THE SKATE. 65 



the conus arteriosus, each row containing five somewhat 

 semilunar flaps, opening towards the synangium. 



XXXVI. Pass a. probe, directed outwards and backwards, into 

 either of the cut ends of the sinus venosus : carefully cut away 

 the cartilage of the shoulder girdle and other tissues until the 

 end of the probe is brought into view : it will be found to 

 have passed into 



149. A small chamber, the precaval sinus (Figs. 17 and 21, pes), 

 situated in the antero-lateral angle of the abdominal cavity. In its 

 wall are several apertures : one, situated antero-internally, is the opening 

 of the jugular vein (/»), by which the blood is returned from the 

 head : another, postero-internal in position, is the opening of the 

 cardinal vein (cd) already seen : a third, on the ventral wall of the 

 sinus, puts it in communication with the hepatic sinus. (/^.j) : a fourth, 

 at its anterior end, leads into the sinus venosus (s.v) : a fifth, very 

 small, just internal to the last, into the inferior jugular vein («./«) : a 

 sixth, on the outer wall, into the brachial vein (ir) and lastly, in 

 the middle of the dorsal wall, is a transverse aperture, guarded by two 

 valves, by which the contents of the chief lymphatic trunk (Fig. 1 7, 

 fy) are poured into the sinus. 



150. A sympathetic ganglion (see § 220), a whitish elongated body 

 about a quarter of an inch long, will be found immediately outside the 

 inner wall of the sinus. 



XXXVII. Cut away the remainder of the ventral 

 portion of the shoulder girdle, and cut through 

 the floor of the mouth, by making an incision a 

 little to one side of the median line so as not to 

 injure the ventral aorta, through the oesophagus 

 and basibranchial plates (see § 41), and through 

 both jaws. Fasten back the two halves of the 

 mouth-floor thus separated, so as to expose the 

 whole oral cavity from beneath. Note 



151. The internal branchial clefts, five long vertical 

 fissures, by which the gill pouches communicate with the 

 cavity of the mouth. 



152. The opening of the spiracle into the mouth, just in 

 front of the first gill cleft. 



F 



