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MANUAL OF ZOOLOGY. 



which are nearly of equal size. Finally, attached to the first seg- 

 ment of the head are the eyes, each of which is borne upon an 

 eye-stalk formed by the protopodite. The gill-chamber is formed 

 by a great prolongation downwards of the pleurae of the thoracic 

 segments, and the gills are attached to the bases of the legs. 



As regards the digestive system of the Crustacea, the ali- 

 mentary canal is, with few exceptions, continued straight from 

 the mouth to the aperture of the anus. There are no salivary 

 glands, but a large and well-developed liver is usually present. 

 A heart is generally, but not always, present. In most of the 

 lower forms it is a long vasiform tube, very like the " dorsal 

 vessel " of Insects. In the higher Crustaceans, the course of 

 the circulation is as follows (fig. 138) : The heart is a muscular 

 sac, situated dorsally, beneath the carapace, and it gives origin 



Fig. 138. Diagram of the circulation of the Lobster. The systemic arteries are 

 shaded longitudinally, the veins are dotted, and the branchial vessels are black. 

 h Heart; a a Systemic arteries; bb Branchial vessels; cc Venous sinuses; gg 

 Branchiae ; p Pericardium. 



to six arterial trunks, which convey the aerated blood to all 

 parts of the body. The terminations of the arteries open into 

 a series of irregular venous sinuses, whence the blood is 

 collected into a principal ventral sinus, and distributed to the 

 branchiae, where it undergoes aeration. From the gills the now 

 aerated blood is carried by a series of branchial vessels to a 

 large sac, which is badly termed the " pericardium," and which 

 envelops and surrounds the heart. The arterial blood gains 

 access to the cavity of the heart by means of six pairs of 

 valvular fissures, which allow of the ingress of the blood, but 

 prevent regurgitation. A portion of the venous blood, how- 

 ever, is not sent to the branchiae, but is returned directly, 

 without aeration, to the pericardium ; so that the heart finally 

 distributes to the body a mixture of venous and arterial blood. 



