GENERAL ZOOLOGY 



Stomach 



Liver 



Brain 



Buccal mass, 

 jaws, and radula 



Gonad 



Siphon 



Fig. 13.18. The squid, Loligo pealeii: internal anatomy as shown by a sagittal section of the 

 body. (Photoa;raph of a model, courtesy American Museum of Natural History.) 



The circulatory system is remarkably well developed in comparison with 

 that of other mollusks (Fig. 13.19). The large, median systemic heart gives 

 off an anterior and a posterior aorta, whose branches are distributed among 

 the organs. Symmetrically placed anterior and posterior collecting vessels 

 conduct blood from the tissues to a pair of branchial hearts, accessory pulsatile 

 organs lying at the bases of the gills. These hearts propel blood through the 

 vessels of the gills and into the systemic heart for redistribution. The 

 cephalopod vascular system is noteworthy in that it is a closed system with 

 capillaries, in contrast to the open systems of other mollusks. 



The excretory organs are a pair of large, glandular nephridia lying just 

 below the heart. These structures are richly vascularized and apparently re- 

 move wastes from the circulating blood. They open into the mantle cavity at 

 a pair of nephridiopores. 



Squids are dioecious. The large gonad occupies a major portion of the 

 posterior end of the visceral mass, and the gametes are conducted to the out- 

 side through a complex series of ducts with specialized accessory glands. In 

 the male the spermatozoa are formed into bundles, or spermatophores, which 

 at the time of mating are transferred into the mantle cavity of the female. 

 Fertilization occurs in the oviduct, and the zygotes are embedded in finger- 

 like, gelatinous masses which are extruded and attached to the substratum. 

 The egg of the squid contains a large amount of yolk, and the young animal 

 hatches as a miniature adult able to swim and capture food. 



The nervous system of the squid can be correlated with the typical mol- 

 luscan plan, but it is very highly developed and specialized. It centers in a 

 large ganglionic mass, the brain, lying in the head and enclosed by a cartilag- 

 inous endoskeleton. The brain probably represents the basic three pairs of 

 ganglia, as found in other mollusks, but it has been shown that the brain of 

 the squid contains specialized areas devoted to sensory, motor, and associative 

 functions. Nerves radiate from the brain to all parts of the body. One pair 

 of large nerves connects the brain with a pair of stellate ganglia on the inner 

 surface of the mantle, and from these ganglia nerves are distributed to the 



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