588 THE INVERTEBRATA 



funnel or siphon, a muscular organ, originally bilobed, used for the 

 expulsion of water from the mantle cavity; one or two pairs of typical 

 ctenidia; coelom sometimes exceedingly well developed, the genital 

 part being continuous with the pericardium; typically a chambered 

 shell in the last chamber of which the animal lives, though in most 

 modern representatives it is reduced and internal or wholly absent; 

 nervous system greatly centralized and eyes of great size and often 

 complex type; eggs heavily yolked and development direct. 



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

 branchiata) there are two pairs of ctenidia and a w^ell-developed 

 external shell, w'hile 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 will best be 

 understood 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 many 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 wdth 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 Sepioidea. Decapoda with specially modified 4th 



pair of tentacles which can be retracted into pits; eyes with 

 a cornea, internal shell sometimes with phragmocone bent 

 ventrally: fins not united posteriorly; shore and bottom 

 living forms. Spirula, Sepia, Sepiola. 



(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 wdth 

 phosphorescent organs; some gigantic forms, like Archi- 

 teuthis, 60 feet long. 



