IV. VERTEBRATA: PISCES, TELEOSTEI. 575 



The hyoid arch always bears an operculum and branchiostegal 

 membrane, but there is no opercular gill. The gills of the 

 comb-like type, are confined to the four anterior gill arches, but 

 they may be reduced to even two and one-half pairs of demi- 

 branchs. Instead of a conus (present in Butrinus), the bulbus 

 arteriosus is well developed; a spiral valve is lacking, but pyloric 

 appendages are common. A swim bladder is usually present, but 

 its duct is frequently closed. 



The teleosts are distinguished from all vertebrates except the cyclo- 

 stomes and perhaps some ganoids in that the nephridial system does not 

 form part of the sexual ducts. The eggs and milt are deposited through 

 the abdominal pores or by special canals developed from the body cavity. 

 Copulation occurs in only a few viviparous forms (Embiotoeidae, Garribu- 

 sia, etc.). The rule is that males and females deposit their reproductive 

 products in the water at the same time, and this leads to the enormous 

 schools of herring and other fishes which occur yearly at certain times. 

 This also explains the ease with which artificial impregnation in fish 

 culture is performed. 



In rare instances the males care for the young, as in the case of the 

 sticklebacks ; more noticeable are the conditions in the lophobranchs (sea 

 horses and pipe fish), where the males receive the eggs in a brood pouch on 

 the ventral surface. A metamorphosis is known only in the eel-like fishes, 

 the larvae of which originally described as distinct under the name Lepto- 

 cephalus are flat, transparent forms with colorless blood, enormous tails, 

 and extremely small trunk. These larvae normally occur in the sea at the 

 depth of some hundred fathoms. The fresh-water eels go to the ocean 

 for propagation. On the other hand many salt-water fish go to fresh 

 water for reproduction. 



The classification of the fishes is yet in an unsettled state, partly owing 

 to the large number of forms, partly to the fact that the groups intergrade. 

 Most European writers recognize six divisions, Physostomi, Anacanthini, 

 Pharyngognathi, Acanthopteri, Chaetognathi, and Lophobranchii. Our 

 authorities separate the Ostariophysi from the Physostomi, the Pediculati 

 and Hemibranchii from the Acanthopteri, and unite the Anacanthini and 

 some of the Pharyngognathi with the Acanthopteri and make a distinct 

 group, Synentognathi, of the others. The characters on which these divi- 

 sions are based are less convenient for the tyro than those adopted here. 



Order I. Physostomi. 



The character to which this name refers is not readily seen 

 without dissection, the persistence of the duct of the swim bladder. 

 This is, however, correlated with the soft character of the fin 

 rays (few exceptions) and the abdominal position of the ventral 

 fins. The Ostariophysi are remarkable in having a chain of bones 

 connecting the swim bladder with the ear. More than a third of 

 the food fishes and nearly all of the fresh-water fishes belong here. 



