120 



ZOOLOGY 



SECT 



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. 803) are 

 nearly cylindrical, the tail-region compressed or flattened from 

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

 large basin-like depression, the buccal funnel (buc.f.), surrounded with 



9 * 



br.cl.i 



FIG. 803. Petromyzon fluviatilis. Ventral (A), lateral (B), and dorsal (CO views of the head. 

 br. cl. 1, first gill-cleft ; buc.f. buccal funnel ; eye, eye ; mth. mouth ; na. ap. nasal aperture ; 

 p. papillae ; pn. pineal area ; tl.t2.t3. teeth of buccal funnel ; 1 4. teeth of tongue. (After 

 W. K. Parker.) 



papillae (p) and beset internally with yellow, horny teeth (t 1 I ->). 

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

 " tongue " (t4), 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 

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

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

 eyes. On the ventral surface, marking the junction between trunk 

 and tail, is the very small anus (Fig. 812, a.), lying in a slight depres- 

 sion, and having immediately behind it a small papilla pierced at 

 its extremity by the urinogenital aperture (z.). There is no trace 



