46 A B I o L O G Y O F CRUSTACEA 



must eventually be pushed back into the heart. In order for the 

 crustacean heart to suck in blood it must have a mechanism for 

 actively expanding after it has contracted. This is provided in two 

 ways. The elasticity of the heart tends to return it to its normal 

 shape, and, of more importance, the heart is connected to the hard 

 outer covering of the body by elastic fibres. When the heart con- 



Fig. 26. Diagrams showing the heart and main arteries in certain 

 Crustacea. A, a calanoid copepod. B, Daphnia. C, a myodocopan 

 ostracod. D, an isopod, Ligia. E, an amphipod, Gammarus. F, 



drawn t( 



a crab, Carcinus. The outlines are not all 



Gammarus. 

 to the same scale. 



tracts these fibres are stretched, and their elasticity pulls the heart 

 back to its shape before contraction, filling it with blood from the 

 pericardial cavity. Valves at the beginnings of the arteries prevent 

 blood from flowing back into the heart from the arteries. 



The circulatory system of a crustacean is obviously not one in 

 which any great pressure can be developed. The fact that the arteries 



