140 ZOOLOGY SECT. 



an air-bladder, which in certain exceptional cases acquires the 

 function of a lung or chamber for breathing air. The hypophysis 

 is not in any way connected with the nasal chambers and lies 

 within the cranial cavity. There is a pair of nasal chambers which 

 only exceptionally communicate internally with the mouth cavity. 

 The auditory labyrinth contains the three typical semicircular 

 canals. The kidney is a persistent mesonephros. 



The first two sub-classes are nearly related to one another and 

 are frequently regarded as sections of a single sub-class the 

 ChondricWiyes or Cartilaginous Fishes. 



Sub-Class I. Elasmobranchii. 



The sub-class Elasmobranchii comprises the Sharks, Dog-fishes, 

 and Rays. The skeleton of these fishes, like that of the Cyclo- 

 stomes, is composed essentially of cartilage, and, though there 

 may be calcification of the substance of the cartilage, true bony 

 tissue, such as is found in all higher groups, is not present. The 

 dermal fin-rays, supported on the cartilaginous skeleton of the fin, 

 are of horn-like constitution. There is never (in recent forms) an 

 operculum or gill-cover. A cloaca is present, the external opening 

 of which serves as a common outlet for the rectum and the renal 

 and reproductive ducts. Among some of the fossil representatives 

 of this group are to be found the most primitive of all known Fishes. 



1. EXAMPLE OF THE SUB-CLASS : THE DOG-FISH (Scyllium 

 canicula or Hemiscyllium modestum). 



General External Features. The general shape of the body 

 (Fig. 821) may be roughly described as fusiform ; at the anterior, 

 or head-end it is broader and depressed ; posteriorly it tapers 

 gradually and is compressed from side to side. The head termi- 

 nates anteriorly in a short, blunt snout. The tail is narrow and 

 bent upwards towards the extremity. The colour is grey with 

 brown markings, or dark-brown above, lighter underneath* The 

 entire surface is covered closely with very minute hard placoid 

 scales or dermal teeth, rather larger on the upper surface than 

 on the lower. These are pointed, with the points directed some- 

 what backwards, so that the surface appears rougher when the 

 hand is passed over it forwards than when it is passed in the opposite 

 direction. When examined closely each scale is found to be a 

 minute spine situated on a broader base. The spine consists 

 of dentine covered with a layer of enamel ; the base is composed 

 of bone-like substance, and the whole scale has thus the same 

 essential structure as a tooth. Along each side of the head and 

 body runs a faint depressed longitudinal line or slight narrow 

 groove the lateral line, marking the position of the lateral line 

 canal, which contains integumentary sense-organs. 



