124 



ZO< )L(KiY 



SRfT. 



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 connnon fresh- water Lamprey {F. jluv'm- 

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

 pride, or lesser fresh-water Lamprey (P. ])/rfnrri), 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 Gcotria, 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 



buc.f 



B 



naa/' ^ 



hr.cli 



Fid. 703.— Petromyzon marinus. Ycntialc(A), lateral (15), mA dorsal (C) views of the liciul. 



///. (•/. 1. first t,'ill-i'left ; In'r. /. Imceal funnel ; fw, ej-e ; iiitk. nmuth ; na. up. nasal aiiertnre ; 

 p. paijilUe ; pn. pineal area; «'. I-. t'K teeth of Imeeal fmmel ; l^. teeth of tongue, (.\fter 

 W. K. Parker.) 



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

 basin-like depression, the lucval funnel (iw./.), surrounded with 

 papilla? {p) and beset internally with yellow, horny teeth {p- — ^^). 

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

 "tongue" (;!■*), also bearing horny teeth, and having immediately 

 above it the narrow month {^mtli.). On the dorsal surface of the head 

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

 transparent area of skin (2^n.) indicates the position of the pineal 

 organ. The paired eyes have no e\'elids, but are covered by a trans- 

 parent area of skin. The gill-slits {Jir. <l. I) are seven pairs of small 

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



