204 AN INTRODUCTION TO ZOOLOGY 



the blood to enter the heart, but prevent it from flowing back into 

 the pericardial sinus. 



Respiratory System. Between the branchiostegites and the 

 body wall are the branchial chambers containing the gills. At 

 the anterior end of the branchial chamber is a channel in which the 

 scaphognathite of the second maxilla (Fig. 102, A, ep.) moves back 

 and forth, forcing the water out through the anterior opening. 

 Water flows in through the posterior opening of the branchial 

 chamber. 



GILLS. There are two rows of gills, named according to their 

 points of attachment. The outermost, the podobranchia, are 

 fastened to the coxopodites of certain appendages (see Table 

 VIII) and the inner double row, the arthrobranchia, arise from 

 the membranes at the bases of these appendages. In Astacus 

 there is a third row, the pleurobranckice, attached to the walls of 

 the thorax. The number and arrangement of these gills are 

 shown in Table VIII (169). The podobranchiae consist of a 



TABLE VIII 



THE NUMBER AND POSITION OF THE GILLS OF THE CRAYFISH 



(Cambarns) 



SEGMENT PODOBRAN- ARTHROBRANCHI.E TOTAL 



CHLE NUMBER 



Anterior Posterior 



VI o (ep.) ooo (ep.) 



VII i 



VIII i 



LX i 



X i 



XI i 



XII i 



o 2 



3 

 3 

 3 

 3 

 3 



6(ep). 



basal plate covered with delicate setae and a central axis bearing 

 a thin, longitudinally folded corrugated plate on its distal end, 

 and a feather-like group of branchial filaments. The arthrobran- 

 chiae have a central stem on either side of which extends a number 

 of filaments, causing the entire structure to resemble a plume. 



