PETKOMYZON. 



521 



The best - known species, 

 Bdellostoma dombeyi, resembles 

 the hag in many ways. It lives 

 at the bottom of the sea, at 

 depths of a hundred fathoms or 

 more, and is often found inside 

 caught halibut, etc. The gill- 

 pouches have separate openings, 

 and are extraordinarily variable 

 in number, from six to fourteen 

 on either side a variability per- 

 haps pointing to ancestral reduc- 

 tion from a larger number (cf. 

 Amphioxus}. Large eggs are 

 laid on a shelly or rocky bottom, 

 become connected by polar 

 hooks in chains or clusters, are 

 fertilised after deposition, and 

 exhibit meroblastic discoidal 

 segmentation and direct devel- 

 opment. Ayers' experiments 

 show that the removal of one or 

 both ears in this form does not 

 materially affect equilibration. 



SECOND TYPE 

 Petromyzon The Lamprey 



There are three British 

 species the sea lamprey 

 (Petromyzon marinus\ over 

 3 ft. in length; the river 

 lampern (P. fluviatilis\ 

 nearly 2 ft. long; and the 

 small lampern or "stone- 

 grig " (P. planeri). They 

 eat worms, small crustace- 

 ans, insect larvae, dead 

 animals, etc. ; but they also 

 attach themselves to living 

 fishes, and scrape holes in 

 their skin. As their names 

 suggest, they also fix their 

 mouths to stones, and some 

 draw these together into 

 nests. 



FIG. 275. Bdellostoma stouti (Cali- 

 fornian hag), enveloped in sheath 

 of mucus. After Bashford Dean. 



.,Barbules .,eyes m. mucus; ^-., eggs. 



