396 



GASTEROPODA. 



fleshy, and by the position of the eyes at the 

 extremity of so long a stem these can be readily 

 directed to different objects. In no case, how- 

 ever, can they be retracted within the body so 

 as to be quite enclosed in the visceral cavity. In 

 the terrestrial Gasteropods the eyes are gene- 

 rally placed at the extremity of the superior 

 horns, a position which manifestly extends the 

 range of vision, and moreover, in consequence 

 of the structure which we have described when 

 speaking of the organs of touch, may be com- 

 pletely drawn within the body. In Jig. 189, 6, 

 the eye of Vaginulus is seen at the extremity of 

 the upper tentacle, and the origin of the optic 

 nerve (c) from the brain (d), as well as the 

 convolutions which it makes to allow of its 

 adaptation to the varying length of the tentacle, 

 and the bulb in which it terminates behind the 

 eyeball (b*),a.re sufficiently displayed. ~Lnjig. 

 192, 6, the eye of the snail exhibiting the same 

 circumstances has been represented, and the 

 apparatus by which the movements of the whole 

 organ are effected is so clearly shewn as to 

 render further description superfluous. 



Generative system. The description of the 

 generative apparatus of the Gasteropoda forms 

 one of the most remarkable parts of their 

 history, and the complication which it presents 

 in some orders is probably unique in the ani- 

 mal kingdom. The class may be divided, as 

 far as relates to this function, into three great 

 divisions: 1st. Hermaphrodite and self-im- 

 pregnating ; 2d. Hermaphrodite, but recipro- 

 cally impregnating each other by mutual 

 copulation ; 3d. Sexes distinct, the female 

 being impregnated by copulation with the 

 male. We shall consider each of these divi- 

 sions in the order in which they have been 

 enumerated. The lowest orders approximate 

 the Conchifera in most parts of their organisa- 

 tion, and in the arrangement of their generative 

 system we need not be surprised to see a 

 manifest resemblance. The Scutibranchiate 

 and Cydobranchiute orders, therefore, present 

 this great distinguishing character, which more 

 than any other detaches them from the others, 

 namely, that every individual being furnished 

 both with ovigerous and impregnating organs 

 is sufficient to the impregnation of its own 

 ova. Nothing, in truth, can be more simple 

 than such an arrangement. The ovary is found, 

 when empty, embedded in the substance of 

 the liver, but at certain epochs it becomes so 

 much distended with ova as to cover in great 

 part the rest of the viscera ; from this ovary 

 arises a simple canal or oviduct, which termi- 

 nates after a short course in the neighbourhood 

 of the anus. No trace of accessory apparatus 

 has been found, and the only part to which 

 the office of a testis is assignable is the tube 

 through which the ova are discharged, which 

 probably furnishes a secretion subservient to 

 the impregnation of the eggs. Such is the 

 structure of the generative system in Haliotis, 

 Patella, and others of the orders to which 

 these respectively belong, exhibiting a simpli- 

 city of parts widely different from what is 

 found in the division which next presents itself 

 to our notice. The second type of the genera- 



tive apparatus is common to the Nudibran- 

 chiate, Infer abranchiate, Tectibranchiate, and 

 Inoperculated pulmonary orders; in all of which 

 every individual is provided with both male and 

 female organs of copulation, and, accordingly, 

 mutual impregnation is effected by the congress 

 of two individuals, or in a few instances by 

 the combination of several. We shall select 

 the common snail ( Helix pomatia ) as the most 

 familiar illustration of the general arrangement 

 of the parts composing this double apparatus, 

 leaving the varieties which it presents to sub- 

 sequent notice. The admirable plate of Cuvier, 

 of which fig. 190 is a copy, represents the 

 whole system with that clearness and fidelity 

 so characteristic of all the laborious contribu- 

 tions to science which we owe to his indefati- 

 gable industry. The female portion consists 

 of the ovary, the oviduct, and an enlarged 

 portion of the oviduct which forms a receptacle 

 for the ova, and is called by Cuvier the womb 

 (la ma trice). The ovary (q) is a racemose 

 mass embedded in that portion of the liver 

 which is enclosed in the last spire of the body, 

 i. e. that part which is placed nearest to the 

 apex of the shell ; from this proceeds a slender 

 oviduct (r), folded in zigzag curves, and vari- 

 ously convoluted : it commences by many 

 small branches derived from the ovary, and 

 terminates in a mass (*), regarded by Cuvier 

 as the testis, in which it becomes so attenuated 

 that it is difficult to trace it ; emerging, how- 

 ever, from this mass, it expands into the 

 womb (), which is a long, capacious, and 

 sacculated canal, and capable of much dis- 

 tension, in which the eggs are retained until 

 they have acquired their full development : 

 this viscus opens into the common generative 

 cavity at e, Jig. 194. 



The male organs consist of a testicle, vas 

 deferens, and penis. The testicle (s,Jig. 190) 

 appears to be composed of two distinct portions, 

 the larger of which is soft and homogeneous in 

 texture, but the smaller has a granulated ap- 

 pearance ; the latter (M) runs along the womb 

 like a mesentery, connecting its folds as far as 

 the termination of that viscus. The testicle 

 varies much in size at different periods, being 

 generally very small, but during the season of 

 love it dilates so as to fill nearly half of the 

 visceral cavity, at which time the womb like- 

 wise is much enlarged. From the testicle 

 arises its vas deferens or excretory duct, which 

 terminates in the penis near the base of that 

 organ. The penis (fig. 194, n) is a most sin- 

 gular instrument, resembling a long hollow 

 whip-lash, formed of circular fibres, and, like 

 the tentacles, capable of complete inversion, 

 which in fact occurs whenever it is protruded 

 from the body ; it is also furnished with a re- 

 tractor muscle (Jig. 190, w), serving to draw it 

 back again after copulation is accomplished. 

 The penis is not perforated at its extremity, 

 but the vas deferens terminates within it by a 

 small aperture, which of course during the 

 inversion of the organ opens externally at 

 about one-third of the length of the penis from 

 its root ; the aperture by which the vas defe- 

 rens thus opens upon the exterior of the penis, 



