STRUCTURE OF THE OVUM IN THE INVERTEBRATA. 189 



here limit myself to these few remarks, in which I have 

 endeavoured to point out the principal points of comparison 

 between the ova of these animals and those of the higher 

 classes. I am unable to enter into any details respecting the 

 various forms of the ovaries, and the relations of their several 

 excretory ducts, in the very numerous subdivisions, of the 

 Yermes. 



All the MOLLUSCA possess well-developed glandular organs 

 for the formation of the ova, consisting of numerous acinous 

 follicles, the epithelial cells of which undergo development into 

 ova. A peculiar gland, called the hermaphrodite, is widely 

 distributed through the class, in which, and sometimes even in 

 the same follicles, both ova and seminal corpuscles are found, 

 which, as Eisig (136) has very recently described in the case 

 of the Lymnseus auricularis, proceed from the epithelial cells 

 lining the glandular follicles. Both sexual products are con- 

 sequently discharged through the same excretory duct. In 

 Helix and Limax, so far as I have been able to see, the larger 

 ova that are adherent to the parietes of the acini are separated 

 from the remaining cells by a special thin-walled capsule. I 

 have not been successful in tracing out the precise mode in 

 which these capsules are developed, nor have I been able to 

 demonstrate the development of an epithelium on their inner 

 surface, by the presence of which these capsules are rendered 

 completely analogous to the Graafian follicle of a vertebrate 

 animal. For information respecting the development of the 

 ova in Mollusca, I must refer the reader to the works of 

 Semper (106), Claparede (29), Baudelot (11), Eisig, and others. 



In the ARTHROPODA, which, in my opinion, stand next to the 

 Vertebrata in the development of their female generative 

 organs, structures may be demonstrated that precisely corre- 

 spond to the Graafian follicles. 



Fig. 195 exhibits in a semi-diagrammatic manner a portion 

 of the ovaiian tubes of the vanessa urticae. The narrower 

 portion (a) is the terminal portion of the long tubular ovary that 

 is connected to the dorsal vessel by a delicate ligament (g). 

 Occupying the interior of the extremity of this at a 1 , just as in 

 the ovarian tubes of the Ascaridse, transparent germinal vesicles, 

 with nucleoli, are found imbedded in a diffused and delicate 



