428 



FISHES 



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 circumscribe 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 

 A „ at once follows. Owing to 



the small amount of food- 



na.ap 



br.i 



yolk which they contain 

 the eggs of the Lamprey 

 (e.g. P. planeri) are small, 

 measuring about I'l — 1'2 

 mm. in length, and from 

 0-9 — I'O 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 

 larva known as the " Am- 

 mocoetes." At this stage 

 of its development the larva 

 lacks several of the most 

 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 AmjjJiioxus and a very primitive 

 Craniate. The mouth of Ammocoetes is bounded laterally and in 

 front by a curious hood-like 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 pairs 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- 



FlG. 244. — Head of the Ammocoetes of P. Jlnvia- 

 tUls. A, ventral view; B, side view. &r. 1, 

 First brancliial aperture ; eye, eye ; l.l, lower 

 lip ; na.ap, naso-pituitary aperture ; u.l, upper 

 lip. (From Parker and Haswell, after W. K. 

 Parker. ) 



