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[VOL. I. 



as are also the nephridia. The gills, moreover, are not situ- 

 ated upon the body wall, as in other Cephalopoda, but upon 

 the inner side of the mantle-fold, near its junction with the 

 body. If the mantle be split along the mid-ventral line it is 

 plain that the gills hang from the mantle, while the sacs of 

 the nephridia lie wholly within the mantle-fold ; a small por- 

 tion of the posterior venous appendages hanging into the body 

 cavity back of the mantle. The heart lies just back of the 

 mantle-fold. The branchial veins run for some distance in the 

 mantle-fold. The position of the venous appendages within 



FIG. 5. The lines of attachment of the mantle to the shell spread upon a flat surface, i. Dorsal 

 aponeurotic band. 2. End of shell-muscle. 3. Anterior ventral aponeurotic band. 4. Posterior 

 ventral aponeurotic band. 5. Position of siphuncle. 



the mantle-fold necessarily causes the posterior portion of the 

 mantle to be very much thickened. Owing to this position 

 of these organs the posterior part of the mantle-fold has also 

 lost its musculature to a great extent. When the mantle is 

 reverted these parts sink into the body cavity, and the gills 

 appear to be situated upon the body wall. It is probably due 

 to these conditions that previous observers have failed to see 

 the true relations of the parts noted. The gills are, however, 

 plainly situated upon the mantle, since they are at the sides 

 of the nephridia, where the mantle is thin and very muscular. 



