318 



Mollusca. 



posterior end of the body, runs through the whole length of the shell.* 

 In some extinct nautiloid forms the shell was straight 

 (Orthoceras) ; in others slightly curved or rolled into an incom- 

 plete spiral; in others it formed a perfect spiral, of which the 

 successive coils did not touch. In the living genus Spirula, one of 

 the decapodous Cephalopods, there is a spiral multilocular 

 shell (Fig. 264 A) like that of the Nautilus, but the individual turns 

 do not touch, and the shell is coiled in the opposite direction, the 

 convexity corresponding to the dorsal surface ; only a small portion 

 of the body is contained in the shell, of which the outer chamber 

 is small, and the shell itself is completely enclosed by folds of skin 

 which have wrapped round and coalesced. In certain extinct 

 decapodous forms there is also a multilocular shell (Fig. 264 B, C), 

 but this is usually straight and drawn out into a plate-like portion 

 anteriorly; in living Decapoda (with the exception of Spirula) 

 this piece is almost the only part of the shell which persists; 

 the posterior spiral, which in extinct forms was concamerated, has 

 become quite rudimentary or is entirely absent. In these forms the 



A B C D E 



Fig. 264. Diagrammatic figures of various Cephalopod shells viewed from the side. 

 A Spirula ; B Spiruliwstra (extinct) ; C Belemnites (extinct) ; in B and C the posterior 

 portion of the shell is prolonged in a strong spiny process ; D Conoteuthis (extinct) ; 

 E Ommatoslrephes (living) . p plate-like portion of the shell. — Orig. 



* It is assumed that the septa are formed by the animal's withdrawing from the 

 outermost as the shell grows at the mouth, and secreting a new septum in front of the 

 last formed ; the cord elongates simultaneously. 



