xii THYLUM CIIORDATA 373 



eventually at the point where the narrow portion of the 

 intestine passes into the wide portion. 



Connected with the rectum on its dorsal aspect is an oval 

 gland the rectal gland (ret. gl.} about three-quarters of an 

 inch in length. 



The spleen (spl.} is a dark-red or purple body attached to 

 the convexity 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-pouches. Each gill-pouch is an 

 antero-posteriorly compressed cavity opening internally by 

 the internal branchial aperture (Fig. 202, /. br. a.i 5) into 

 the pharynx and externally by the gill-slit or external branchial 

 aperture. 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 hemibranck) or half gill, consisting of a series of close- 

 set parallel folds or plaits of highly vascular mucous mem- 

 brane. Separating adjoining gill-pouches and supporting 

 the gills, are a series of broad interbranchial septa, each 

 containing the corresponding branchial arch with its con- 

 nected 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 wall only. On the anterior wall of the spiracle 

 is a rudimentary gill the pseudo-branch or spiracular gill 

 in the form of a few slight ridges. 



The heart (Fig. 211), 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. It 

 consists of four chambers sinus venoms (s.v^ auricle (au.) t 



