ON THE ANATOMY OP NAUTILUS POMPILIUS. 191 



of the coelom just as does the viacero-pericardial pore into the 

 pericardium, and, like it, is situated raesiad to the kidney- 

 opening. The only striking difference is, that this pore is 

 normally rather farther apart from its corresponding kidney- 

 opening than is the viscero-pericardial pore. The latter is 

 normally quite close to its kidney-opening, but its distance from 

 it is very variable and may reach 3 mm. 



It appears to me that there can be no question as to the 

 homology of the two sets of apertures. In the genital segment, 

 however, the migration of the ccelomic aperture has gone a little 

 further beyond the bounds of the kidney-sac. Each ccelomic 

 duct, plus its kidney-sac, would on this view correspond to an 

 ordinary " nephridium," i.e., a tube leading from the coelom to 

 the exterior, part of the wall of which has taken on an excretory 

 function. In the Dibranchs, in correlation with the disappear- 

 ance of the anterior gill, the corresponding kidney-sac has 

 disappeared, while its ccelomic duct persists as the genital duct. 

 The genital ducts of the Cephalopoda in general then are 

 nephridia", minus their excretory sacs. 



III. The Male Genital Ducts and Penis. 



The general disposition of the genital apparatus in the male 

 is shown in fig. 3 (p. 192). As is well known, only the duct of 

 the right side is functional in Nautilus. On the left side there 

 is the "pyriform sac" of Owen, shown by Lankester and Bourne 

 to represent the left genital duct, although the question was 

 left open by them — whether it represented only the genital 

 duct, or the genital duct together with the genital gland of the 

 same side-. 



From the large ccelomic aperture the genital duct passes 

 through the quadrangular " accessory gland " composed of 



1 Pelseneer asserts that the genital ducts of Cephalopods are nephridia— 

 without, however, qualifying his statement or supporting it by evidence. 



2 That it represents only the duct appears to me to be shown by the condi- 

 tion in the very young animal, in which the inner part of the genital duct has 

 exactly the appearance of the pyriform sac in the adult— the rudiment of the 

 gonad being quite distinct and apparently median and unpaired. 



17—2 



