40 THB NAUTILUS. 



The following are the essential characters of the anatomy. 



Anal opening closed above by the union of the inner edges of the 

 mantle, without leaving a supraanal opening. The upper, closed 

 portion is rather long, over 1^ times as long as the anal opening. 

 The latter is rather large, about as large as the branchial opening, 

 separated from the branchial opening by the solid union of the inner 

 mantle edges. Inner mantle edge of anal opening smooth, that of the 

 branchial opening with rather small papillae. In front of the 

 branchial opening, the inner mantle edges are unconnected and 

 smooth. 



Palpi longer than wide, with curved lower edge, posteriorly with 

 a short truncation, but not produced into a point, and not connected. 



Gills long and narrow, the inner the wider. Outer gill becoming 

 gradually lower anteriorly, and ending at a point behind and above 

 the posterior end of the palpi (at the highest point of the mantle 

 attachment line). Inner gill hardly lower anteriorly, and its 

 anterior insertion fills the entire space between the anterior end of the 

 outer gill and the palpi, so that there is no gap between the posterior 

 end of the inner palpus and the anterior end of the inner gill. Edge 

 of inner gill with a longitudinal furrow, that of the outer gill without 

 this furrow. Outer lamina of outer gill entirely connected with the 

 mantle. Inner lamina of inner gill free from the abdominal sac, 

 except at its anterior end; posteriorly, behind the foot, the two inner 

 laminae of the inner gills are entirely connected, thus forming the 

 larger anterior part of the diaphragm. AH" four gills /a^Z short of the 

 posterior mantle edge, and the posterior continuation of the diaphragm 

 is formed by the bridge %miting the mantle edges, which, from the 

 mantle edge, extends to a certain distance forward (or inward). 

 This bridge, the mantle part of the diaphragm, is rather short; in the 

 dissected female, the soft parts of which are about 68 mm. long, it is 

 4 mm. long, while the gill part of the diaphragm is much longer, 

 about 29 mm. 



Water tubes of both gills well developed, rather regular and quite 

 distinct from base to edge of gill, but rather wide, and the septa are 

 remote from each other. The latter are heavy and strong; in the 

 male they are similarly developed in both gills, and represent simple 

 lines of union of the tissue of the two laminae, generally with a large 

 blood vessel in the middle. In the female, the structure of the outer 

 gill is identical with that of the male; but the inner gill alone is 



