340 MONCECIOUS MOLLUSCA. 



opinions of spontaneous generations, this no one has ven- 

 tured to apply to any of the class of Mollusks, which are 

 too well known in this respect to be longer the subjects of 

 the dreams of speculative experimentalists. 



Indeed, you will find in no class of animals more variety 

 or curious complexity of the generative system than in the 

 Mollusca ; and it is a subject which has greatly exercised 

 the comparative anatomist. It would, however, be impos- 

 sible to give you a correct view of this structure and its 

 numerous variations, without entering into details incon- 

 sistent with my plan ; and I mean in consequence to confine 

 myself very much to such facts as have reference to function 

 — to their manner of depositing and protecting their ova 

 and young, and to some phenomena exhibited by these in 

 their developement. 



In order to give some arrangement to our facts we may 

 divide molluscous animals into (1) The Monoecious, in which 

 there is no distinction of sex, but where every individual 

 of the species has the same structure of its reproductive 

 organs ; (2) The Dioecious, in which the sexes are distinct 

 as in the higher animals ; and (3) The Hermaphroditical or 

 Androgynous, where each individual has the male and female 

 parts conjoined in its person, but to whose propagation a 

 sexual union is nevertheless necessary. 



I. MONOECIOUS MOLLUSCA. 



The Tunicata, with a few exceptions, are monoecious. 

 In the genera which are permanently fixed to their sites, the 

 ovum, previous to its expulsion from the matrix, has under- 

 gone such a degree of developement, that, on its birth, it 

 has the form and semblance of a minute tadpole, and is 

 endowed with very considerable locomotive powers. (Fig. 

 69.) We are indebted to M. Sars, * and to Audouin and 

 Milne-Edwards for the discovery of this singular larva 

 in the compound Tunicata ; -j- and Sir John G. Dalyell has 

 shown that the larva of the single species is precisely similar. 

 He calls it a spinula from its peculiar shape ; and he tells us 

 that it has the strongest resemblance to the tadpole, not 

 merely in figure, but also in motion. " A large head, almost 

 opaque, with a black internal speck, declines into an atten- 

 uated flattened tail, with indistinct indications of segments 

 and fins, or cilia. It wriggles through the water chiefly by 



* Beskriv. 69, pi. 12, fig. 34. 

 t Litt. de la Fiance, i. 72, 73. 



