220 THE DOG-FISH 



its dorsal wall by the wide basi-branohial plate and 

 the inner ends of the last hypo-branchials. 



2. The heart is a single tube twisted on itself in an 



S-shape, and dilated to form a series of chambers 

 separated by transverse constrictions. 



Determine the position and general characters of the 

 chambers in order from behind forwards, pressing them aside 

 when necessary, but making no fv/rther dissection. 



a. The sinus venosus is a thin-walled transversely 



placed tube, attached along its whole length to 

 the posterior and dorsal wall of the pericardial 

 cavity. It receives laterally the veins returning 

 blood from the body, and opens in front into the 

 auricle by a median aperture. 



To expose the sinus venosus, press the -hea/rt slightly for- 

 wards. 



b. The auricle is a large thin- walled sac, occupying the 



dorsal half of the pericardial cavity. It is tri- 

 angular in shape, with its posterior angles pro- 

 duced into processes which, in the natural position, 

 of the parts, project laterally at the sides of the 

 ventricle. 



To expose the auricle, push the globular ventricle to one 

 side. 



c. The ventricle is a thick-walled globular sac lying 



ventral to the auricle, and forming the most co^i- 

 spicuous part of the heart. 



d. The conus arteriosus is a cylindrical tube, running 



forwards from the ventricle to the anterior end of 

 the pericardial cavity. 



3. The perioardio-coelomic canal is a median canal about 



an inch long,- opening in front into the hinder end of 

 the pericardial cavity dorsal to the sinus venosus, 

 and running back along the ventral surface of the 

 oesophagus, to open behind into the body-cavity. Its 

 anterior opening is large and conspicuous ; the open- 



