150 ZOOLOGY SECT. 



branch is borne on the posterior surface of the hyoid arch. The 

 last gill-pouch differs from the rest in having gill-plaits on its 

 anterior wall only. On the anterior wall of the spiracle is a vestigial 

 gill the pseudobranch or spiracular gill in the form of a few slight 

 ridges. 



Blood-system. The heart is situated in the pericardial 

 cavity, on the ventral aspect of the body, in front of the 

 pectoral arch, and between the two series of branchial pouches. 

 The dorsal wall of the pericardial cavity is supported by the 

 basibranchial cartilage. Placing it in communication with the 

 abdominal cavity is a canal the pericardio-peritoneal canal. 

 The heart (Fig. 827) consists of four chambers sinus venosus 

 (sin.), auricle (aur.), ventricle (vent.), and conus arteriosus (con.), 

 through which the blood passes in the order given. The sinus 



doe 



I 



FiG. 829. The heart arid branchial arteries of Scyllium, from the side. af. br.i $, afferent 

 branchial arteries ; au. auricle ; c. a. conus arteriosus ; cl. l 5 , branchial clefts ; cor. coronary 

 artery ; d. ao. dorsal aorta ; d. c. dorsal carotid artery ; ef. br.i 9, efferent branchial 

 arteries ; ep. br. 1 4 , epibranchial arteries ; mn. mandibular artery ; sp. spiracle ; s. cl. sub- 

 clavian artery ; s. v. sinus venosus ; v. ventricle ; v. ao. ventral aorta ; v. c. ventral carotid 

 artery. (From Parker's Practical Zoology.) 



venosus is a thin-walled, transverse, tubular chamber, into the ends 

 of which the great veins open. It communicates with the auricle 

 by an aperture, the sinu-auricular aperture. The auricle is a large, 

 three-cornered, thin-walled chamber, situated in front of the sinus 

 venosus and dorsal to the ventricle. Its apex is directed forwards, 

 and its lateral angles project at the sides of the ventricle : it commu- 

 nicates with the ventricle by a slit-like aperture guarded by a two- 

 lipped valve. The ventricle is a thick- walled, globular chamber, 

 forming the most conspicuous part of the heart when looked at 

 from the ventral surface. From it the conus arteriosus runs 

 forwards as a median stout tube to the anterior end of the peri- 

 cardial cavity, where it gives off the ventral aorta. It contains 

 two transverse rows of valves, anterior and posterior, the former 



