176 JEEEMIAH S. FERGUSON 



taking care to prevent the escape of the fluid through the coro- 

 nary vessels into the sinus venosus. 



The hypobranchial system of vessels is so important for the thy- 

 roid gland as to deserve more than passing mention. T. J. Parker 

 ('86) has described this system in connection with his much 

 quoted work on the circulation in Mustelus antarcticus, but he 

 makes no mention of its relation to the thyroid gland, in fact, he 

 mentions neither the gland nor the thyroid artery; the gland 

 was apparently not observed. 



The main trunk of the hypobranchial system, in the species 

 which I have examined is the median hypobranchial aitery ; it is 

 formed by the commissural arteries coming from the ventral ends 

 of the loops formed by the efferent branchial vessels which receive 

 blood from the gill capillaries. These loops surround each bran- 

 chial cleft, and within the gill they lie parallel to the afferent 

 branchial arteries; they are just antero-internal to the afferent 

 vessels. The ventral efferent vessels are very much smaller than 

 either the dorsal efferent or the afferent (fig. 5). Opposite the 

 ventral end of the second and third branchial clefts (sometimes 

 only the second or the third) each loop gives off a commissural 

 branch which passes inward and somewhat backward to the ven- 

 tral surface of the ventral aorta where these vessels, with consider^ 

 able variations, unite with their fellows of the opposite side to 

 form a median hypobranchial artery which is frequently double 

 so as to form a sort of elongated arterial circle. Sometimes the 

 vessels fail to unite in front so that instead of a median hypobran- 

 chial there is a right and left hypobranchial artery, one on either 

 side of the ventral aorta (fig. 5). Frequently the vessels so unite 

 as to form an annular anastomosis which encircles the aorta, 

 but portions of the ring may be absent. Some of these variations 

 are indicated in figs. 5 and 6. The varied arrangements of these 

 vessels are all indications of a more or less complete fusion of the 

 commissural vessels to form a median hypobranchial artery. This 

 vessel terminates posteriorly in a small sinus-like dilatation, single 

 or double as the case may be. From this sinus the coronary vessels 

 arise either as a median vessel which promptly divides, or as two 

 or three independent vessels. From this same sinus a small paired 



