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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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 Yertebrated 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 Mollusca, the circulation 
