THE CRAYFISH 131 



relations as, though it is of less extent than, that of Apus. 

 There is a large perivisceral sinus in the cephalothorax, whose 

 relations again are fundamentally the same as those of Apus, 

 and a system of branchio-cardiac canals running from the 

 bases of the gills up the sides of the thorax, and converging 

 to enter the pericardial sinus. After traversing the various 

 organs of the body the blood is returned to a large ventral 

 blood sinus in the thorax, and thence is conducted to the 

 gills. The last-named structures are attached either to the 

 coxopodites of the thoracic limbs, or to the arthrodial mem- 

 branes of the thoracic limbs, or to the walls of the thorax, 

 and according to position are known as podobranchise, arthro- 

 branchiae, or pleurobranchise. There is a functional podo- 

 branch on the coxopodite of every limb from the seventh 

 (second maxilliped) to the twelfth (third pareiopod) inclusive. 

 The first maxilliped does not bear a functional gill, but its 

 membranous epipodite is the representative of an aborted gill. 

 There is a single arthrobranch on the arthrodial membrane of 

 the seventh limb, and two arthrobranchs on the arthrodial 

 membranes of each of the next succeeding five limbs viz. 

 the eighth, ninth, tenth, eleventh and twelfth. The thirteenth 

 limb bears neither podobranch nor arthrobranch, but a single 

 functional pleurobranch is attached to the wall of the thirteenth 

 segment, and the walls of segments ten, eleven, and twelve 

 bear each a minute filament or rudimentary pleurobranch.* 

 Thus, there are eighteen functional and four rudimentary gills 

 on each side of the body. They lie close against the thoracic 

 wall, covered over by the branchiostegite, and are bathed by 

 the respiratory current flowing in at the hinder end and out 

 at the front end of the branchial chamber. This current is 

 produced by the scooping or paddling motion of the scapho- 

 gnathite of the second maxilla. The branchial chamber is, 

 for the most part, deep from above downwards, but narrow 

 from side to side. Anteriorly, however, as it follows the line 

 of the cervical groove, it becomes much shallower from above 

 downwards, and at the same time wider from side to side, 

 forming a groove or channel which opens just below the base 

 of the second antenna. In this groove lies the scaphognathite, 



* This is true for the "Ecrevisse & pieds rouges," Astacus nobilis. I 

 have never found more than two rudimentary pleurobranchs in the common 

 English crayfish, A. torrentium. G. C. B. 



