HEART OF IXYEUTEBBATES. 



61 



/Ascaris), but also by the movements of other organs, e.g., the 

 alimentary canal (Cyclops). At a higher stage of development a 

 rudiment of the central organ of the circulation appears, in that a 

 special section of the blood path acquires a muscular investment, 

 and as a pulsating heart, comparable to a force and suction-pump. 



FIG. 52. Male of Branchipus stagnalis with many- 

 chambered heart or dorsal vessel Sg, the lateral 

 openings in which are repeated in every seg- 

 ment. D, intestine ; M, mandible ; Sd, shell 

 gland ; Sr, branchial appendage of the llth pair 

 of legs ; T, testis. 



FIG. 53. Heart of a Copepod 

 (Calanella) with an ante- 

 rior artery, A. Os, cstia ; 

 V, valves at the arterial 

 ostium ; M, muscle. 



maintains a continuous circulation of the blood. The heart is either 

 sac-shaped, with two lateral or one anterior slit-like opening (Daphnia, 

 Calanus) (fig. 51), or elongated and divided into successive chambers 

 and perforated by many pairs of slit-like openings (Insects, Apus) 

 (fig. 52). As a rule, each chamber possesses a pair of laterally placed 



