42 8 FISHES CHAP. 
females. Much energy is spent by both sexes in moving stones 
by lifting them with the buccal funnel, but it is not always 
clear that this is done to cireumscribe the nest, or to remove 
impeding obstacles. Eventually, a male attaches himself to the 
back of the head of a female, who at the same time is holding 
fast to a stone. The male then rotates its body so that the 
urino-genital papilla is brought near the genital orifice of the 
female, and the simultaneous extrusion of eggs and spermatozoa 
at once follows. Owing to 
the small amount of food- 
yolk which they contain 
the eggs of the Lamprey 
(e.g. P. planeri) are small, 
measuring about 1°1—1:2 
mm. in length, and from 
0-9-—1:0 mm. in width. 
There is a micropyle at 
the animal pole of the egg, 
but the characteristic horny 
egg-case and the _ polar 
hooks of the Myxinoids 
are both wanting. The 
embryo hatches out as a 
Fic. 244.—Head of the ene tee P. fluvia- larva known as the “Am- 
tilis. A, ventral view; B, side view. 6r.1, mocoetes.” At this stage 
Fist brineial apetnre; eye; 1 lover of its development the larva 
lip. (From Parker and Haswell, after W. K. lacks several of the most 
Parker) striking features which 
characterise the adult, and it is highly probable that the Ammo- 
coetes represents a stage in the evolution of Vertebrates in some 
respects intermediate between Amphiowus and a very primitive 
Craniate. The mouth of Ammocoetes is bounded laterally and in 
front by a curious hood-lke upper lip, and behind by a short 
transverse lower lip (Fig. 244). The eyes are deeply seated and 
rudimentary, and as visual organs they are useless, but the parietal 
eye is well developed. As in the adult, there are seven pairg of 
gill-sacs, but they open internally into a pharynx, directly con- 
tinuous behind with the rest of the alimentary canal, and there 
is no dorsal oesophagus. Like the skull, the branchial basket is 
still very rudimentary. The dorsal and caudal fins are con- 
