516 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



as in the case of the Macruran and Brachyuran heart 

 owing to the extension of the postero-inferior angle. 

 Viewed from behind, it has a distinct sinistral inclina- 

 tion. The walls are thick and strongly muscular, and 

 the cavity in older specimens is almost filled by the 

 strands of muscle which pass across it. The blood enters 

 the heart by three pairs of ostia provided with flaps 

 opening inwards — one pair placed antero-dorsally, one 

 latero-ventrally, and one postero-laterally. Seven 

 vessels leave the heart, three passing forwards, three 

 downwards, and one backwards. 



The heart-beats are regular in any one individual, 

 but there is some variation in different specimens. The 

 contractions are very explosive, and each contraction 

 draws in the anterior end of the heart, stretching the 

 two lateral and median arteries at the same time. 



The Pericardium, whose walls have the same general 

 outline as the organ they enclose, is a fairly spacious 

 thin-walled cavity extending from the cervical groove 

 anteriorly to the beginning of the eighth thoracic somite 

 posteriorly. It rests ventrally on the mid-gut, and 

 dorsally is applied closely to the carapace. On either 

 side of the posterior part of the pericardium there is a 

 shallow cavity. These possibly represent the " poches 

 pericardiales " of Brachyura. The blood is brought to 

 the pericardium from the gills, and passes through the 

 ostia to the heart. 



The various spaces in the body which are filled with 

 blood do not represent a true coelom. They are 

 morphologically a part of the vascular system which has 

 become greatly distended and which has been termed 

 by Lankester a haemocoel. 



A portion of the renal organ and the gonadial sacs 

 may possibly represent the true coelom of other animals. 



