THE NAUTILUS. 



101 



anterior end of the outer gill and the posterior base of the palpi. 

 However, this gill is not actually connected with the palpi, but its 

 anterior end begins immediately behind the posterior end of the 

 palpi. 



Outer lamina of outer gills entirely connected with the mantle; 

 inner lamina of inner gill entirely connected with abdominal sac. 

 Posteriorly to the foot, the two inner laminae of the inner gills are 

 entirely connected, thus forming a diaphragm completely separating 

 the suprabranchial canals from the branchial cavity. This gill- 



FIGURE 1. 



.a 



Anatomy of Hyridella australis (Lamark) (female). Side view of soft parts 

 after removal of left half of mantle, a, anal opening; b, branchial opening; 

 m, solid mantle connection between anal and branchial opening; x, hole by 

 which the cloacal and branchial chambers communicate. 



diaphragm does not extend entirely to the posterior margin of the 

 mantle, and the posterior part of the diaphragm is formed by the 

 bridge (m) which unites the two mantle margins and separates the 

 anal from the branchial opening. But there is a small median hole 

 (x) between the posterior end of the gills and this bridge, connecting 

 the cloacal cavity (and anal opening} with the branchial cavity. 



In all four gills of the male the two laminae are connected by 

 very faint, distant, and often incomplete and interrupted septa, run- 

 ning in the direction of the gill-filaments. The outer gill of the 

 female, and the most anterior and most posterior parts of the inner 

 gill have the same structure; the rest of the inner gill of the female 

 (its larger middle part) has marsupial structure (see figure), with 

 strongly developed interlamellar connections. In the sterile female 

 at hand, the solid parts are slightly elongated in the direction of the 



