94 SURVEY OF THE REPRODUCTIVE PROCESS 



Mollusca, it may be remarked, do some times, though 

 rarely, occur of parasitic habits, as, for instance, Stylifer 

 astericola. 



Of the gamomorphic phase of the life history of Mol- 

 lusca, little need here be said. Alternation, as has been 

 remarked, is unknown, except the presumed parasite of the 

 Synapta may furnish an example. Hermaphroditism is not 

 uncommon among the Mollusca, but self-impregnation must 

 be rare, if it occurs at all. In some cases, indeed, it is evi- 

 dently made impossible by the conformation of the parts, 

 and in others by the sexual elements not ripening simul- 

 taneously in the same individual. 



8. REPRODUCTION IN THE HELMINTHA. 



Among the lower vermiform tribes, the Helmintha are of 

 especial interest for characteristic examples of the leading 

 modifications of alternation, and for the peculiar relations 

 of the sexes, which are here more closely associated than 

 perhaps anywhere else in the animal kingdom. The species 

 are frequently not only bisexual, but self-impregnating, and 

 in some the eggs may be said to be impregnated in the very 

 act of formation, the germinal vesicles, vitelline matter, and 

 spermatozoa being first commingled, before they become in- 

 vested with a vitelline membrane to form the ovum.* 



* It is perhaps more correct to regard the so-called germinal vesicles with 

 their granular investments as ova which have not acquired a proper wall, 

 or in which, as in those of the bird, the original ovum-wall has had but a 

 temporary existence, and to consider the matter from the so-called "vitel- 

 ligenous organ," added in conjunction with the spermatozoa, as a sort of 

 adventitious or food-yolk. See Siebold's Compar. Anat., I., 115. 

 Huxley in Medical Times, XIII., 133-131. Claparede (Ann. of Nat. 

 Hist., 2d Ser., XVIII., 298 N.) Thomson in Cyclopaedia of Anat. and 

 Physiol. Supp. [120] (Ovum). In Pelodytes hermaphroditus, according 

 to Schneider, in the same generative tube spermatozoids first make their 

 appearance, and then eggs, and fecundation is effected at once. Annals 

 Nat. Hist., 3d Ser., V., 506. 



