382 



COMPARATIVE ANATOMY, 



generative gland is, moreover, always unpaired, and is placed, and 

 opens, asymmetrically, so that, as compared with the Lamellibran- 

 chiata, it must have undergone unilateral degeneration. 



The hermaphrodite gland varies greatly in character. It is 



always made up of a 

 number of lobes (Fig. 

 203, A), which form the 

 ovarian germs at their 

 furthermost blind ends 

 (a), while the seminal 

 masses are developed at 

 some distance from the 

 end (/)). These spots are 

 not, however, separated 

 from one another, but 

 the common cavity of a 

 lobule is the spot at 

 which the various pro- 

 ducts are developed. 

 They are, therefore, cells 

 derived from epithelial 

 structures, which become 

 ova at one point, and at 

 another give rise to seminal filaments. This double production does 

 not seem to take place simultaneously, as a rule, so that the same 

 lobule or the same gland produces ova at one time, and sperm at 

 another. 



A differentiation may be made out in the lobules when the 

 ovarian portions form diverticula (B, a) ; in this case they are 

 grouped in a rosette around the seminal portion (b), and have the 

 appearance of secondary acini. The variations in the forms of the 

 hermaphrodite gland are due to the way in which the separate 

 lobules are united together ; thus each lobule may have its proper 

 duct, and the whole gland look like a greatly-ramified oi^gan (Opis- 

 thobranchiata) ; or the acini may open in a row on one side of a duct, 

 as in some Pteropoda (Cymbulia, Tiedemannia) ; or they may be 

 grouped into racemose or lobate masses of glands, of which there 

 may be a number (Phyllirhoe), or they may form a single, more or 

 less compact, gland (some Pteropoda, as Pneumodermon, Hyalea ; 

 most of the Opisthobranchiata and Pulmonata). 



Fig. 203. Follicles of the hermaphrodite 

 glands of Gastropoda. A Of Helix horten- 

 s i s. The ova (aa) are developed on the wall of the 

 follicle; and the seminal masses (b) internally. J5 0f 

 Aeolidia. The seminal portion (b) of a follicle is 

 beset peripherally by ovarian saccules (a), c Com- 

 mon efferent duct. 



§295. 



The efferent ducts may be arranged in one or other of these 

 modes in the hermaphrodite Gastropoda. 



1) There is a common efferent duct for the semen and ova, which 

 represents, therefore, a vas deferens and oviduct, and carries both 

 sets of products from the hermaphrodite gland to the generative 

 orifice. A cascal diverticulum forms the uterus, and also serves for 



