3i6 



CHARACTERS OF INVERTEBRATE ANIMALS 



large enough to contain the entire body of the animal. It is 

 mainly secreted by the inner surfaces of two of the arms, which 

 are dilated at their ends into large lappets. By means of these 

 the animal holds on to the shell, which is not attached to it by 

 any muscular or fibrous tissue. 



Sub-class 2 — Pearly Nautilus (Tetrabranchiata) 



The only living representative of this is the Pearly Nautilus, 

 of which the best -known species {Nautilus pompilius) (fig. i8i) 

 has a wide distribution in the Indian and Pacific Oceans. The 



animal is enclosed 

 in a large spiral 

 shell, of which the 

 coiled - up portion 

 projects forwards 

 towards the ani- 

 mal's front end. 

 The body does not 

 occupy all the shell, 

 for a considerable 

 part of this is di- 

 vided into a series 

 of gas - containing 

 chambers by means 

 of curved parti- 

 tions, concave to- 

 wards the external 

 aperture. The last and broadest part iorms a body-chamber in 

 which the animal is contained, the rounded end of its visceral 

 hump resting against the concave surface of the last partition. 

 The shell grows in size by successive additions to its aperture 

 or mouth in accordance with the growth of the animal, and at 

 the same time the older part of it is from time to time cut 

 off by formation of a new partition, the body slipping forwards, 

 as it were, so as to permit of this. The old chambered part 

 of the shell is not, however, entirely devoid of soft parts, for 

 each partition is perforated in the middle by a hole continuous 

 with a short tube directed away from the body-chamber, and in 

 this way a hollow structure know as the siphuncle is constituted. 



Fig. i8i. — Pearly Nautilus [Nautilus pompilius). The left half of its 

 shell has been removed 



