CEPHALOPODA 525 



the septa forming the oblique calcareous partitions of the cuttle bone, 

 while the other side forms a minute lip in which the septa are crowded 

 together (Fig, 385 D'). Thesiphuncle (p. 526) is a short wide funnel in 

 between. 



In Loligo there is only a horny petty which represents the pro- 

 ostracum, while in the Octopoda there is no skeleton at all. 



The Dibranchiata are specialized in two ways. The first is for a 

 pelagic life; their bodies become elongated, fins develop and they 

 become transparent. They may, exceptionally, develop such speed in 

 the water that they take off from the surface and glide for considerable 

 distances through the air, in the manner of the flying fish, aided by 

 their spreading fins {Todarodes Sagittarius). Loligo (Fig. 386 B) is a 

 well-known example of the pelagic type and may be seen in aquaria 

 swimming in troops, keeping their distances and turning with military 

 precision. 



The second mode of specialization is for a semisedentary life on 

 the bottom. In this the body is short and the arms, which are much 

 larger and more mobile than in the other type, are used for crawling. 

 Octopus (Fig. 386 A) hides itself among stones and seeks its prey only 

 at night. Sepia and Sepiola, though capable of active movement, 

 spend long periods of rest half-covered with sand, assuming by means 

 of chromatophore expansion brown ripple-marking on their mantles. 

 The most sedentary form is the flattened Opisthoteiithis , which is almost 

 radially symmetrical and has a remarkable resemblance to a starfish ; 

 the arms are all joined together and form a suctorial disc by which 

 the animal applies itself to a rock. 



Order TETR ABRANCHI ATA 



Cephalopoda with well-developed calcareous shells. Living forms 

 with two pairs of ctenidia and kidneys ; tentacles very numerous, 

 without suckers ; eye simple ; chromatophores absent ; funnel in 

 two halves. 



Suborder Nautiloidea, with membranous protoconch, central 

 siphuncle and simple suture line, e.g. Nautilus, Orthoceras. 



Suborder Ammonoidea, with calcareous protoconch, marginal 

 siphuncle and usually complicated suture line, e.g. Phylloceras, 

 Baculites. 



A brief description of Nautilus, the only surviving cephalopod with 

 an external chambered shell, must be given here. The shell is coiled 

 in a plane spiral ; the earliest formed portion was membranous and is 

 represented by a small central space. In the ammonoids there is a 

 calcareous chamber, iht protoconch, in this position. Succeeding this 

 are the numerous chambers, separated from each other by the curved 



