166 ESSENTIALS OF ZOOLOGY 



The food is rapidly conveyed from the mouth to the stomach. 

 In .the stomach it is subjected to an acid digestion by the 

 action of the gastric juice, and in the intestine to an alkaline 

 digestion by the products of the liver and pancreas. The 

 digested products are absorbed by the vessels of the intestine, 

 and the undigested residue is egested at the cloaca. The 

 liver, the pancreas, and the other glands of the alimentary 

 canal are developed as outgrowths of the primitive endo- 

 derm. 



The respiratory organs are the gills. The spiracle possesses 

 a rudimentary gill on its front wall, and it is supported 

 by the spiracular cartilage. The first four branchial clefts 

 bear gill filaments on both the anterior and the posterior 

 walls, the fifth cleft only on the front wall. The cartilaginous 

 gill arches intervene between the clefts, and support the gill 

 folds internally. So that from the point of view of the arches 

 the hyoid arch bears one set of gill filaments, or a hemibranch, 

 the next four a pair of gills each, and the fifth arch does not 

 support a gill. The gills are contained in a spacious gill chamber 

 which is interrupted by folds dividing the chamber into a 

 series of clefts and which bear the filaments on the opposing 

 faces. The gill filaments are further supported by gill rays 

 which spring from the arches. These are internal gills, and are 

 endodermal in origin. In the embryo the external margins of 

 the gills are produced as fine long outgrowths forming external 

 gills which protrude from the apertures of the gills into the 

 cavity of the egg. 



In fishes, generally, the water is introduced to the gills by 

 the mouth, and expelled after passing over the gills. In the 

 skate the water is carried in through the spiracle, and it may 

 be passed out from the gills or returned to the spiracle for 

 discharge. The blood is carried in the venous condition to the 

 gills, and it comes into close relationship with the water by 

 being spread over the thin epithelial surface of the gill filaments. 

 The oxygen borne by the water is taken up in exchange for 

 carbon dioxide, and other products of metabolism which can be 

 readily dealt with at such a surface, and this is respiration. 

 The haemoglobin of the blood bears the oxygen to all parts 

 of the body. 



