456 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



pliunculated shell. The terminal chamber is much larger than 

 any of the rest, and the body of the animal can be almost 

 completely retracted into it. When, as in the only existing 

 genus, JVautilus^ {Fig. 130), the shell is coiled into a flat, 

 symmetrical spiral, its apex lies on the anterior face of the 

 body, and the outermost chamber, into which the whole body 

 can be retracted, is consequently posterior to the axis of the 

 helix. In JVautilus, the brachial processes are short, and pos- 

 sess no acetabula such as exist in the Dibranchiata, but the 

 margins of the foot are produced externally into a sort of 

 sheath, which, in front, has the form of a broad hood with a 

 tuberculated surface ; while, at the sides, it is divided into 

 many processes of unequal lengths. Behind, the halves of 

 the sheath are separated throughout the greater part of their 

 length by a wide interval, but are united above by a thick 



ntx 



Fig 



c\e\ ch, black part of the shell under the mantle p' ; kn, process of the cartilagi- 

 nous skeleton into the funnel. (After Keferstein.) 



muscular isthmus. The central portion of the sheath is a 

 broad, triangular, hood-like plate, the apex of which is free. 

 It contains two long, narrow cavities, each of which lodges a 

 tentacle. The tentacle consists of a slender stem, on which 



1 Owen, " Memoir on the Pearlv Nautilus," 1832. Van derHoeven, " An- 

 nales des Sciences Naturelles," 1856. Keferstein in Bronn's " Klassen u. 

 Ordnungen." 



