26 A Guide to the Zoological Collections 



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great veins of the body open ; (2) a thin -wal led auricle 

 that receives the blood — valves preventing regurgita- 

 tion — from the venous sinus ; (3) a thick-walled muscular 

 ventricle, which receives the blood — valves intervening — 

 from the auricle and propels it to the gills. 



The Gills of the Sting-ray exemplify the type of the 

 Sub-class Elasmobranchii. They are contained in sepa- 

 rate gill-pouches, of which there are five pairs, arranged 

 in two rows, one row on either side of the pharynx. 

 Each gill-pouch has a separate opening, not only into 

 the pharynx but also to the exterior. The gill-pouches 

 are supported by the hyoid and branchial arches and 

 their "branchial rays" before mentioned. Each pouch 

 contains two half-gills, except the last, which contains 

 only one half -gill. Besides the five pair of gill-pouches, 

 there is a pair of spiracles which open into the pharynx, 

 and also to the exterior just behind the eyes : they do 

 not bear functional gills, but they are of use in respira- 

 tion by admitting water into the pharynx: they are 

 homologous with gill-clefts, being in fact the pair of 

 clefts between the jaws and the hyoid bone, that is to 

 say between the first two primitive branchial arches. 



In the Heart of the Sting-ray (Elasmobranch type) the 

 ventricle ends in a muscular, pulsatile, many-valved 

 arterial bulb, or arterial cone, from which a single large 

 branchial artery issues and runs forward between the 

 two rows of gill- pouches, giving off a branch to each 

 pouch. These branches of the branchial artery break 

 up and ramify in the gills for the purification of the 

 blood. The purified blood is collected by a correspond- 

 ing series of arteries, which finally unite to form a 

 single large blood-vessel— the dorsal aorta — that runs 



