116 



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. fluma- 

 tilis), about 60 cm. in length : and the Sand-pride, or lesser 

 fresh-water Lamprey (P. branchialis), not exceeding 30 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. 749) are 

 nearly cylindrical, the tail-region compressed or flattened from 



FIG. 749. Petromyzon xnarinus. Ventral (A), lateral (B), and dorsal (C) views of the head.. 

 br. cl. 1, first gill-cleft ; buc. f. buccal funnel ; <=?/(-, eye ; /nth. mouth ; na. ap. nasal aperture :, 

 V. papillae ; pn. pineal area ; fi. t-. &. teeth of buccal funnel ; t*. teeth of tongue. (After 

 W. K. Parker.) 



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

 large basin-like depression, the buccal funnel (hue./.), surrounded 

 with papillae (p.) and beset internally with yellow, horny teeth 

 (t l t 3 ). At the bottom of the funnel projects the end of the 

 tongue (t*), also bearing teeth, arid 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 trans- 

 parent area of skin (pn.} indicates the position of the pineal organ.. 

 The paired eyes have no eyelids, but are covered by a transparent, 

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

 tures on the sides of the head, the first a little behind the eyes. 

 On the ventral surface, marking the junction between trunk and 



