520 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



lost except part of the right one in the second segment. 

 A new artery arises from this branch ; it assumes the 

 function of an inferior vessel and develops, into the 

 prominent ventral division of the superior abdominal 

 artery present in the adult. 



This modification may have been due primarily to 

 the animal's assumption of a shell, and secondarily to 

 the fact that nearly all the organs of the body — which 

 in other Decapod Crustacea are supplied by separate 

 thoracic arteries — are to be found in their bulk extended 

 into the abdomen. One may conjecture, in the first 

 place, that the pressure of the shell on the under-surface 

 of the body would constrict a ventral vessel, especially 

 at the peduncle, in such a way as to make its output of 

 blood inefficient. In the second place, a vessel so 

 remotely connected with the heart as the inferior 

 abdominal, under these disadvantageous conditions, 

 would be of little value in maintaining an efficient flow 

 of blood through the large number of slender ramifying 

 vessels required by the abdominal organs. Its place 

 would tend to be filled by one more directly in communi- 

 cation with the means of maintaining the circulation. 

 Thus the superior abdominal artery, whose size and 

 proximity to the heart qualify it for the task of providing 

 a large quantity of blood, has come to monopolise the 

 supply to the abdomen. 



Blood Sinuses and Veins The whole of the space 



inside the body walls might be theoretically considered 

 as one large sinus containing impure blood. The 

 presence of the viscera divide this space up into several 

 smaller sinuses, which are, however, all connected with 

 each other. 



Above and in front of the stomach there is a distinct 

 blood space — the Dorsal sinus. The main sinus with 



