140 SUB-CLASS ELASMOBRAXCHII. 



from epithelial outgrowths of the embryonic branchial passages. 



Body cavity. The pericardial cavity communicates with 

 the general body-cavity by a canal which leaves it dorsal to the 

 sinus venosus and passing along th3 ventral side of the oesophagus 

 opens behind by a slit-like aperture into the body-cavity. This 

 canal may divide in a Y-shaped manner behind, but usually only 

 one of the limbs is complete. 



The body-cavity may communicate with the exterior by abdo- 

 minal pores, with the kidney-tubules by persistent nephrostomes 

 and with the cloaca by the oviducts. 



Abdominal pores are usually present, though there is consider- 

 able variability as to these openings in closely allied species or 

 even in the same species at different periods of life. They are 

 placed on each side of the cloacal opening, either on the surface 

 or at the bottom of a pouch (cloacal pouch). They lead into 

 that part of the body cavity which is placed on each side of the 

 cloaca (peritoneal canals). 



The vascular system is arranged in the usual piscine manner. 

 The pericardial cavity is placed dorsal to the median union of 

 the coracoicl cartilages of the shoulder girdle. Its walls are 

 stiffened by that skeletal structure and by the basibranchial 

 plate which lies dorsal to it. It thus happens that its walls are 

 unyielding and when the ventricle contracts, blood flows from 

 the great venous sinuses outside the pericardium into the large 

 auricle. The ventricle is provided with a muscular conus arteri- 

 osus in which there are from two to five or even more rows of 

 semilunar valves. The ventral aorta sends branches to the hyoid 

 arch, which has a demibranch on its hinder face, and to all the 

 branchial arches except the last. The carotids arise from the 

 dorsal system and the internal carotids anastomose as they enter 

 the cranial cavity. Tlie efferent vessel coming from the hyoid 

 demibranch gives off near its ventral end an artery which passes 

 forward ventrally to the spiracle on to the mandibular arch. It 

 passes near the front wall of the spiracle, giving off vessels to the 

 pseudo branch and then pierces the cranial wall to join the internal 

 carotid artery within the skull. 



The great veins are much dilated into sinuses as they approach 

 the heart. This applies to the anterior and posterior cardinals 

 and to the hepatic veins. Moreover the anterior cardinal is 

 dilated round the eye-ball forming the space (a kind of hsemo- 



