CEPHALOPODA. 



55 



sic and Cretaceous rocks. They are commonly called 

 " Ehyucholites," and genera such as Bhynchoteuthis have 

 been founded upon, them (fig. 446). 



The ink-bag is a special gland possessed by the Cuttle- 

 fishes, for the purpose of secreting an inky fluid, which the 

 animal can discharge into the water, so as to enable it to 

 escape when menaced or pursued. The secretion of the 

 ink-bag consists of finely-divided particles of carbon sus- 

 pended in fluid, and it is extremely indestructible. The 

 ink-bag, with its contained secretion, is not uncommonly 

 found in the fossil condition ; but it has only been observed 

 in strata of Secondary age. In the Tetrabranchiate Cepha- 

 lopods, in whicli there is an external shell, and this means 

 of defence is not needed, there is no ink-bag. 



The shell of the Cephalopoda is sometimes external, some- 

 times internal. The internal skeleton is known as the 



Fig. ii6.~Rhiinchoimthis Asticriana. Lower Greensaiid (Cietaceous). 



" cuttle-bone," " sepiostaire," or " pen " {gladius), and may be 

 either corneous or calcareous (fig. 447, a and h). In some 

 cases it is rendered complex by the addition of a chambered 

 portion or " phragmacone," which is to be regarded as a 

 visceral skeleton or " splanchnoskeleton." In Sinrida the 

 phragmacone (fig. 447, c and d) is the sole internal skeleton, 

 and is coiled into a spiral, the coils of which lie in one 

 plane, and are near one another, but not in contact. It 

 thus resembles the shell of the Pearly Nautilus, but it is 

 internal, and differs, therefore, entirely from the external 

 shell of the latter. The only living Cephalopods which are 

 provided with an external shell are the Paper Nautilus 



