COMPARATIVE ANATOMY AND PHYSIOLOGY. 103 



be termed the branchial ganglion. Another is found at the base of the foot, 

 which may be called the pedal ganglion. And there is sometimes another, 

 which especially supplies the mantle with nerves ; and this may be called the 

 pallcal ganglion. The distribution of their nerves to the different organs 

 would alone indicate their respective functions ; but these are placed beyond 

 doubt by that very great variety in the disposition of these organs which is 

 characteristic of the Mollusca. The development of the sensory organs, the 

 situation of the gills, the structure and position of the foot, the conformation 

 and uses of the mantle, are well known to differ, in the most obvious manner, 

 in genera which are closely allied to each other. Hence the anatomist is able, 

 by the discovery of corresponding changes in the nervous system, to satisfy 

 himself of the particular functions of its different centres.* 



133. It is only in the higher tribes, however, that this separation of function 

 is evident ; and it may be especially noticed in the class GASTEROPODA ; which 

 is so named from the presence of a kind of foot, or locomotive organ, on the 

 under side of the body, this being formed by a thickening of the muscular 

 part of the mantle in that situation. Of the animals belonging to this class, 

 some form univalve shells, whilst others are entirely shell-less. They are much 

 superior in general organization to the animals inhabiting bivalve shells, which 

 are included in the class Conchifera; and this superiority manifests itself 

 strongly in the development of the powers of locomotion and sensation. The 

 Conchifera belong to the group of bicephalous, or headless Mollusca ; the 

 mouth not being placed upon a prominent part of the body, nor guarded with 

 organs of special sensation. The lowest form of this group consists of the 

 class Tunicata ; composed of animals in which the whole body is enclosed in 

 a tunic or bag, having but two orifices, through one of which the water is 

 drawn in by ciliary action, whilst through the other it is expelled. This bag 

 forms a large chamber, the lining of which is devoted to the respiratory 

 function ; and at the bottom of it lies the mass of the viscera, on which is the 

 true mouth or entrance to the stomach. A part of the water which is taken 

 into the respiratory chamber flows into this, and passes through the in- 

 testinal canal ; being discharged along with that which has only served the 

 purpose of aerating the blood. These animals have no power of motion, 

 but such as is effected by the general contraction of the respiratory sac ; that 

 is effected by a single ganglion placed between its orifices, w r hich is there- 

 fore chiefly a branchial ganglion, and is the only nervous centre they possess. 

 Although none of the GASTEROPODA are able to execute very active move- 

 ments, few are entirely fixed; all are more or less dependent upon the exer- 

 cise of these powers for their supply of food ; and the higher tribes employ 

 them also in the perpetuation of the race, since the connection of two 

 individuals is in them an essential part of this function. Although the foot 

 is the chief instrument of locomotion, some of the naked aquatic species have 

 other means of propelling themselves. These move through the water by the 

 undulations of their whole bodies, like the leech, or the vermiform fishes : and 

 a few appear materially assisted by an expansion of the mantle on the ante- 

 rior part of the body, which contains muscular fibres, and seems to act as a 

 fin. In every division of the Animal Kingdom, we find the development of 

 special sensory organs to bear a close relation with that of the locomotive appa- 

 ratus. In the present instance, we observe an evident example of this general 

 rule. The organs of vision, which, when existing at all among the Conchi- 

 fera, were very imperfect, are here almost constant and more highly deve- 

 loped; rudimentary organs of hearing maybe detected; the tentacula are more 



* See Mr. Garner on the Nervous System of the Mollusca, in the Linnasan Transac- 

 tions, vol. xvii. 



