CHAP.xxxvr.] MOLLUSCA. 533 



is formed, which is projected during copulation. The dart may be looked 

 upon as an arrangement for producing sexual excitement, for each snail has 

 been seen to prick the other just before coition. 



Amongst the Cephalopoda, the highest of the Mollusca, the sexes are always 

 distinct. Connected with the excretory duct of the ovary is an apparatus 

 which furnishes a secretion by which the eggs are bound together, and a firm 

 horny covering formed for their protection. 



The testicle, consists of an oblong organ, situated at the bottom of the cavity 

 of the mantle. Connected with the excretory tube, is a sac in which some 

 very complicated organs, containing the sperm, are developed, from which the 

 contents are expelled by a very remarkable projectile apparatus. Coition 

 appears to take place simply by one animal applying itself to the other. A 

 true intromission of the penis seems hardly possible. 



One of the most curious phenomena which has been discovered, in connec- 

 tion with the generation of some of the members of this class, must be briefly 

 noticed here. On the male Argonaut is developed a curious elongated body, 

 termed Hectocotylus, which communicates with the testicle of the Argonaut 

 by a duct. Before this body had been proved to belong to the male Argo- 

 naut, and had only been seen upon the female, it was looked upon as a para- 

 site, and Cuvier described it as one of the Trematoda. The Hectocotylus 

 which is to be regarded as one of the arms of the animal metamorphosed in 

 a peculiar way, at length becomes filled with spermatic fluid, and drops off. 

 It is now independent, and comes into contact with the female Argonaut, 

 which it impregnates. In this point it resembles, " as also by its movements, 

 by a kind of circulation, and by the long duration of its life after detachment, 

 a true male animal " (H. Miiller). 



Among different families of the Crustacea, the arrangement of the genera- 

 tive organs varies much. In most, the sexes are distinct ; but one class, 

 Cirrhipoda, is hermaphrodite. Some Crustacea, again, are almost exclusively 

 females ; and these for many generations produce females, and at very long 

 intervals only, males. Some females, again, lay two kinds of eggs, one of 

 which becomes developed spontaneously, while the other requires to be 

 fecundated by the spermatic fluid. The female of the Daphnia, towards the 

 close of the year, produces two eggs, which must be looked upon in the light of 

 gemmaB, or buds, as they contain no germinal vesicle. These are the so-called 

 hybernating eggs, and are developed without the fecundating influence of 

 the sperm. 



Among the Crustacea, the genital organs are usually double, and symme- 

 trical in both sexes. Connected with the efferent duct of the female organs are 

 some glands, which secrete a viscid substance, by which the eggs are glued 

 together in clusters to the posterior abdominal feet, as occurs, for the most 

 part, among the Decapoda (lobster, etc.) ; but in those species in which these 

 organs are deficient, there is formed a marsupium, or sort of pouch, connected 

 with the lower surface of the thorax, into which the eggs are received and 

 retained until the young escape. The greater number of the Cirrhipoda are 

 hermaphrodite ; but it has been shown by Goodsir that this is not a uni- 

 versal characteristic of this class. 



Among the different families of the large cla.ss Insecta, the arrangement of 

 the generative organs presents great variety. The sexes are always distinct, 

 and impregnation is invariably effected by copulation ; hence, the external 



