CEPHALOPODA. 45 



Septibranchia. (Fig. 25, d.) 



The members of this order differ from other Pelecypods in case 204. 

 having the gill-plates represented by a muscular septum. They are 

 provided with two respiratory siphons and two adductor muscles, 

 and the edges of the mantle-lobes are connected at three points. 

 The families Verticordiidce, Foromt/idce, Cetoconchidce and CuspidariidcR 

 belong to this order. The species are all small, without colour-mark- 

 ings, are world-wide in their distribution and occur at all depths. 



Class v.— CEPHALOPODA.^^ 



This Class includes the Octopus or Polypus, Cuttlefish, Squid, Cases 205- 

 Spirula, the Paper and Pearly Nautilus. The body of the animal con- waii-case 

 sists of a muscular sac, in the cavity of which the viscera are placed. 

 In front of the body projects the head, which, in species belonging to 

 the two-gilled section of the Class, is surrounded by eight or ten fleshy 

 arms. A wide aperture below the head admits the water to the gills 

 or branchise, which are situated in the interior of the sac, whilst a 

 short tube, the so-called funnel or siphuncle, projects from the open- 



Fig. 39. 



A, the upper, B, the lower beak of Arcliiteutliis monachus; one-third 

 natural size. 



ing of the mantle — the water and various excretions being expelled 

 through this tube, especially also an ink-like fluid, which is dis- 

 charged by nearly all Cephalopods when disturbed, in order to 

 darken the water and thus escape their enemies. The centre of 

 the head, between the bases of the arms, is occupied by the mouth, 

 which is armed with two horny jaws, similar in shape to the 

 beak of a parrot (Fig. 39). In Nautilus the cutting edges are 

 covered with a calcareous deposit. The two large eyes are placed on 

 the sides of the head. The arms or feet are more or less elongate, 

 capable of movement in any direction, and, except in Nautilus, 

 furnished on one side with numerous suckers, by means of which the 



* From tlie Greek : kephale, head, and pous, foot. 



