BLOOD AND ITS CIRCULATION 47 



open into comparatively large spaces prevents the development of 

 high pressures. In the lobster the pressure in the main artery when 

 the heart contracts is only 7-35 mm. of mercury; in man the 

 corresponding figure would be 1 20. 



The rate of beating of the crustacean heart is remarkably variable 

 (Table 2), not only in different kinds of Crustacea, but within one 

 species. Sometimes, as in Daphnia, the heart can stop for short 

 periods with no great harm. Many external factors influence the 

 rate of beat, producing effects that are quite different from any- 

 thing in the mammalian heart. The hearts of some of the trans- 

 parent Crustacea beat quicker in response to an increase in light 

 intensity, and slow down when the light is reduced. An increase 

 in temperature has a similar effect to an increase in light intensity. 

 The rate of beat may be doubled by a rise of io° C. These effects of 

 light and heat show that the crustacean heart is much more at the 

 mercy of the elements than the heart of a mammal, which is con- 

 cealed beneath an opaque outer covering, and is housed in a body 

 which maintains a fairly constant temperature. 



Table 2 



Rate of heart beat in various Crustacea. The figures are very 

 approximate, and are subject to variation with the size of the 

 individuals. 



(Data extracted from Schwartzkopff, 1955) 



Temperature Beats per 



