XII 



PHYLUM MOLLUSC A 



737 



plete tube. There is an external, spiral, 'chambered shell. There 

 are four ctenidia, four nephridia, and four auricles. The ink-gland 

 is absent. 



This sub-class includes only one living genus, Nautilus, but 

 the Ammonites and other extinct forms are usually referred to it. 



Systematic Position of the Examples. 



The genus Sepia is a member of the family Sepiidm of the order 

 Decapoda, which is distinguished from the seven other families of 

 the order by the combination of the following features : The 

 body is compressed and comparatively broad ; the fins are narrow 

 and elongated ; the internal shell consists almost entirely of cal- 

 careous material. 



Nautilus is the sole living representative of the sub-class Tetra- 

 branchiata. 



3. GENERAL ORGANISATION. 



The uniformity of structure among the Dibranchiate Cephalopoda 

 is very great, and, as already stated, Nautilus is the only living 





PIG. 646. Octopus vulgaris. A, at rest ; B, in motion ; /. funnel, the arrow showing the 



direction of the propelling current through the water. (From Cooke, after Merculiano.) 



member of the Tetrabranchiata, so that comparatively little has to 

 l>e said to supplement the descriptions of these two examples. 



External Features. The general external shape differs very 

 little in the different members of the Dibranchiata : the body in 

 some is more elongated, in others, less ; the degree of compression 



VOL. i 3 B 





