CHORD ATA — VERTEBRATA — FISHES 339 



Class C, Pisces (Fishes) 



Cold-blooded, aquatic vertebrates, both marine and fresh 

 water. The organs of locomotion are the paired fins and the 

 flexible tail. Gills are the chief, usually the only, organs of 

 respiration, and are attached to the gill arches throughout life. 

 The notochord is more or less completely replaced by cartilagi- 

 nous or bony vertebrae. The skull is well developed; in its 

 embryonic growth each of the two bars of the first (man- 

 dibular) visceral arch divides into a dorsal and a ventral por- 

 tion. The former, the palatoquadrate cartilage, is situated 

 forward and, uniting in the median line with the corresponding 

 portion of the opposite bar, forms the upper jaw. The latter, 

 Meckels cartilage, similarly extends forward and, uniting with 

 its fellow, forms the lower jaw. The second (hyoid) visceral 

 arch likewise divides into a dorsal and a ventral portion. The 

 latter amongst other uses serves to support the tongue. The 

 former (hyomandibular) unites the jaws mth the skull in most 

 Elasmobranchii and Teleostomi ; hence most of these are called 

 hyostylic fishes ; in other fishes, e.g. the Holocephali and Dip- 

 neusti, the hyomandibular is atrophied and the jaws are fused 

 with the skull by means of the quadrate bone, hence these are 

 called autostylic fishes. Most fishes lay eggs, but in some forms 

 (e.g. Mustelus of the Elasmobranchii and the blenny of the 

 Teleostomi) the young are developed within the mother and 

 brought forth alive ; they usually develop lying free within 

 the uterus, but exceptionally are attached to its walls. The 

 eggs, when produced, vary much in size ; where they are minute, 

 i.e. with little yolk, they are exceedingly numerous, and where 

 large, few. 



Development of fins. — Fishes, known from the Silurian to the 

 present, are conceived to have originally possessed (i) a median 

 fold of skin extending along the back from the base of the 

 head around the tail almost to the anus, giving rise by division 

 to the dorsal, caudal and anal fins, and (2) a horizontal fold 



