xin PHYLUM CHORDATA 225 



arid there are many well-known species, such as the Cod and the 

 Herring, which exhibit the hermaphrodite condition as an occa- 

 sional variation. 



Reproduction and Development. Most Teleostomi are 

 oviparous, the eggs being impregnated after they are laid, but 

 in some Teleostei, such as the Viviparous Blenny (Zoarces), internal 

 impregnation takes place ; the young are developed in the hollow 

 ovary and are brought forth alive. Many instances of parental 

 care of the young are known, the most familiar being that of the 

 male Stickleback (Gasterosteus), which constructs a nest of weeds, 

 fastened together by a glutinous secretion of the kidneys, and 

 jealously guards the developing young. In the Sea-horse (Hippo- 

 campus) and the Pipe-fish (Syngnatlms) the young are developed 

 in a pouch (Fig. 832, brd. p.) on the abdomen of the male. In 

 the Siluroid Aspredo the eggs are pressed into the soft spongy 

 skin of the belly and thus carried about by the parent. The 

 ova are 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 

 they become cemented to the surface of a rock. 



In all the Ganoids hitherto investigated segmentation is com- 

 plete, but very unequal (Fig. 848) : the megameres are immense 

 as compared with the micromeres, and 

 the process may be said to be inter- 

 mediate between the holoblastic and 

 meroblastic types. In Teleostei, on 

 the other hand, segmentation is al- 

 ways partial and discoidal. The general 

 features of development are much the 

 same as in the Trout, except that in 

 the Sturgeon there is an open medul- 

 lary groove. There is frequently a 

 metamorphosis : in Lepidosteus, for in- 

 stance, the newly hatched young 1 is 



j j ..i J T . T J 1 . 1 FIG. 848. Segmentation in Lepi- 



provided with a sucking-disc, and the dosteus. (After Baifom- and 



proportions of the head are quite dif- 

 ferent 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 zosea-stage of some 

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



VOL. II Q 



