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CHAPTER XLII. 



DIVISIONS OF THE CEPHALOPODA. 

 Order I. Tetrabranchiata. 



The Tetrabranchiate Cephalopods are distinguished by the posses- 

 sion of an external many -chambered shell, the last and largest chamber 

 of which lodges the body of the animal. The chambers are separated 

 by calcareous partitions or " septa" perforated by a membranous tube 

 or " siphuncle." The head is furnished with numerous fleshy " ten- 

 tacles" which are not provided with suckers ; the branchice are four 

 in number, two on each side of the body ; the funnel does not form a 

 complete tube ; and there is no ink-bag. 



This order of the Cephalopods is represented at the present day 

 by the single genus Nautilus, with four living species, and we may 

 take the well-known Pearly Nautilus (N pompilius) as the type of 

 the division. The body of the Pearly Nautilus consists (fig. 738) 

 of a posterior visceral mass and an anterior cephalic region, the 

 whole being contained in the capacious outermost chamber (body- 

 chamber) of the shell, from which the head can be protruded at 

 will. The shell is rolled up into a flat spiral, and the position of 

 the animal is such that the " funnel " (fig. 738,/) is turned towards 

 the convex side of the shell, which is thus the ventral side, the 

 concave side being dorsal. The visceral mass is included in the 

 mantle, which is thin behind, but becomes thicker in front, 

 being reflected on the dorsal side as a kind of collar over the con- 

 vexity of the preceding whorl, which it invests with a black shelly 

 deposit. The animal is attached to the shell by a double shell- 

 muscle, while the posterior extremity gives off a membranous tube 

 or " siphuncle," which pierces the successive septa centrally, and is 

 continued backwards through the entire series of air-chambers. 



The front portion of the "foot" surrounds the animal's mouth, 

 and is furnished with a number of cylindrical, retractile " tentacles," 

 which are not provided with suckers. Dorsally a portion of the 



