MOLLUSCS 105 



The body is enclosed like that of a snail in a loose 

 skinny sac — the mantle — whilst from its lower margin 

 underneath the head protrudes a syphon pipe which can 

 exhale the sea-water vital to its existence, and which is 

 passed in an unending stream over the gills. By " breath- 

 ing hurriedly " the outrush of water can send the animal 

 travelling backwards at a high speed ; whilst when 

 occasion arises it may also be used to pump water over the 

 egg capsules or to hollow out a depression in the sand. 



Reproduction is effected in a most remarkable manner. 

 The male element in most species becomes segregated 

 in a special extension of one tentacle, and this, becoming 

 detached, leads an independent existence until a female 

 is found. This organ was at first mistaken for a new 

 species of animal and was by some early naturalists even 

 described as a worm. The Cephalopods all lay eggs 

 enclosed in membraneous capsules, usually anchored to 

 weeds or rock. They are incubated by the female, who 

 blows water over them until the young — miniature repro- 

 ductions of their parents — emerge. 



In many deep-sea species light organs are present, 

 whilst the ink or sepia, which as such would be valueless 

 where all is already dark, takes the form of a luminous 

 fluid, which when discharged effectually baffles pursuers. 



Highly constituted as they are, Cephalopods show a 

 remarkable degree of intelligence at times. Many kinds 

 are gregarious, whilst others unite to form schools during 

 the migratory periods. 



The class is divided into two orders : the Tetrabranchia — 

 including numerous species of extinct Ammonites, and the 

 living Nautilus — distinguished by having two pairs of 



