48 ABIOLOGYOFCRUSTACEA 



As well as being influenced by external factors the crustacean 

 heart is also controlled by an internal mechanism. The analysis of 

 this mechanism has provided some surprises. A comparison with 

 the mammalian heart will illustrate these. The rhythm of a mam- 

 malian heart originates in a patch of muscular tissue in the heart, 

 and spreads over the rest of the heart from this pacemaker. The 

 heart continues beating with a steady rhythm when all the nerves 

 leading to it are cut. A heart of this type, where the rhythm 

 originates in the muscular tissue of the heart, is termed myogenic. 

 The nerves which supply the heart merely provide a fine adjusting 

 mechanism. One set of nerves (sympathetic) accelerates the heart, 

 while the other (parasympathetic) slows it down. Most Crustacea 

 have a heart of a different type, in which the beat originates in 

 nerve cells on the wall of the heart. This type of heart is termed 

 neurogenic. The two types of heart differ in their reactions to 

 certain drugs. The mammalian heart is accelerated by adrenaline, 

 which is produced at the ends of the sympathetic nerves, while it 

 is slowed by acetylcholine, which is produced by the para- 

 sympathetic fibres. The hearts of most Crustacea are accelerated by 

 acetylcholine. Surprisingly they are also accelerated by adrenaline, 

 so that here the two substances are not antagonistic. Exceptions to 

 the general rule among the Crustacea are found in the Cladocera. 

 Acetycholine slows the heart of Daphnia, so that it appears to be 

 myogenic, but the situation is not identical with that in the 

 mammals, for no clear effect of adrenaline has been found. 



LITERATURE 



Fox, H. M. (1948). The haemoglobin of Daphnia. Proc. Roy. Soc. 



B - l 35 : I95' 212 - 

 Fox, H. M. (1955). The effect of oxygen on the concentration of 



haem in invertebrates. Proc. Roy. Soc. B. 143: 203-214. 

 Fox, H. M. (1957). Haemoglobin in Crustacea. New Biology 24: 



60-77. 

 Gilchrist, B. M. (1954). Haemoglobin in Artemia. Proc. Roy. Soc. 



B. 143: 136-146. 

 Green, J. (1955). Haemoglobin in the fat-cells of Daphnia. Quart J. 



Micr. Sci. 96: 173-176. 



