RESPIRATORY SYSTEM OF THE SKATE. 421 



side ; the stomach lying rather to the left is bent upon itself; 

 the large brownish liver is trilobed, and has an associated 

 gall-bladder from which the bile-duct extends to the duo- 

 denum — the part of the gut immediately succeeding the 

 stomach ; the whitish pancreas lies in the duodenal loop 

 between stomach and intestine, and its duct opens opposite 

 the bile-duct ; the intestine contains an internal spiral fold — 

 a membrane which increases the absorptive surface ; a small 

 rectal gland of unknown significance is attached to the 

 terminal or rectal portion of the gut ; the end of the gullet 

 and the anterior portion of the stomach and the rectum are 

 supported by folds of peritoneum, — the membrane which 

 lines the body-cavity, — the rest of the gut lies freely ; into 

 the terminal chamber or cloaca the rectum, the ureters, and 

 the genital ducts all open ; an abdominal pore opens on each 

 side of the cloacal aperture. 



Respiratory System. — The first apparent gill-clefts — the 

 spiracles — open dorsally behind the eyes. Each contains 

 a rudimentary gill, supported by a spiracular cartilage. 

 Through the spiracles water may enter or leave the 

 mouth. 



There are five pairs of gill cavities, separated by parti- 

 tions, and with ventral apertures. The first cavity is 

 bounded anteriorly by the hyoid arch, posteriorly by the 

 first branchial arch. The hyoid arch bears branchial fila- 

 ments on its posterior surface; the first four branchial 

 arches bear gill - filaments on both surfaces ; the fifth 

 branchial arch bears none. Each of the first four branchial 

 arches bears a half gill on each side ; thus, including the gill- 

 filaments borne on the posterior side of the hyoid, there are 

 four and a half gills. The absence of an operculum or gill- 

 cover is obvious. 



The anterior innominate artery or branch of the ventral 

 aorta supplies the first half gill of the hyoid, and the two 

 half gills borne by the first branchial. The posterior inno- 

 minate artery supplies the gills borne by the second, third, 

 and fourth branchial arches. 



Circulatory System.— The impure blood from the body 

 enters the heart by a bow-shaped sinus venosus, which leads 

 into a large thin-walled auricle. Thence through a bivalved 

 aperture the blood passes into a smaller mus<;ular ventricle. 



