TELEOSTS. 



253 



throat. A spiral valve is absent except in the single genus 

 Cheirocentrus, while the conus arteriosus is represented only in 

 the genus Butrinus (see, however, p. 227). The bulbus aortae 

 is large. The pallium of the cerebrum is non-nervous in char- 

 acter (Fig. 53), and the optic nerves cross (Fig. 63) and never 

 unite in a chiasma. 



An interesting discovery has recently been made, that in 

 many if not in all teleosts breathing-valves exist, one pair at 



Fig. 255. Five stages in the development o( the ciinner ( Clenolabrus). A, 

 two cells, resting upon the yolk; 5, surface view of the eight-celled stage; C, the 

 blastoderm covers about one-third of the yolk, the segmentation cavity (i) showing 

 through ; the embryo (f) is outlined, while the blastoderm is margined by a thicker 

 rim (r) of inturned entoderm; D, the blastoderm has covered three-quarters of 

 the yolk; the three primary brain regions are differentiated, and myotomes have 

 appeared ; E, a stage shortly before hatching, the yolk having been largely absorbed, 

 and the tail having grown out. 



the mouth, the other behind the gills, so arranged that when the 

 mouth cavity is enlarged water can only flow in through the 

 mouth ; when contracted, it can only escape through the gills. 

 The eggs of the teleosts are peculiar in the almost complete 

 separation between yolk and protoplasmic portions ; the latter 

 alone dividing in the early stages of development, and giving 



