736 ZOOLOGY SECT. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Cephalopoda are bilaterally symmetrical Mollusca, which 

 have the propodium displaced forwards to the neighbourhood of 

 the mouth, and divided into a series of arms bearing suckers, or of 

 lobes bearing tentacles, while the metapodium forms a funnel for 

 the egress of water from the mantle-cavity. The visceral mass is 

 symmetrical and not coiled. The mantle encloses posteriorly and 

 ventrally a large mantle-cavity, in which are situated the ctenidia 

 and the nephridial, reproductive, and anal apertures. The shell 

 may be absent or rudimentary ; when present and well developed, 

 it may be internal or external, undivided or divided internally by 

 septa into a series of chambers. There is an internal cartilaginous 

 skeleton, supporting and protecting the nerve-centres, and giving 

 attachment to muscles. The mouth is provided with a pair of horny 

 jaws, and an odontophore is present. In the majority there is an 

 ink-gland with a duct opening into the rectum. The ctenidia and 

 nephridia are either two or four in number. The nervous 

 system is highly developed ; and the principal 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 propodium assumes the character of 

 a circlet of either eight or ten arms bearing suckers, 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 nephridia, and two branchio-cardiac 

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



ORDER 1. DECAPOD A. 



Dibranchiata possessing ten arms, with stalked suckers, provided 

 \vith 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 Octopi and the Argonauts. 



Sub-Class II. Tetrabranchiata. 



Cephalopoda, in which the propodium has the character of lobes 

 bearing numerous tentacles. The funnel does not form a com- 



