PHYSIOLOGY OF THE INVERTEBRATA. 



199 



legs. The blood contained in the above-mentioned sinus 

 Hows from behind forward, or in an opposite direction to 

 that contained in the arterial sinus, and for the most part is 

 introduced into the heart through its posterior extremity, 

 though some would also seem to enter the posterior pair of 

 the lateral valvular openings. 



Sars says that the course of the blood within the several 

 limbs is not easy to examine in Cjjdesthcria hislopi, for they 

 (the limbs) are concealed for the greater part by the shell. 

 In the antennas, however, which at times are more or less 

 completely exserted beyond the shell, the blood can be dis- 

 tinctly seen passing along the upper edge of each branch to 

 the extremity, then turning 

 round, and flowing back along 

 the lower edge to the scape 

 (Sars). 



In the higher Crustacea, the 

 heart and circulatory appa- 

 ratus are far better defined 

 than in the lower orders of 

 this class. The heart of Ho- 

 iiw/rus is a powerful quadrate 

 organ, and the arteries are 

 large, definite in number and 

 in distribution. There are 

 conti-actile expansions (" gill- 

 hearts") at the base of the 

 blood-vessels conducting the 



blood to the branchias. The heart consists of a single con- 

 tractile cavity, and the arteries in the higher Crustacea are 

 closed tubes ; but the venous blood passes back through the 

 interstices between the organs of the body, until it reaches 

 certain cavities or reservoirs situated at the bases of the limbs 

 (Fig. 43); therefore the venous blood bathes all the organs. 

 From the reservoirs or sinuses the blood passes to the bran- 

 chia?, where it becomes aerated by contact with the water, 



Fig. 43. — Vertical Section of a 

 Crustacean, showing the Course 

 OF the Blood. 



a = gill, b = vessels which collect the 

 aerated blood from gills, c = vessels 

 conducting venous blood to gills. 

 d = heart, c = carapace. / — bran- 

 chiocardiac vessels. £' = sternum. 

 A = venous sinus. 



