GASTEROPODA. 



397 



when that organ is protruded, is sufficiently 

 distinct, admitting with facility an ordinary 

 bristle (^/zg.194, I). On slitting up the penis 

 as it usually lies retracted into the visceral 

 cavity, its inner membrane is found gathered 

 into longitudinal folds, and this provision is 

 needful to allow of that distension which must 

 occur during its erection, at which time this 

 lining membrane becomes the external integu- 

 ment of the protruded organ. 



These parts would seem sufficient in them- 

 selves to fulfil the functions belonging to the 

 organs of both sexes, nevertheless we find 

 others superadded, the uses of which are not 

 so readily assignable; these are the bladder, 

 as it is called by Cuvier, the multifid vesicles, 

 and the sac of the dart. 



The sac ^Ahich has been called the bladder 

 (fig- 190, z, Jig. 194, o) is invariably present; 

 it consists of a round vesicle, variable in size, 

 communicating by means of a canal, generally 

 of considerable length and diameter, with the 

 termination of the matrix : it is usually found 

 filled with a thick and viscid brownish matter, 

 and is generally supposed to furnish an enve- 

 lope to the eggs as they escape from the con- 

 voluted oviduct, an opinion, however, as we 

 shall afterwards see, which is not without op- 

 ponents. 



The multifid vesicles (fig. 190, x, fig. 194, 

 c) are much less constantly met with, and are 

 in fact almost peculiar to the snail ; they are 

 two groups of cceca, each composed of about 

 thirty blind tubes, which after uniting into 

 larger canals ultimately form a principal duct 

 on each side, through which the secretion 

 which they furnish is poured at a little distance 

 below the orifice leading to the bladder into 

 the passage by which the ova are expelled. 

 The fluid furnished by these curious glandular 

 appendages is white and milky, but as this 

 secretion is almost peculiar to the genus Helir, 

 its use is extremely problematic. 



The sac of the dart (Jig. 190, y, fig. 194, b) 

 is another part of the generative apparatus only 

 found in the snail, and from the extraordinary 

 instrument which it conceals is perhaps the 

 most singular appendage to the generative sys- 

 tem met with in any class of animals. It is 

 an oblong sac with strong muscular walls 

 opening by a special aperture into the common 

 generative cavity, like which it is capable of 

 complete inversion. On opening it, its cavity 

 is seen to be quadrangular, and at its bottom 

 projects a four-sided fleshy tubercle, which 

 secretes the curious weapon that this sac is 

 destined to conceal. This (fig. 194, b) con- 

 sists of a four-sided calcareous and apparently 

 crystalline spike, about five lines in length, 

 which grows by successive layers deposited at 

 its base from the surface of the fleshy tubercle 

 to which it is attached : it will be evident that 

 when the sac is everted, the dart contained 

 within it will be protruded externally. This 

 dart, if broken off from its place of attachment, 

 is speedily renewed. 



To complete our description of the parts 

 composing this complex organisation, it remains 

 only to mention the common generative cavity 



Fig. 194. 



Generative organs of Helix Pomatia. 



(Jig. 194, a), into which the others open; this, 

 when in its ordinary position, is a muscular 

 bag, opening externally by a large aperture 

 near the upper tentacle on the right side of the 

 neck, whilst at its bottom are seen the orifices 

 of three distinct passages, one leading to the 

 penis, one to the female organs, and a third 

 to the sac of the dart. This cavity, like that 

 of the dart, is capable of inversion, which is 

 effected partly by the action of its muscular 

 walls, aided in all probability by a kind of 

 temporary erection, and when thus turned in- 

 side out, the orifices leading to the penis, the 

 womb, and the sac of the dart of course 

 become external. 



In order to understand the functions of these 

 various parts it will be necessary to describe at 

 length the singular mode hi which copulation 

 is effected. When two snails, amorously dis- 

 posed, approach each other, they begin their 

 blandishments by rubbing the surfaces of their 

 bodies together, touching successively every 

 part. This preliminary testimony of affection 

 lasts for several hours, gradually exciting the 

 animals to more effective demonstrations. At 

 the end of this time the generative orifice, 

 placed on the right side of the neck, is 

 seen to dilate, and the common generative 

 cavity becoming gradually inverted displays ex- 

 ternally the three apertures which open into it. 

 This being effected, an encounter of a truly 

 unique character commences; the opening 

 leading to the sac of the dart next expands, 

 and that organ undergoing a similar inversion 

 displays the dart affixed to its bottom. A 

 series of manoeuvres may then be witnessed of 

 an unaccountable description ; each snail, in 

 turn, inspired with an alacrity perfectly foreign 

 to its ordinary sluggish movements, striving 

 with his dart to prick the body of his associate, 

 which with equal promptitude endeavours to 



