igo PHYSIOLOGY OF THE INVERTEBRATA. 



branches which, also anastomose and give rise to a fine net- 

 work of blood-vessels situated on the organs of generation, 

 nephridia, and in the muscular mesodermic layer. The red 

 blood contained in these vessels has already been described. 

 In the Polychceta the perivisceral cavity is continued into 

 all the important appendages of the body, ■ consequently they 

 are filled, with blood. " The circulation of this fluid is 

 effected partly by the contraction of the body and its 

 appendages, partly by the vibratile cilia, with which a greater 



Fig. 38.— Diagram of the Pseodo-h^mal Vessels of Hirudo. 

 {After Gratiolet.) 



a — dorsal vessel and branches, b = lateral vessel and branches. 

 c = ventral vessel and branches, d = branches. 



or less extent of the walls of the perivisceral cavity is covered. 

 In a great number of Polyclweta no part of the body is 

 specially adapted to perform the function of respiration, the 

 aeration of the blood probably taking place wherever the 

 integument is sufficiently thin ; and, even when distinct 

 branchiae ordinarily exist, members of the same family may 

 be deprived of them." 



