222 ELEMENTARY BIOLOGY. [CHAP. 



I. The circulatory organs. 



1. Dissect away the greater portion of one branchiostegfte, 

 so as to expose the branchial chamber and gills un- 

 injured. Place the animal on its side and drill a 

 small hole in the roof of the carapace near the heart ; 

 insert the point of a cannula into the same, inject 

 and examine under water. Upon displacing the 

 branchiae there will be seen — 



a. The branchio-cardiac canals ; six well-defined chan- 

 nels, running parallel with each other immediately 

 beneath the inner wall of the branchial chamber. 



h. Remove the podobranchiae and turn back the gills 

 remaining in situ. The stem of each will be seen 

 to lodge a vessel {efferent bra?ichial vessel) — those of 

 a given set of branchiae uniting to form one branchio- 

 cardiac trunk. 



c. Cut all the gills short and follow the branchio- 

 cardiac canals upwards. They converge towards the 

 region of the heart, to enter the pericardial sinus — a 

 spacious chamber now largely filled with injection. 



As a rule, the trunks related to the ambulatory 

 appendages 2, 3, and 4 enter the pericardium in- 

 dependently : the last of them receives the posterior 

 trunk of the series, which brings in the blood from 

 the pleurobranchia alone ; the upper ends of the 

 three are connected by short anastomoses. The an- 

 terior trunk is formed by the union of those related 

 to the second and third maxillipedes and the chela. 



2. Dissect a specimen which has been at least 24 hours 

 in alcohol, under water, from the tergal aspect, cutting 

 away carefully with a pair of scissors the mid-thoracic 

 portion of the carapace. 



