16 GILBERT C. BOURNE. 



and the pericardium is clearly shown in fig. 24, and figs. 23 

 to 25 show that the lower end of the canal is, in fact, con- 

 tinuous with the gonaduct, and opens along with it into the 

 kidney, close to the external orifice of the latter. It should 

 be noted as a peculiar feature in Incisura that there is no 

 distinct duct to the right kidney; its simple slit-like opening 

 into the mantle-cavity is a Pectinibranch character. 



The gonad, in both sexes, is a simple tubular sti-ucture lying 

 to the left side of the stomach, and in the case of the ovary 

 partly embracing this organ. The anteiior end of the gonad 

 extends as far forward as the posterior limit of the mantle- 

 cavity and ends blindly below the first bend of the rectum. 

 The cavity of the ovary, in all my specimens, is filled with ova 

 in all stages of development, the ripe ova being very large 

 relativel}' to the size of the animal, and abundantly supplied 

 with yolk-granules. The testis, in the single male I have 

 been able to examine, is very small, and I think the individual 

 must have been a spent one, as the cavity of the testis only 

 contained a few free spermatozoa and I could find no trace of 

 spermatogenesis. 



The course of the gonaduct and its connection with the 

 right kidney has been correctly but all too briefly described 

 and insufficiently figured by Pelseneer. He only says of it : 

 *'La glande genitale est unique et occupe le soinmet de la 

 masse viscerale. Elle n'a pas d'orifice exterieure ; son con- 

 duit ai-rive au rein droit." But it would be difficult for any- 

 body to guess the course of the gonaduct before its arrival 

 at the right kidney by an inspection of his fig. 115, perfectly 

 correct as it is. As shown in fig. 4 the ovary, which in 

 the more anterior and broader part of the visceral mass 

 is on the left side of the stomach, extends into the narrow 

 commencement of the terminal whorl of the spire, and here 

 its posterior end is produced from left to right into a 

 fairly spacious thin-walled sac which lies between the upper 

 and lower of the two posterior lobes of the liver extend- 

 ing into the spire. The walls of this sac are not lined by a 

 germinal epithelium but its cavity often contains a ripe ovum. 



