134 ZOOLOGY SECT. 



parachordals and auditory capsules. Just in advance of the anterior 

 region is a ring-shaped opening surrounded by cirri (c.), and con- 

 sidered to be the nasal aperture. The posterior region gives off 

 paired plates (#.) which may perhaps correspond with the dorsal 

 longitudinal bars of the branchial basket in the Lamprey. 



CLASS II. PISCES. 



The Pisces, including the cartilaginous and bony Fishes and 

 the Dipnoi, are Craniata which have the organs both of re- 

 spiration 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 com- 

 pletely 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 semicircular 

 canals. The kidney is a persistent mesonephros. 



Sub-Class I. Elasmobranchii. 



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

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

 stomata, is composed essentially of cartilage, and, though there may 

 be ossifications in the substance of the cartilage, distinct bones, 

 such as are found in all higher groups, with the exception of the 

 Holocephali, are 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. There 

 is a cloaca, 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. 



