154 



ZOOLOGY 



SISCT 



and runs in it for about half an inch, opening eventually at the 

 point where the small intestine passes into the colon. 



Connected with the rectum on its dorsal aspect is an oval gland 

 — the redal gland {red. gl.) — about three-quarters of an inch in 

 length. 



The spleen (spl.) is a dark-red or purple body attached to the con- 

 vexity of the U-shaped stomach and sending a narrow lobe along 

 the right-hand limb. 



The organs of respiration in the Dog-fish are the gills, situated 

 in the five gill-pouehes. Each gill-pouch (Fig. 818) is an antero- 

 posteriorly compressed cavity opening internally into the pharynx 

 and externally by the corres,ponding gill-slit. The walls of the 

 pouches are supported by the branchial and hyoid arches with 

 their rays, the first pouch being situated 

 between the hyoid and first branchial 

 arches, the last between the fourth and 

 fifth branchial arches. On the anterior and 

 posterior walls of the pouches are the gills, 

 each hemibranc|h consisting of a sepes of 

 close-set parallel folds or plaits of highly 

 vascular mucoi.s membrane. Separating 

 adjoining gill-J ouches, and supporting the 

 gills, are a senes of broad interhranchial 

 septa, each containing the corresponding 

 branchial arch with its connected branchial 

 rays. The most anterior hemibranch is 

 borne' on the posterior surface of the hyoid 

 arch. The last gill-pouch differs from the 

 rest in having gill-plaits on its anterior w^U only. On the anterior 

 wall of the spiracle is a vestigial gill — the psetidcbratnfh or 

 spiracular gill — in the form of a few slightXridges. 



Blood-system.— The heart is situated in the pericardial cavity, ^ 

 on' the ventral aspect of the body, in front of the pectoral arch, and 

 between the two series of branchial pouches. Its dorsal wall is 

 supported by the basibranchial cartilage. Placing it in communi- 

 cation with the abdominal cavity is a canal — the pericardia-peri- 

 toneal caned. The heart (Fig. 817) consists of four chambers — sinus 

 venosus {sin. ven.), auricle (cmr.), ventricle (vent.), and conus arteriosus 

 {eon.), through which the blood passes in the order given. 1h.e sinus 

 venosus is a thin-walled, transverse, tubular chamber, into the ends 

 of which the great veins open. It communicates with the auricle 

 by an aperture, the sinu-auricular aperture. The auricle is a large, 

 triangular, thin-walled chamber, situated in front of the sinus veno- 

 sus and dorsal to the ventricle. Its apex is directed forwards, and 

 its lateral angles project at the sides of the ventricle : it commu- 

 nicates with the ventricle by a slit-like aperture guarded by a two- 

 lipped valve. The ventricle is a thick-walled, globular chamber, 



Fio. SIS.— Hemiscylliuin. 



Branchial sue exposed from 

 the outside. 



