i III. FORMATION OF THE ORGANS — THK GENITAL ORGANS. 228 



fn cases in which, finally, the penis becomes shifted towards the 

 female genital aperture, and the two join to form a common atrium, as 

 in the Stylommatophora (Fig. [04 A'), the rudiment of the ectodermal 

 parts form from a common rudiment. In these cases, we have only 

 t" distinguish between the mesodermal rudiment of the hermaphro- 

 dite gland (or the hermaphrodite organ) and the ectodermal rudiment 

 of the primary and secondary ducts and copulatory apparatus. 



The significance of these processes is still little understood, and it is doubt- 

 ful if ontogeny will throw much light upon the subject. ' Summaries and 

 critical descriptions of these ontogenetic processes are given by Rouzaud, 

 Brock, Sempeb (No. 1171, Schiemenz (No. 107), and Klotz (No. 54). 



a. 



<J3. 



C. 



-tn>. 



Fig. I'M. .I-/-;, diagrams illustrating the manner in which the genital apparatus opens 

 out at different ontogenetic stages. .1. in a dioecious Gastropod ; B-M, in herma- 

 phrodite Gastropods, os, ovoseminal duct ; //. penis; vd, vas deferens; 5 and J 

 genital apertures or the terminal portions of the corresponding efferent ducts. 



[The interpretation of the complicated conditions met with in the herma- 

 phrodite genitalia of the Opisthohranchia and Pulmonata is one of those 

 difficult problems upon which ontogeny throws little light. We think there 

 can be little doubt, that it will be found more profitable to leave the onto- 

 genetic side alone and accept the obvious and comparatively simple interpre- 

 tation offered by the study of tbe comparative anatomy of these organs. 



In existing Gastropoda, we seem to have every stage in the development of 

 the secondary genital ducts preserved to us, so that, starting from the simple 

 condition of the Diotocardia, where the gonads discharge by the still functional 

 right kidney, we pass to the Monotocardia. where the right kidney has lost 

 its excretory function and serves solely to transmit the genital products, and 

 where also we find a secondary duct appearing in the male in the form of a 

 groove leading forward to a non-introvertible penis. A condition similar to 

 this is found in many of the hermaphroditic Tectibranchia, which, how- 

 ever, generally show an albumen-gland and a sperinetheca, while the open 



