166 THE SPINY DOGFISH— THE SKATE 



thick radial ; the metapterygium bears several slender radials. These radials 

 bear the horny fin-rays. 



[In the Skate {Raia) the pectoral girdle is attached to the anterior vertebral plate, 

 and there are three articular facets for the basal pieces of the fin, namely, a large 

 propterygium, a small mesopterygium, and a large metapterygium.] 



Racliala 



Propterygium 



Coracoirl 



. fontanelle Meftopterygium 



,..,„. _ I Coracoid 



Anterior facet - — 



Middle facet 



Posterior facet 



Fig. 55.— Dorsal View of the Pectoral Girdle and Fin Skeleton of 

 the Skate {Raia). From Masterraan's Text-book of Zoology. 



B. The Vascular System. 



1. The Heart and the Afferent Branchial Vessels. 



Carefully remove the middle portion of the pectoral girdle so as to expose 

 the heart. Extend the opening by removing (in thin slices) the ventral wall of 

 the pericardial cavity. Note that the heart, situated in the pericardial cavity, 

 is composed of four parts. Posteriorly and dorsally is the small thin-walled 

 sinus venosus, opening in front into a single large and thin-walled atrium or 

 auricle. The auricle communicates with a thick-walled ventricle, from which 

 is given off in front a tubular chamber with thick muscular walls, the conus 

 arteriosus. 



To see the sinus venosus properly, press the ventricle forwards. 



Cut away or tear apart the covering muscles in front of the conus so as to 

 expose the ventral aorta. This vessel is a continuation of the conus arteriosus, 

 and passes forward between the gills. Trace the \entral aorta forward to near 

 the middle of the lower jaw, and by carefully clearing away the tissue and 

 muscles from its vicinity, expose the five afferent branchial vessels (on each side) 

 from the aorta to the gills. Trace out these arteries, noting that the first or 

 anterior branchial artery goes to the half-gill of the hyoid arch, and the other 



