30 SHELLS AND SHELL- FISH. PART I. 



Acephala (Dithyra), or all four together, as in Tritonia. 

 The brain is always composed of two of these lobes, 

 which are generally connected, as in Aplysia, or some- 

 times separate, as in Haliotis and Patella : the remaining 

 two ganglions of the collar send off nerves to the or- 

 gans of respiration, &c., and are either united together, 

 as in the common slug and Patella, or are separate, as 

 in Aplysia." It is time, however, to quit these ge- 

 neralities, and enter into more definite details. 



(26.) A rapid survey of the general peculiarities of the 

 class will not, however, be here misplaced. We have said 

 that these animals have no distinct feet, or any organs 

 which, by forming appendages to their body, supply their 

 want. It is, however, customary although the appella- 

 tion is certainly incorrect to call that part of the body 

 which is next to the ground, and which is generally smooth 

 and flattened, the foot, when, in fact, it is merely the 

 belly, assuming only the office of a foot, and the shape 

 of a sucker : by alternately contracting and extending 

 this part, the animal moves forward, slowly, indeed, 

 but very surely. The aquatic species are able to crawl 

 on marine rocks, even when the sea is greatly agitated ; 

 and every one knows the tenacity with which a slug 

 will adhere to the substance from which we attempt to 

 remove it. This is at once explained by the smooth 

 disc of the belly acting as a sucker, like the tail of the 

 leech. This mode of progression is almost universal 

 among the Gasteropoda, or univalve shell-fish ; but, in 

 the bivalves, the body of the animal is frequently length- 

 ened behind, so as to assume a much more analogous 

 appearance of a foot than what we find in others. The 

 common cockle is one of the best, as it is one of the 

 most familiar, instances of this structure. Those who 

 have watched these animals at the ebb of tide, know 

 that they leap very frequently ; but on their method of 

 doing this two opinions have been given : some main- 

 tain that the foot, which, when protruded, forms a 

 sharp angle, is the chief organ made use of; while 

 Bothers assert that this motion is caused by the sudden 



