PHYLUM MOLLUSCA 221 



cavity and forced out of the siphon (s) so powerfully that 

 the whole animal is propelled through the water backward. 

 This movement is accelerated and guided by the lateral 

 fins, thus making cephalopods expert swimmers. 



The food of these molluscs consists of small fish and other 

 marine animals. After being captured, the prey is held 

 with the suckers on the arms, torn apart with the parrot- 

 like beak, and ground up somewhat by the radula before 

 passing down to the stomach. On account of the contrac- 

 tions of the mantle while swimming, all the internal organs 

 are subjected to great pressure and food would be regurgi- 

 tated through the mouth if special valves were not present. 

 Valves are also present in the blood-vessels to prevent back- 

 flow, and there are two accessory hearts which pump 

 blood through the plume-like gills. 



All cephalopods have large eyes on the sides of the head. 

 These resemble those of vertebrates in their general plan, 

 but differ markedly in details of structure and in develop- 

 ment. The eyes enable squids and octopi to see with great 

 acuity, and the pursuit of active animals for food is there- 

 fore easy. When a squid or an octopus is set upon by an 

 enemy, a great cloud of ink is squirted into the water through 

 the anus. Its movements are thus obscured, and escape 

 is made possible. 



The breeding habits of cephalopods are very peculiar. 

 A male takes a spermatophore, or packet of sperm, on one 

 of his arms and places it within the mantle cavity of the 

 female. When a female lays her eggs she ruptures the sper- 

 matophore, a great cloud of spermatozoa swarm over them, 

 and fertilization is thus insured. In some species the male 

 breaks off the tip of his arm and leaves it in the female 

 with the spermatophore enclosed. When this arm tip was 

 first discovered by one of the older zoologists, it was de- 

 scribed as a parasitic worm which was believed to live 

 within the female. A female squid or octopus deposits her 

 eggs on the bottom of the ocean or attaches them to a 



