PETROMYZON. 
The best-known species, 
Bdellostoma dombeyz, 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 merdblastic 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 
Letromyzon—The Lamprey 
There are three British 
species—the sea lamprey 
(Petromyzon marinus), over 
3 ft. in length; the river 
lampern (Pf. ftuviatilis), 
nearly 2 ft. long; and the 
small lampern or “stone- 
grig” (P. planeri). They 
eat worms, small crustace- 
ans, insect larve, 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. 
521 
ss 
a. 
Teh 0, 
Fic. 275.—Bdellostoma stout? (Cali- 
fornian hag), enveloped in sheath 
of mucus.—After Bashford Dean. 
4., Barbules ¢.,eyes m. mucus; eg., eggs. 
