GASTEROPODA. 417 



nothing to what has been said concerning the senses of these Mol- 

 lusca in the description of the snail, already given as an example 

 of the general structure of the entire class. 



(453.) We now approach an inquiry of much interest as con- 

 cerns the economy of the animals before us ; namely, the varied 

 forms of their organs of reproduction, and the character of the gene- 

 rative system belonging to each order. This investigation, however, 

 is one of no ordinary difficulty ; for so numerous are the modifica- 

 tions of structure observable in almost every genus, that, were we 

 not strictly to confine ourselves to the study of the most prominent 

 and important features of this portion of their history, the patience 

 of the student would be severely put to the test in following us 

 through all the details connected with so extensive a subject. 



(454.) The three lowest orders of the Gasteropoda are still, in 

 many particulars, more or less allied to the CONCHIFERA ; but 

 more especially this is the case in the organization of their gene- 

 rative apparatus. The Cyclobranchiata, Scutibranchiata, and 

 Tubulibranchiata, like the inhabitants of bivalve shells, are all 

 hermaphrodite and self-impregnating.* A large granular ovary 

 is in all these orders imbedded in the mass of the liver, and 

 from this a duct leads to an external orifice situated in the vicinity 

 of the anus : if impregnation is in such animals essential to fecun- 

 dity, the fertilizing secretion must be furnished by the glandular 

 walls of the oviduct, as no male organs have as yet been disco- 

 vered. 



(455.) The Pectinibranchiata, on the contrary, are all dioeci- 

 ous ; the sexes being distinct, and intercourse between the male 

 and female necessary for the impregnation of the latter. 



The male is generally at once distinguished by the penis appended 

 to the right side of the neck (Jig. 193, g), an organ which is fre- 

 quently of enormous proportions ; so large, indeed, that, it being 

 impossible that it should be retracted into the body, it is generally 

 simply folded back into the branchial chamber. The testicle is 

 imbedded in the mass of the liver, and lodged in the inmost recesses 

 of the shell. It gives origin to a long and very tortuous vas de- 

 ferens, which is at first extremely slender, but on emerging from 

 the mass of the viscera it becomes thicker, running along the right 

 side of the body until it enters the penis, and, having made many 



* The announcement of the discovery of Spermatozoa in individuals belonging to 

 these orders, mentioned in a former page, will, perhaps, materially modify the opinions 

 of physiologists upon this point. 



