VEETEBRATA. 179 



Myxinida. Petromyzontidce. 



Myxine = Gastrobranchus Petromyzon (Lamprey). 



(Hag). Geotria. 



Bdellostoma. Mordacia. 



Order III. TELEOSTEI. 



TELEOSTOMI. 



Skeleton osseous; cranium of many bones, provided with a 

 lower jaw. Gills free, pectinate, protected by a bony gill-cover 

 [operculum], with gill-membrane and rays. Body covered with 

 imbricated scales. Bulbus arteriosus not rhythmically contrac- 

 tile. 



The scales are sometimes embedded in the skin, as in the Murse- 

 nidiB, and there are no true scales in the Siluridse. They are, 

 however, mostly present, thin and flexible, either with their edges 

 entire (cycloid) or with their posterior edges toothed (ctenoid). 

 They are marked with concentric and radiating lines. In some 

 the scales are partially ossified, e. g. the Tunny. " Argentine " is 

 the name of the silvery matter of the scale ; the brighter colours 

 are due to pigment-cells in the skin. 



The skull is very complex, owing to the number of bones, which 

 are, however, only centres of ossification. Four branchial arches 

 on each side, articulated to the base of the cranium, support the 

 gills. The operculum or gill-cover, attached to the hyoid arches, 

 id composed of four flat bones, below which are the bony rays sus- 

 taining the branchiostegous or gill-membrane. 



The muscular system of these fishes is made up of a series of 

 vertical flakes (myotomes) corresponding in number with the ver- 

 tebrae ; they are connected together by a gelatinous tissue, which 

 is dissolved by boiling. The vertebrae are often very numerous 

 (236 in Gymnotus) ; they either gradually diminish in size to 

 the end (Muraenidae), or end, as a rule, in a compressed series of 

 anchylosed bones, from which the caudal rays proceed. A caudal 

 fin so formed has almost always two equal lobes (homocercal). 



Lcptocephalus and Hyoprorus are probably, according to 

 Gunther, the offspring of Mursenids arrested in their develop- 

 ment in early life, yet continuing to grow without corresponding 

 development, and never attaining the character of perfect animals. 

 The former, indeed, is now said to be the larval form of the 

 confer. 



Nearly all the ordinary fishes (about 9000 species) are contained 



