REL\TTONS OF KIDNEYS IN HALIOTIS TUBERCULATA. 85 



spermatic duct (s. d.) running to the penis along the floor of 

 the mantle cavity (fig. 10) or situated internally (6g. 8) has 

 probably been formed by the covering in of such a groove. A 

 groove of this kind is found in the male Tanganyikia, but the 

 penis is absent (fig. 5, h). 



From these facts Mr. Moore has argued that the common 

 ancestor of Tsenioglossa and Opisthobranchs possessed some 

 accessory reproductive organ which had probably become 

 separated from the genital duct and remained connected 

 with the genital opening by means of a groove, which tended 

 to become covered over. This accessory reproductive organ 

 was somewhat variable, as it is lost in all TaBnioglossate 

 females except the two named, though the groove is retained 

 in a few. In Opisthobranchs, which are originally female, 

 and in male T^nioglossa this organ becomes the penis, while 

 the groove is very often covered over and thus transformed 

 into a duct. The development of such an accessory repro- 

 ductive organ de novo is a difficulty further enhanced by the 

 presence of the groove, but it is still premature, perhaps, even 

 to suggest that possibly its ancestor is the anterior lobe of 

 the Diotocard right kidney. It is interesting to note that 

 Ty phobia horei has a penis (fig. 7 p) which is extruded 

 apparently via the genital aperture, and which is placed as an 

 anterior dilatation on the reproductive duct. 



To sum up, it will be most appropriate to give a brief 

 statement of the views of previous workers and of the chief 

 points raised in this paper. 



The first theory is that of Professor Ray Lankester. He 

 believed that the excretory aperture of Monotocards is the left 

 kidney opening of Diotocard s, but he thought also, from its 

 position with regard to the rectum, that the Monotocard 

 kidney (7/ was the Diotocard left (7 l). Since these views 

 were stated, the supposed absence of a pericardial pore of 

 the right kidney has been used as evidence of the degeneracy 

 of this organ, and, consequently, in favour of homologising 

 the Monotocard kidney with the Diotocard left. Erlanger's 

 work on the development of Palndina is also quoted in 



