CEPHALOPODA 513 



The Cephalopoda fall into two groups, in one of which (Tetra- 

 branchiata) there are two pairs of ctenidia and a well-developed 

 external shell, while the members of the other (Dibranchiata) have 

 one pair of ctenidia and either one internal shell or none at all. Of the 

 Tetrabranchiata Nautilus is the only living member; of the Dibran- 

 chiata, Sepia, a common form in the Mediterranean and elsewhere, is 

 a convenient type. The organization of the group w411 best be under- 

 stood from a description of these examples. As Sepia is the more 

 easily obtained we shall describe it first and in more detail, though it 

 is in some respects less primitive than Nautilus. 



Order DIBRANCHIATA 



Cephalopoda with a single pair of ctenidia and kidneys ; shell in- 

 ternal, enveloped by the mantle and in various degrees of re- 

 duction; 8-10 tentacles; the two halves of the funnel only 

 seen in the embryo ; chromatophores present ; eyes of complex 

 structure. 



Classification 



Suborder Decapoda. Dibranchs with ten tentacles and with a 

 well-developed coelom. Internal shell consisting of phrag- 

 mocone, rostrum and proostracum or very much simplified, 

 (i) Tribe Belemnoidea. Fossils from Mesozoic rocks which 

 have given rise to the following tribes : 



(2) Tribe Myopsida. Decapoda with specially modified 4th 



tentacles; eyes with a cornea, internal shell sometimes with 

 rudiments of calcareous chambers. Spirula, Sepia^ Sepiola, 

 Loligo. 



(3) Tribe Oegopsida. Decapoda with anterior chamber of eye 



open; tentacles usually all alike; suckers often modified to 

 form hooks; shell only represented by a horny gladius; 

 strong swimmers. Includes many abyssal forms with 

 phosphorescent organs; some gigantic forms, like Archi- 

 teuthis, 60 feet long. 

 Suborder Octopoda. Dibranchs with eight tentacles and a reduced 

 coelom. Octopus, Argonauta, Opisthoteuthis . 



Sepia officinalis is a shallow-water form, in which the shell has 

 become internal. The general disposition of the organs remains much 

 as it would be if the animal inhabited the last chamber of a shell like 

 that of Nautilus (cf. Fig. 377 A and B). The whole body is cylindrical. 

 At one end, which would have projected from the shell, is the head 

 with the mouth in the centre and the two relatively enormous eyes at 



