Phylum Molliisca and Minor Eucoelomate Phyla 479 



The "skin" of the squid — as well as that of the other cephalopods — 

 has many cells containing pigment (chrotnatophores) some of which are 

 brown, others reddish yellow. When the animal becomes alarmed it 

 changes its color rapidly by pulling these chromatophores into flat plates 

 with tiny muscles that are attached to their edges. 



The body wall is strengthened by an internal chitinous pen which 

 is secreted by the mantle. This corresponds to the shell of other mol- 

 luscs. Within the mantle cavity are the internal organs. 



The squids are carnivorous, predaceous forms which live on worms, 

 fish, and other animals. They have a pair of horny jaws at the anterior 

 portion of the pharynx for crushing their prey. The gills lie free at the 

 posterior portion of the mantle cavity. The water enters the mantle 

 cavity along the sides of the neck. When the mantle is contracted, 

 valves close off the point where the water entered, and the water is 

 forceably ejected through the siphon. Another valve prevents water from 

 entering the siphon ; thus there is a constant flow of water over the 

 gills, with the water entering along the sides of the mantle cavity and 

 leaving through the siphon. 



While the squid most often swims forward through the water by 

 means of its fins, it can also move very rapidly by means of a modified 

 "jet propulsion." For this latter type of movement, water is forceably 

 ejected from the mantle cavity out through the siphon. By pointing the 

 siphon in the direction opposite to which it wants to go, the squid can 

 dart either forward or backward with great rapidity. 



Anterior to the rectum is the ink sac which contains the dark fluid 

 produced by the ink gland. When the animal is disturbed, this ink is 

 discharged in a cloud through the siphon. The squid can back away 

 under cover of this "smoke screen" and thus escape its enemies. It has 

 also been shown that this ink (at least from the octopus) has a slight 

 paralyzing effect on the olfactory senses of would-be predators. 



The circulatory system is well developed with a systemic heart, 

 blood vessels, and sinuses. There are branchial hearts which pump 

 blood through the gills. Paired kidneys are connected with the peri- 

 cardial cavity and drain into the mantle cavity. 



The nervous system of the cephalopods represents the highest de- 

 velopment of this system among all the invertebrates. There is a con- 

 centration of nerve ganglia in the head region to form a simple brain 

 protected by a "brain box." From this, nerves pass to all parts of the 

 body. The speed of conduction is very rapid and as is true in all nerve 

 fibers is directly correlated with the diameter of the individual fiber. Ex- 



