124 



ZOOLOGY 



SECT. 



1. EXAMPLE OF THE CLASS. THE LAMPREY (Petromyzon), 



Three species of Lamprey are common in the Northern Hemi- 

 sphere : the Sea-lamprey (P. marinus), which attains a length of a 

 metre; the Lampern, or common fresh-water Lamprey (P. fluvia- 

 tilis), which may reach a length of about 90 cm. ; and the Sand- 

 pride, or lesser fresh-water Lamprey (P. planeri), not exceeding 

 45 cm. in length. In the Southern Hemisphere the Lampreys 

 belong to two genera : Mordacia, found on the coasts of Chili and 

 Tasmania, and Geotria, in the rivers of Chili, Australia, and New 

 Zealand. Both genera differ from Petromyzon in minor details 

 only. 



External Characters. The head and trunk (Fig. 793) are 

 nearly cylindrical, the tail-region compressed or flattened from 



Fro. 793. Petromyzon marinus. Yentralr(A), lateral (B), and dorsal (C) views of the head. 

 lii. <'/. 1, first gill-cleft ; huc.f. buccal funnel ; eye, eye; mtk. mouth; iin. up. nasal aperture; 

 p. papillse ; pn. pineal area ; t l . '. < :f . teeth of buccal funnel ; <-. teeth of tongue. (After 

 W. K. Parker.) 



side to side. At the anterior end, and directed downwards, is a large 

 basin-like depression, the buceal funnel (buc. /.), surrounded with 

 papillae (p) and beset internally with yellow, horny teeth (t l t' 3 ). 

 At the bottom of the funnel projects a prominence, the so-called 

 "tongue" ( 4 ), also bearing horny teeth, and having immediately 

 above it the narrow mouth (mth.). On the dorsal surface of the head 

 is the single median nostril (na. ap.), and immediately behind it a 

 transparent area of skin (pn.) indicates the position of the pineal 

 organ. The paired eyes have no eyelids, but are covered by a trans- 

 parent area of skin. The gill-slits (br. d. 1) are seven pairs of small 

 apertures on the sides of the head, the first a little behind the 



