xra PHYLUM CHORDATA 235 



always small as compared with those of Elasmobranchs, never 

 exceeding 5 to 10 mm. in diameter, and being usually much smaller. 

 They are rarely protected by an egg-shell. They are produced in 

 immense numbers, a single female sometimes laying several millions : 

 in such cases the mortality among the unprotected embryos and 

 young is immense. The eggs may be pelagic, i.e. so light as to float 

 when laid, as in the Cod, Haddock, Turbot, Sole, &c. ; or demersal, 

 i.e. so heavy as to sink to the bottom, as in the Herring, Salmon, 

 Trout, &c. In some cases (Chilobranchus) they become cemented 

 to the surface of a rock. 



In all the Ganoids hitherto investigated, with the exception of 

 Lepidosteus, segmentation is complete. In Acipenser and Amia 

 (Fig. 909, A and B) it is very unequal, the megameres being immense 

 . as compared with the micromeres : in Polypterus it is subequal 

 at first, becoming unequal later : the process may be said to be 

 intermediate between the holoblastic and meroblastic types. In 

 Lepidosteus (Fig. 909, C) the segmentation is meroblastic, the 

 fissures not extending much beyond the equator of the egg. In 

 Teleostei segmen- 

 tation is always 

 partial and dis- 

 coidal. The 

 general features of 

 development are 

 much the same as 

 in the Trout, ex- 

 cept that in the FIG - 91 - Polypterus bichir. Head of advanced larva. EG. 

 external gill. (From Dean, after Steindachner.) 



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 zoaea-stage of some Crustacea, serve a defensive purpose. 

 The pelagic larvae of Eels are strongly compressed, perfectly trans- 

 parent, and have colourless blood. They are sometimes known as 

 " Glass-fish," and were formerly placed in the genus Leptocephalus, 

 their real nature being unknown. The young of the Crossopterygii 

 (or at least Polypterus, Fig. 910) have external gills, as in Dipnoi 

 and Amphibia (vide infra), and the same holds good of Cobitis, 

 Heterotis, and Gymnarchus 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 



