*90 ZOOLOGY 



Order 1. — Decapoda. 



Dibranchiata possessing ten arms, with stalked suckers provided 

 with horny rims, and with a well-developed internal shell. 



This order includes the Cuttle-fishes, Squids, Spirula, and others, 

 as well as the extinct Belemnites. 



Order 2. — Octopoda. 



Dibranchiata provided with eight arms, the suckers on which are 

 sessile and devoid of horny rims : with or without slight vestiges 

 of an internal shell. An external shell, secreted by a specially- 

 modified pair of arms, is present in the female Argonaut only. 



This order includes the Octopods and the Argonauts. 



Sub-Class II.— Tetrabranchiata. 



Cephalopoda in which the main part of the foot has the 

 character of lobes bearing numerous tentacles. The funnel does 

 not form a complete tube. There is an external, spiral, chambered 

 shell. There are four ctenidia, four nephridia, and four auricles. 

 An 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 Sepiidos 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 calcareous 

 material. 



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

 branchiata. 



3. General Organisation. 



The uniformity of structure among the Dibranchiate Cephal- 

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

 living member of the Tetrabranchiata, so that comparatively little 

 has to be said to supplement the descriptions of the 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 

 likewise varies. Fins may be absent, and the animal may pro- 



