334 NERVOUS SYSTEM OF MOLLUSCA. 



is usually furnished, indeed, with feelers or tentacula, which are 

 nothing else than prolonged lips, ard which serve to distinguish 

 and select the food ; but the eyes, if any are present, are gene- 

 rally placed elsewhere, in such a position as to be of the most 

 effectual guidance in the movements of the body. In the lowest 

 Mollusca, we lose all traces of any special organs of sense ; and 

 it appears as if the sense of feeling, possessed in all probability 

 by the body in general, but peculiarly by the lips or tentacula, 

 is the only one through which the animal can receive any inform- 

 ation of its condition. 



873. The conformation of the Nervous System is very much 

 what might be anticipated from the facts just stated. It by no 

 means displays the same complexity, or seems to possess the 

 same importance as a prominent feature in the composition of a 

 Molluscous, as it does in that of an Articulated animal ; and its 

 arrangement is not marked by any regular characters, but varies 

 with the disposition of the organs with which it is connected. 

 Thus we have a single ganglion, or a pair of ganglia, situated in 

 the head, where this exists as a distinct part ; and these ganglia, 

 which seem to be the principal seat of the instincts of the animal, 

 serve to direct those movements which are not reflex. The gills, 

 the pharynx, the foot, and other organs, usually have their own 

 distinct ganglia ; and these, which are all connected with the 

 cephalic ganglia (those situated in the head), seem to be the 

 centres of the reflex actions of the several parts ( ANIM. PHYSIOL. 

 439, and ZOOLOGY, 53, and Figs. 35 and 36). In the 

 lowest Mollusca we find but a single ganglion, which seems in 

 some degree to combine all the functions just mentioned, but to 

 be particularly connected with the respiratory apparatus. 



874. The Mollusca are, for the most part, extremely vora- 

 cious ; and are not particular in their selection of food. It is in 

 those which possess most power of locomotion, that we see (as 

 might be expected) the greatest exercise of choice ; those which 

 are dependent for their aliment upon the casual supplies brought 

 by the sea, being obliged to take what they can thus 

 Their digestive apparatus is always highly developed. W< 

 uniformly meet with a large liver ; and frequently with salivary 



