144 ZOOLOGY sect. 



CLASS II,— PISCES. 



The Pisces, including the cartilaginous Fishes, the bony Fishes, 

 and the Dipnoi, are Craniata which have the organs both of 

 respiration and of locomotion adapted for an aquatic mode of life. 

 The chief, and in the majority the only, organs of respiration 

 are the gills, which are in the form of series of vascular processes 

 attached to the branchial arches and persisting throughout life. 

 The organs of locomotion are the paired pectoral and pelvic fins, and 

 the unpaired dorsal, ventral, and caudal ; these are all supported 

 by fin-rays of dermal origin. A dermal exoskeleton is usually 

 present. In the endoskeleton the notochord is usually more or 

 less completely replaced by vertebrae ; there is a well-developed 

 skull and a system of well-formed visceral arches, of which the first 

 forms upper and lower jaws, the latter movably articulating with 

 the skull, and both nearly always bearing teeth. There is frequently 

 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 semi- 

 circular 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 

 Ghonclrichthycs or Cartilaginous Fishes. 



Sub-Class I. — ElasmobrancMi. 



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 recfent 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 repre- 

 sentatives of this group are to be found the most primitive 

 of all known Fishes. 



1. — Example of the Sub-Class : The Dog-Fish (Scyllmm 

 canicula or Hemiscy Ilium modeshtm). 



Greneral external features. — The general shape of the body 

 (Fig. 811) may be roughly described as fusiforum ; at the anterior, 

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



