CEPHALOPODA. 45 
Septibranchia. (Fig. 25, d.) 
The members of this order differ from other Lamellibranchs 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 Poromyidcz and Cuspidariidce, constitute this order. 
The species are all small, without colour-markings, 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- 
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 branchiae, which are situated in the interior of the sac, whilst a 
short tube, the so-called funnel or siphuncle, projects from the open- 
Fiff. 39. 
208. 
A, the upper, B, the lower beak of Arcliiteuthis 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 all Cephalopods (except Nautilus) when disturbed, in 
order to darken the water and thus escape their enemies. The 
centre of the head, between the base of the arms, is occupied by the 
mouth, which is armed with two horny or calcareous jaws, similar 
to the beak of a parrot (Fig. 39). 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 the Greek : kephale, head, and pons, foot. 
