242 ZOOLOGY SECT. 



with the micromeres, and the process may be said to be 

 intermediate between the holoblastic and meroblastic types. 

 In Teleostei, on the other hand, segmentation is always partial 

 and discoidal. The general features of development are much 

 the same as in the Trout, except that in the Sturgeon and 

 Polypterus, as in Craniates in general, there is an open medullary 

 groove which becomes closed in to form a medullary canal. 

 There is frequently a metamorphosis: in Lepidosteus, for instance, 

 the newly hatched young is provided with a sucking-disc, and the 

 proportions of the head are quite different from those of the adult. 

 In the larval Sturgeon provisional teeth are present, and in many 

 Teleostei the young differ from the adult in the presence of large 

 spines, which probably, like the spines in the zoasa-stage of some 

 Crustacea, serve a defensive purpose. The larvae of Eels are 

 strongly compressed, perfectly transparent, and have colourless 

 blood. They are sometimes known as " Glass-fish/' and were 

 formerly placed in the genus Leptocephalus, their real nature being 



FIG. 899. Polypterus bichir. Head of advanced larva ; EG. external gill. (From Dean, 



after Steindachner. ) 



unknown. The young of the Crossopterygii (or at least 

 Polypterus) have external gills, as in Dipnoi and Amphibia 

 (vide infra), and the same holds good of Colitis, ffeterotis, and 

 G-ymnarchus among the Teleostei. 



The Geographical Distribution of the Ganoid Teleostomi is 

 curiously limited : they are all essentially fresh-water forms 

 although some Sturgeons are found in the sea and are almost 

 exclusively inhabitants of the Northern Hemisphere, and especially 

 of the Holarctic Region. The Chondrostei occur in the rivers of 

 Europe, Asia, and North America : one genus of Sturgeons 

 (Seaphirhynehus) lives in the Mississippi and in the rivers of 

 Central Asia, but not in the intermediate regions : in the same 

 way Polyodon is found only in the Mississippi, while the closely- 

 allied Psephurus is found in the Yangtse-kiang and Hoangho a 

 striking instance of discontinuous distribution. Amia is found 

 in the fresh waters of the United States ; Lepidosteus extends 

 also into Central America and Cuba. Polypterus lives in the 

 Upper Nile and some other tropical African rivers ; Calamichthys 

 in the Old Calabar River. 



