ORGANS OP LOCOMOTION AND SENSATION. 333 



by the presence of a set of arms or members disposed in a cir- 

 cular or radiating manner around tbe mouth ; and these arms 

 are frequently the chief instruments of locomotion, as well as of 

 the prehension of food, being furnished with a strong connecting 

 membrane, that forms a sort of circular fin, by the aid of which 

 the animal swims backwards (Fig. 645). In other members of 

 this group, the arms are less developed, but the body is elon- 

 gated into a fish-like form ; and it is furnished at its edges with 

 fins much resembling the dorsal and anal fins of Fishes, and 

 supported by the cartilaginous internal skeleton already men- 

 tioned (Fig. 640). From this general sketch it is evident that 

 the appendages for locomotion do not possess, in the Molluscous 

 series, anything like the same regularity of development which 

 they manifest in the two preceding ; and that locomotion forms 

 a much smaller part of the life they are destined to lead. In 

 fact, the word sluggish^ which is founded upon the well-known 

 habits of a naked (or shell-less) terrestrial Mollusk, very well 

 expresses the general character of the group in this respect. 



944. The amount of development of the organs of sense 

 in Mollusca, varies as much as the character of the locomotive 

 apparatus. Thus in the highest class, comprehending the Cuttle- 

 fish and its allies, the head is furnished with a pair of large well- 

 formed eyes, constructed upon the same general plan as those of 

 Vertebrata. There is also an apparatus for Hearing, much 

 resembling that which exists among the lowest Fishes ; and 

 there is reason to believe that an organ of Smell is also present. 

 The senses of Taste and Touch appear to be very acute. It is 

 in such animals that we should expect to find the organs of 

 sensation most developed, on account of their rapid locomotion 

 and voracious habits. In the Gasteropods, which are not so 

 much distinguished in these respects, we find the organs of sense 

 less developed ; but it is seldom that either of them is altogether 

 absent.* Descending lower, however, we no longer find these 

 organs situated upon a prominent part of the body ; nor are they 

 usually placed in the near neighbourhood of the mouth. This 



* An apparatus for hearing, though of a very simple kind, has been detect- 

 ed in a large number of this class. 



