1 88 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the return of the blood to the sinus, as well as an 

 irregular number of smaller holes. In the occasionally 

 parasitic copepods (Coiycseus) and the degenerated 

 Cirripedes there is no heart. 



The vessels arising from the heart likewise differ 

 considerably in their arrangement ; in the Entomos- 

 traca there is an anterior artery only, which may branch 



ji a, 



m 



era 



Fig. 82. Diagrams to show the arrangement of the great Blood-vessels 

 in the fresh-water Mussel (c) ; aud the Fish (D). 



more or less at its free end ; but the greater part of 

 the blood makes its way through definite spaces without 

 distinct enclosing walls, the so-called lacunae.* In 

 the Malacostraca a posterior aortic artery is given off, 

 in addition to the anterior; and in the crayfish, for 

 example, we may further distinguish two pairs of 

 anteriorly directed trunks ; the antennary, which 



* Our knowledge of the vascular system of Arthropoda, or 

 Molluscs, is in an unsatisfactory condition; the " lacunar con- 

 nection between the arteries and veins, which is confidently de- 

 scribed and discussed by all zoologists, has never yet been demon- 

 strated to exist in a manner satisfying the requirements of modern 

 histology " (Lankester). 



