228 



METAZOAN PHYLA 



A third cephalopod type is the chambered, or pearly, nautilus (Fig. 

 139), which lives on the bottom of the sea near certain islands in the 

 south Pacific Ocean. The shell is coiled in one plane and is composed of 

 a series of compartments, each representing a chamber which has in the 



Tentcrc/es 



Dor sat/ /obe 



S/joh. 



3/phuncle 



She// masc/e 



A^arrHe 



Parf/Won 

 befweer? chambers 



Fig. 139. — A female chambered nautilus. {From Hertwig and Kingdey, "Manual oj 

 Zoology," after Ludwig and Leunis, by the courtesy of Henry Holt & Company.) The shell 

 is bisected, but the animal is not. 



past been occupied by the animal, but which has been deserted as it 

 and the shell have grown larger. The animal lives in the outermost 

 compartment. The partitions are concave toward the mouth of the 

 shell. These compartments are stated to be filled with gas, which gives 



buoyancy to counteract the weight of 

 the shell. Through the center of 

 them and piercing the successive 

 partitions clear back to the end of 

 the coil is a tube which lodges a 

 cylindrical mass of tissue known as 

 the siphuncle. 



Another type of cephalopod is 

 the paper nautilus (Fig. 140). By 

 means of glands in certain tentacles 

 the female secretes a small and deli- 

 cately coiled shell which is really a 

 basket for the eggs, and which, because 



"Text-book of Zoology:' by the courtesy of ^f j^g method of formation, is not 

 The Macmillan Company.) X about }-'i. , , .,i ,i i n r j.i. 



homologous with the shells ot other 

 mollusks. The animal swims in this shell as if in a boat, propelling itself 

 by means of the tentacles. 



Among the cephalopods are the largest of the invertebrates. The 

 giant squid reaches a total length of over 50 feet, including the tentacles, 



Fig. 140. — Paper nautilus, Argo- 

 nauta argo Linnaeus. Female, swim- 

 ming. {From Claus and Sedgwick, 



