xn PHYLUM MOLLUSCA 767 



In the majority there is an ink-gland with a duct opening into 

 the rectum. The ctenidia and kidneys are either two or four in 

 number. The nervous system is highly developed ; and the prin- 

 cipal nerve-ganglia are aggregated together around the oesophagus. 

 The sexes are separate; the segmentation of the ovum is meroblastic, 

 and there is no metamorphosis. 



Sub-Class I. Dibranchiata. 



Cephalopoda in which the main part of the foot assumes the 

 character of a circlet of either eight or ten arms, bearing suckers, 

 and surrounding the mouth. The funnel forms a complete tube. 

 The shell is usually internal ; when external its cavity is not divided 

 by septa. There are two ctenidia, two kidneys, and two branchio- 

 cardiac vessels or auricles. An ink-gland and duct are present. 



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 kidneys, 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 Sepiidcs 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. 



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