PERICAEDIAL CAVITY 219 



2. The pseudobranchise. 



Slit up the spiracle on the left side in the same way as the 

 gill-clefts. 



The pseudobranch is a rudimentary gill, consist- 

 ing of about half a dozen small parallel folds, on the 

 anterior wall of the spiracle. Its presence shows 

 that the spiracle is really a gill-cleft. 



VI. DISSECTION OF THE CIRCULATORY SYSTEM. 



A. The Pericardial Cavity and Heart. 

 1. The pericardial cavity. 



Lay the dog-fish on its back under water, and pin it down 

 through the fins. Open the pericardial cavity by a mid- 

 ventral incision in front of the pectoral girdle, taking care not 

 to injure the heart. Cut away the ventral wall of the cavity 

 so as to expose it fully. Cut away also the middle half-inch 

 of the pectoral girdle, taking care not to injure the sinus 

 venosus, which lies along its dorsal border. 



The pericardial cavity is median, and lies between 

 the gills: it is triangular in shape, with the apex 

 directed forwards, and is almost completely filled by 

 the heart, of which the globular ventricle is the most 

 prominent portion. Its ventral wall is stiffened by 

 the anterior prolongation of the pectoral girdle, and 



FIG. 37. Scyllium canicula. A longitudinal and vertical section through 

 the head and anterior part of the body. The section of the brain 

 is taken a little to one side of the median plane, and the spinal 

 cord has been cut across a short way behind the brain. The cut 

 surfaces of the cartilaginous skeleton are dotted. (A. M. M.) 



A, roof of the cranium. B, internasal septum. C, mucous canals. D, fold 

 of the wall of the hemisphere in which lies the choroid plexns. E, aperture 

 leading from the cerebral vesicle to the olfactory vesicle. F, third ventricle. 

 Gr, pineal body. TT, optic lobe. I, optic chiasma. IP, intervertebral neural 

 plate. K, pituitary body. L, cerebellum. M, lateral lobe of cerebellum. 

 MN, lower jaw. MX, upper jaw. K", interorbital sinus; H"P, vertebral 

 neural plate. N"S, neural spine. O, basi-hyal, supporting the tongue. 

 P, pericardial cavity. PC, pericardio-ccelomic canal. Q, sinus venosns. 

 E, auricle. S, ventricle. T, conus arteriosus. TE, teeth. U, cardiac aorta. 

 V, aperture of afferent branchial vessel. VC, vertebral centrum. "W, first 

 branchial cleft. X, spiracle, or hyo-maudibular cleft. Y, notochord. Z, spinal 

 cord. 



