OKGANS OF EEPKODTJCTION. 549 



behind the eyes, upon the posterior portion of the two anterior ganglia, 

 and are at once recognizable by their sharp outline and very con- 

 siderable size, which surpasses that of the eyes themselves. The proper 

 auditory nerves are wanting, the vesicles of hearing being lodged in 

 little excavations in the ganglia themselves. These vesicles, which are 

 of a round or oval shape, consist of a thin vitreous-looking membrane 

 (but which is sufficiently tough to resist considerable pressure), and 

 contain a fluid in which is suspended a minute rounded otolith. 



(1464.) We now approach an inquiry of much interest as concerns 

 the economy of the animals before us namely the varied forms of 

 their organs of reproduction, and the character of the generative system 

 belonging to each order. This investigation, however, is one of no 

 ordinary difficulty ; for so numerous are the modifications 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. 



(1465.) The three lowest orders of the GASTEKOPODA are still, in many 

 particulars, more or less allied to the CONCHIPEEA ; but more especially 

 this is the case in the organization of their generative apparatus. The 

 Cyclolranchiata, Scutibrancliiata, and Tubulibranchiata, like the inhabit- 

 ants 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 

 fecundity, the fertilizing secretion must be furnished by the glandular 

 walls of the oviduct, as no male organs have as yet been discovered. 



(1466.) The Pectinibranchiata, on the contrary, are all dioecious, the 

 sexes being distinct, and intercourse between the male and female 

 necessary for the impregnation of the latter. 



(1467.) The male is generally at once distinguished by the penis, ap- 

 pended to the right side of the neck (fig. 275, g), an organ which is fre- 

 quently of enormous proportions so large, indeed, that, it being impos- 

 sible 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 deferens, which is at first 

 extremely slender, but on emerging from the mass of the viscera be- 

 comes thicker, running along the right side of the body until it enters 

 the penis, and, having made many zigzag folds, reaches the extremity 

 of that organ, where it terminates by a small orifice. 



* 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. 



