22 



COMPARATIVE ANATOMY 



the posterior mantle cavity there are two or four ctenidia. The heart has two 

 or four auricles, and there are two or four kidneys. Gonad unpaired, with single 

 or paired ducts. The sensory organs are highly developed, especially the eyes, 

 which lie anteriorly and laterally on the "head" (Kopffuss). The jaws and 

 radula are powerful. There is sometimes a shell, either external or internal. An 

 ink-bag is generally present. The Cephalopoda are large, highly-developed marine 

 carnivora. Dioecious. 



Order 1. Tetrabranchia. 



An external chambered shell, the animal inhabiting the last (and largest) 

 chamber. It is symmetrical, and exogastrically coiled. The mouth is surrounded 

 by numerous tentacles without suckers, which rise from large lobes and can be 



Fig. 32.— Nautilus Pompilius, after Owen. Median section of shell, a, Cephalic hood ; 

 6, tentacles; c, infundibuluni (siphon); d, eye ; c. projection caused by nidanrental gland; /, point 

 of attachment of the adductor muscle ; (/, upper portion of the visceral dome ; h, last (inhabited) 

 chamber of the shell ; i, anterior lobe of the mantle ; k, last chamber but one ; I, siphuncle. 



retracted into special sheaths. There are four ctenidia, four auricles, and four 

 kidneys. The siphon consists of two lateral lobes distinct from one another, which 

 by the apposition of their free edges form a tube. Witliout ink-bag. The eyes are 

 simple pits. The only living form is the Nautilus, radula 2.2.1.2.2 (Fig. 32). 

 The two large divisions of this order, Nautiloidea and Ammonitidea, ^ occur as 

 fossils. 



^ The Ammonitidea, owing to the uncertainty concerning their anatomy, are by 

 many authorities arranged in a separate order, "Ammonea," and placed between the 

 other two. 



