250 CIRCULATION IN INVERTEBRATED CLASSES. 



289. Among the Invertebrated classes generally, the condi- 

 tion of the circulating apparatus differs from that which prevails 



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ap av ap 



Fig. 138. THE SAME, IN THE PERFECT ANIMAL. 



throughout the Vertebrata, in one remarkable feature; 

 namely, that whereas in the latter the blood moves in every 

 part of its course through a set of closed vessels, it meanders 

 in the former through a set of channels or sinuses excavated 

 in the substance of the tissues, and communicating with the 

 " general cavity of the body " in the midst of which the viscera 

 lie. Generally speaking, it is in the venous system that the 

 greatest deficiency exists ; for the heart usually sends forth 

 the blood by definite arterial trunks, which distribute it by 

 its ramifications through the substance of the various parts of 

 the body ; and it is in its course from these to the respiratory 

 organs that it is least restrained within definite boundaries. 

 The degree of this imperfection differs considerably in the 

 several groups of Invertebrata ; for whilst, in the highest Mol- 

 lusca and Articulata, the vascular system is almost as complete 

 as in Vertebrated animals, we find it gradually becoming less 

 and less distinct as we descend, so that in the lower forms of 

 both series it presents itself merely as an extension of the 

 general cavity of the body, and is not furnished with any 

 special organ of impulsion. 



290. In the greater part of the MOLLUSC A, the circulation 



