526 THE INVERTEBRATA 



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 



Fig. 386. 



External appearance of A, Octopus, B, a squid (Loltgo), 

 trate the difference in manner of life. /. funnel. 



to illus- 



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 



