STRUCTURE OF THE TENTACULA. 529 



from the constancy of its occurrence, evidently an organ of importance ; 

 and there seems to be little room to doubt that it is intended to be a re- 

 ceptacle for the seminal fluid, analogous in function to the copulatory 

 pouches we have already met with in Insects and some Crustacea. The 

 reservoir in question, which we have called the spermatheca (fig. 264, t), 

 is in the Snail placed above the stomach ; and the canal derived from it 

 accompanies the sacculated oviduct, which it ultimately joins near its 

 termination, in such a manner that the ova must pass the orifice of its 

 duct as they are expelled from the body. It must nevertheless be con- 

 fessed that the office here assigned to the " bladder " is rather probable 

 than positively established ; for in the Slug, so nearly allied to the Snail 

 in its general organization, the excretory duct of this organ opens into 

 the common generative sac by an aperture distinct from that which 

 leads into the oviduct, although even here the two are closely ap- 

 proximated. Cuvier suggests that perhaps it may furnish some material 

 useful in forming an envelope for the ova ; but experiments are still 

 wanting upon this subject. 



(1397.) There is still another set of organs connected with the canal 

 by which the eggs escape from the oviduct of the Snail ; and these, 

 although peculiar to the genus we are examining, no doubt furnish a 

 secretion of importance to their economy. They are called the multifid 

 vesicles (fig. 264, y}, and are composed of a series of branched casca de- 

 rived from two excretory ducts, by which a milky fluid, secreted by the 

 cyeca, is poured into the egg-passage prior to its termination. 



(1398.) Although it will be convenient to speak in more general 

 terms concerning the nervous system of the GASTEKOPODA than the exa- 

 mination of a particular species would permit, we deem it necessary, 

 before closing our description of the Snail, to describe with some minute- 

 ness the senses possessed by these terrestrial mollusks, and more espe- 

 cially the extraordinary mechanism provided for withdrawing the most 

 important instruments of sensation into the interior of the body when 

 they are not in actual employment. 



(1399.) The only senses that we can expect to meet with in animals 

 deprived of either an external or internal skeleton are those of taste, 

 smell, vision, and touch. 



(1400.) The sense of taste, judging from the structure of their 

 tongue, must be extremely obtuse ; and although these creatures are 

 evidently possessed of smell, it is not easy to point out where their 

 olfactory apparatus is placed. The eyes, however, are now found to 

 present a perfection of structure correspondent with the enlarged brain, 

 and occupy a singular position, being situated at the extremities of the 

 two superior tentacula appended to the head ; while the inferior pair, 

 adapted, as it would seem, more exclusively to the perception of tactile 

 impressions, are deprived of visual organs. Both the upper and lower 

 tentacula are retractile, and can be completely inverted, so as to be 



2* 



