6o2 THE INVERTEBRATA 



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. 41 1 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 Opisthoteuthis ^ 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 TETRABRANCHIATA 



Cephalopoda with well-developed calcareous shells. Living forms 

 with two pairs of ctenidia and kidneys; arms 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. PhylloceraSy 

 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, the protoconch, in this position. Succeeding this 

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

 septa, each one marking a stage in the animal's growth. As the shell 

 is added to, the animal moves forward and from time to time shuts off 

 a space (the chamber) behind it by the secretion of a new septum. The 

 terminal living chamber is much larger than the rest and is occupied 

 by the body of the animal. All the others contain gas (which differs 

 from air in its smaller proportion of oxygen) ; by means of this the 

 heavy shell is buoyed up in the water and the animal can swim freely. 

 The septa are perforated in the middle and traversed by the siphuncle 

 which is a slender tubular prolongation of the visceral hump. It 

 contains blood vessels and probably secretes gas into the chambers to 

 maintain a constant pressure. 



The relations of the different parts of the body in Nautilus are easily 

 compared with those in Sepia (Fig. 402). The shell coils forward over 

 the neck of the animal (exogastric) ; the mantle cavity is posterior as 

 in all cephalopods. In other words differential growth of the vis- 



