Common Crayfish. 101 



interposed between the shell and the heart, and the terg-a of the 

 two anterior post-abdominal segments. The heart in the Deca- 

 podous, as also in some of the lower Crustacea such as Daphnia, 

 differs in its compressed shape from the vasiform structure which 

 is observable in the other classes of Arthropoda, and indeed in such 

 Crustaceans as Squilla and the Hcdriophthalmata. From the an- 

 terior border of the heart a single artery is given off in the middle 

 line, and supplies the eyes and antennules. On either side of this 

 artery is given off the trunk which supplies the antennae; and 

 finally, from either outer angle of the front border of the heart, an 

 artery passes downwards, between the testicular lobe of either side 

 and the intestine, over the pylorus to the hepatic lobes. A slip of 

 blue paper has been placed under the post-abdominal artery, which, 

 passing immediately beneath the middle dorsal line and above the 

 intestine, may be taken as representing the posterior segments of 

 the forms of heart more usually seen in Arthropoda. The sternal 

 artery, which is larger than the post-abdominal, and is given off 

 from the same short trunk, is not seen in this Preparation. Two 

 venous inlets are seen in the anterior fourth of the upper surface 

 of the heart ; the two laterally placed and the two inferiorly placed 

 are not seen in this view. The cavity in which the heart is lodged 

 has been sometimes called ' a pericardium,' and sometimes an 

 'auricle/ It is formed by a reflection of the membrane which 

 lines the general visceral cavity ; and it receives the l)lood returned 

 from the branchiae, so as functionally at least to rei)resent a bran- 

 chial auricle. Six elastic ligaments, the ' alae cordis' pass from 

 the cardiac aspect of this ' pericardiaP or ' auricular' membrane, to 

 attach themselves to the walls of the heart, their main function 

 being probably to reopen by their recoil the six venous orifices 

 which each systole of the ventricle closes. These ligaments may 

 serve also to suspend the heart in the pericardial sinus, but the 

 arteries which pass off from it are probably in these, as in other 

 animals, the chief means whereby the heart is maintained in situ. 



The five arteries arising along the anterior edge of the heart, 

 and the sort of bulbus arteriosus into which the posterior end of 

 the heart tapers, give it the polygonal ajipearance which distin- 

 guishes the heart of the Decapodous Crustaceans from the similarly 

 unilocndar and non-vasiform heart of the Entomostracous order 

 which gives off no arteries. The anterior edge of the heart corre- 



