SCYLLIUM CANICULA 229 



the presence of the internal spiracular opening and five long narrow 

 slits, the internal branchial clefts. Each cleft leads into a branchial 

 pouch, also lined with entoderm, the anterior and posterior walls of 

 which, with the exception of the hinder wall of the last, are thrown 

 into the branchial folds or filaments. The tissue forming the 

 partition between one pouch and the next constitutes the inter- 

 branchial septum, in which lie the cartilaginous rods of the branchial 

 skeleton and the blood-vessels of the gills. One septum with its 

 skeletal and vascular elements and the branchial filaments on each 

 side of it is termed a complete gill or holobranch, so that we find on 

 each side of the fish four complete gills. The filaments on one side 

 of the pouch form a half gill or hemibranch, so that in addition to 

 the four holobranchs there is also on each side of the front wall of 

 the first gih 1 pouch a single hemibranch. The pseudobranch already 

 noted on the anterior wall of the spiracle is, therefore, to be regarded 

 as a spiracular hemibranch which, however, is vestigial and without 

 functional significance. Movements of the branchial region take 

 place during respiration, whereby the water is kept circulating 

 over the highly vascular filaments, which are thus kept supplied with 

 oxygen. The movements are brought about by a fairly complex 

 series of muscles connected with the branchial basket. 



Behind the gill clefts the pharynx passes over into a somewhat 

 narrower tube, the oesophagus, which possesses dark-coloured walls 

 owing to the presence in them of a rich plexus of capillaries, and is 

 quite short. It is lined by a characteristic stratified epithelium. 



The oesophagus leads into the stomach, and the transition from 

 one to the other is marked on the outside by a change in coloration, 

 and on the inside by an alteration in the character of the mucosa, 

 which becomes a simple columnar epithelium of a highly glandular 

 nature. The stomach is a large U-shaped tube, whose proximal, 

 wider end lying to the left is termed the cardiac portion. The distal, 

 somewhat narrower end on the right is known as the pyloric portion, 

 and it runs forward again parallel with the cardiac portion almost 

 to the level of the oesophagus, where it turns back upon itself to form 

 a short swelling, the pyloric enlargement, followed by a marked 

 constriction, the pylorus. The structure of the stomach wall is on 

 the whole very similar to that in Rana, and its circular muscle fibres 

 are very strongly developed in the region of the pylorus to form the 

 pyloric sphincter, a circular constrictor muscle. Internally when the 

 stomach is empty, or only moderately full, the mucosa is thrown 

 into a series of longitudinal folds which disappear when the organ is 

 distended. Inside the pylorus is a ridge of the mucosa termed the 

 pyloric valve which, when the sphincter contracts, enables the 

 stomach to be shut off from the intestine while the food undergoes 



