424 MOLLUSCA. 



alarm the ink -gland, which opens with the anus, pours out its 

 black secretion into the water in the mantle-cavity, so that on the 

 retreat of the animal backwards in the rapid manner just explained, 

 the ejected water is black and produces a murky cloud which 

 completely obscures its movements, and under cover of which it 

 escapes. 



Nautilus alone has an external shell secreted by the mantle. It 

 is spirally coiled, the coil being directed on to the anterior face of 

 the animal, i.e., being of the kincf called exogastric ; moreover, it is 

 divided by transverse septa into chambers. The bulk of the animal 



___ _ Ma 



EK " 



FIG. 338. Nautilus pompilius (original drawing made under direction of Graham Kerr). EK 

 penultimate chamber of the shell, which is in some specimens larger than the ante- 

 penultimate ; H hood ; G groove in the hood ; M shell-muscle ; Ma edge of mantle-fold 

 passing at Ma' on to the anterior face of the visceral sac, where it is in relation with 

 the anterior convexity of the shell-coil ; P eye partly covered by the mantle-fold ; Pr.o 

 preocular tentacle; Pt.o postocular tentacle; part of the outer wall of shell which has 

 not been cut away ; On initial coil of the shell ; S siphon ; S/ postseptal neck ; T tentacu- 

 liferous lobes ; Tr funnel. 



occupies only the last of these chambers, but it is incorrect to say 

 that all the chambers except the last are untenanted by the animal, 

 for there is a delicate process of the dorsal end of the hump the 

 siphon or siphuncle, which is coated with a thin layer of calcareous 

 matter, and passes through all the septa right up to the first chamber. 

 The chambers of the shell contain a gas which is said to have 

 approximately the composition of atmospheric air. 



The retractor muscles of the head and foot are inserted on to the 

 internal wall of the shell, and a small anterior flap of the mantle-fold 



