THE PHYLUM MOLLUSCA 



muscular tissues of the mantle. The hig;hly specialized eyes lie near a pair of 

 large optic ganglia, from which optic nerves enter the brain. 



The entire organization of the squid is thus that of an active, prcdaceous 

 animal. Aside from the cartilaginous brain case, the skeleton consists only 

 of a light, proteinaceous, leaf-like pen enclosed within the tissues of the 

 mantle on the apparently dorsal side of the body. This may be contrasted 

 with the more massive calcareous shells of the nautilus and of the extinct 

 tetrabranchiates. When not moving rapidly through the water, the squid 

 maintains a continuous pumping action of the mantle, drawing water into the 

 mantle cavity and expelling it through the siphon. These same pumping 

 movements are utilized in swimming; the animal moves backward or forward 

 with great rapidity, direction being determined by adjustment of the siphon. 

 The fins and tentacles are used as stabilizers and steering devices. In escaping 

 predators, the squid may release a stream of "ink" from the rectum; this 



Cephalic vein 



Anterior aorta 



Median mantle artery 

 Lateral mantle arteries 



Genital vein 



Gonad 



Gill 



Afferent branchial vein 



Efferent branchial vein 



Anterior mantle vein 

 Systemic heart 



Branchial heart 

 Posterior aorta 

 Posterior mantle vein 

 Genital artery 



Fig. 13.19. Major features of the circulatory system of the squid. Vessels carrying oxygenated 

 blood are clear; those carrying blood that has lost its oxygen to the tissues are stippled. Note 

 the branchial heart at the base of each afTerent branchial vein, functioning to increase pressure 

 and velocity of blood traversing the gill. Compare this system with that of a bivalve, shown in 

 Figure 13.3, p. 374. (Adapted from L. W. Williams, Anatomy of the Common Squid, Leyden, 

 1909.) 



391 



