XII 



PHYLUM MOLLUSC A 



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

 vare four ctenidia, four nephridia, and four auricles. The ink-gl.-md 

 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 Sepiidce 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 



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

 ' direction of the propelling current through the water. (From Cooke, after Mci 



member of the Tetrabranchiata, so that comparatively little has to 

 be 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 



