THE LAMPREY. n 



genital aperture (z),— and lies in an oval depression, in 

 the anterior part of which, just in front of the base of the 

 papilla, is another small aperture, the anus (a). 



29. The two dorsal fins (Figs. 7 and 8), median longi- 

 tudinal folds of the integument : the anterior end of the 

 first is near the middle of the body, that of the second 

 somewhat in front of a vertical line taken through the anus. 



30. The caudal fin, continuous in front with the second 

 dorsal, and continued round the end of the tail on to its 

 ventral edge ; it is diphycercal, that is, evenly distributed 

 above and below the axis of the tail. 



Fig. 5. — Petromyzon marinus. The urinogenital sinus, with the 

 rectum and part of the left kidney (nat. size). 



a, anus : int, intestine : k, left kidney : r, rectnra : u.g.p, urino- 

 genital papilla : u.g.s, urinogenital sinus : ur, left ureter, x, aper- 

 ture of left, and x' aperture of right ureter into urinogenital sinus : 

 y, bristle passed through right abdominal pore : z, bristle passed into 

 urinogenital sinus through its external aperture. 



31. The absence of paired fins, or fore and hind limbs. 



32. The apertures of the sensory tubes, minute 

 punctiform openings on the surface of the head. 



IV. Remove the skin from the anterior part of one 

 side of the body, from the end of the mouth to a 

 short distance behind the gill-slits, and from the 

 middle dorsal to the middle ventral line : make 

 out — 



33. The body muscles, lying beneath the skin, and 



