia cannot in them be properly regarded as representing such an organ.

The viscera in these lamellibranchs are also in relation with the three pairs of ganglia, and not exclusively with any one of them. Filaments to the intestinal canal and the liver are usually given off from the commissures between the anterior and the posterior ganglia; the genital organs are in connection with filaments coming from the commissures between the anterior and the inferior or pedal ganglia; while the branchiæ are in relation with the ganglia at the posterior part of the body.[1]

There is another interesting class of mollusks—the Pteropoda—which, in respect of powers of locomotion and the possession of a distinct head, may, if for no other reasons, be said to lead us on from the comparatively sluggish bivalve Mullusca to the gasteropods and the cephalopods, all of which are distinguished by definite and wide reaching powers of locomotion, and by the possession of a distinct head carrying sense-organs, and a more or less developed brain.

  1. In speaking of the nervous system of the lamellibranchs, I have not alluded to certain small accessory ganglia which exist in some of them in relation with peculiar specially developed contractile structures.