612 MOLLUSC A GENERAL DESCRIPTION. 



employed, and has degenerated into a mere rudiment. Most of the muscles 

 of the body are concerned with the extension or retraction of the different 

 organs of the body, and do not here need special enumeration. 



The nervous system of Mollusca acquires peculiar importance in that it is 

 the last feature to be influenced by the modifications of the organism, and 

 henoe is of extreme value in tracing relationships of the parts. Its principal 

 elements comprise a series of paired nerve centres or ganglia ; of these one 

 pair, the cerebral ganglia, lying above the oesophagus, sends off nerves to 

 (innervates) the head, eyes, and the special organs of sense; another pair, 

 the pedal ganglia, situated below the oesophagus, innervates the foot ; whilst 

 the pleural ganglia, lying one on each side just above the pedal ganglia, form 

 the third pair. These several ganglia are united by nerve cords, so that the 

 whole usually forms a ring or collar round the throat. A nerve cord runs, 

 also, from each pleural ganglion back into the visceral mass, innervating the 

 viscera and various organs : these two ultimately joining form what is known 

 as the visceral commissure. Sometimes these two cords cross over each to the 

 opposite side of the body before uniting, and the loop becomes a figure of 8. 

 Various sense organs are present in the Mollusca. Many of these are situated 

 on the integument, and are probably, like the tentacles, organs of touch. 

 Eyes are found in most forms, sometimes very perfect organs, as in the 

 cuttle-fish ; in snails generally they are less well-developed, and placed near 

 the base of the horns, or on the summit of a special pair: the bivalves being 

 headless, have usually no eyes ; when visual organs are present they occur in 

 some part of the margin of. the mantle and are numerous. In the case of 

 one or two molluscs, eyes are developed over the back. From their dis- 

 crimination of certain food, some Mollusca appear to be capable of tasting, 

 and they certainly can smell. The seat of the olfactory sense is believed to 

 vary, and sometimes to reside in a tentacle, at others to be traced to a special 

 organ called an osphradium, which in marine snails is situated close to the 

 gills and resembles them somewhat in appearance. That molluscs can hear 

 is inferred, rather than known, from the presence of otocysts, small cavities 

 filled with fluid in which grains of shelly material float. These otocysts are 

 situated close to the pedal ganglia and supplied by nerves from the cephalic 

 ganglia. 



The most important feature in connection with the digestive system of 

 Mollusca is the armature of the mouth, and consists of a horny jaw or jaws, 

 and the radula, as it is called. Both, however, are wanting in the bivalves 

 and in a few other isolated forms. 



The cuttle-fish have a pair of jaws, upper and under ; the common snails and 

 limpets, a single upper jaw ; but the majority of the univalve Mollusca have 

 a pair of jaws, right and left. The radula is characteristic of the Mollusca. 

 It consists first of a ribbon-like horny, or chitinous, transparent, yellowish 

 membrane lying on the floor of the mouth, and passing over two cartilages, 

 to which it is attached by special muscles. On the surface of this membrane 

 are set a series of minute, recurved teeth, also formed of very dense, hard 

 chitine. In a few instances there is but a single row of teeth, one behind 

 the other ; usually there are several, sometimes a greafc many, side by side, 

 and row upon row, each tooth repeating the form of the corresponding one 

 in the row immediately in front of it. Only those teeth near the front are 

 in use, the muscles attached to the basal membrane pulling that portion of 

 the radula backwards and forwards over the cartilages, so that the teeth act 

 very much after the manner of a chain-saw, and rasp off portions of the food. 



