EXTERNAL CHARACTERS 



"but in the Hag-Fishes (Myxine and Bdellostoma), where a buccal 

 funnel is not developed, longer tentacle-like structures are present 

 on each side of the mouth. On the upper surface of the head 

 is the single median nostril, or naso-pituitary aperture, placed 

 between the eyes in the Lampreys (Fig. 91, B, C), but at the 

 anterior margin of the 

 head in Myxine and 

 its allies (Fig. 92). In 

 the living Lampreys a 

 semi-transparent area of 

 skin may be noticed 

 behind the nasal organ, 

 which coincides with 

 the position of the more 

 deeply - seated parietal 

 eye. On each side of 

 the body, commencing a 

 short distance behind the 

 eye, is a series of small 

 and almost circular bran- 

 chial clefts (Petromyzon, 

 Bdellostomci). In Myxine, 

 however, the clefts of 

 each side have a single 

 common external aper- 

 ture, situated on the 

 ventral side of the body 

 and some distance be- 

 hind the head (Fig. 

 92, A). At the junc- 

 tion of the trunk with 

 the tail is the anus, 

 behind which is the 

 papilla which carries the urino-genital aperture at its extremity. 

 There are no paired limbs or vestiges of such organs. Median 

 fins are represented in the Lampreys by an anterior dorsal fin 

 and a posterior dorsal fin, the latter being continuous with the 

 caudal fin which fringes the upper and lower margins of the 

 protocercal tail. In Myxine a caudal fin only is present, surround- 

 ing the extremity of the tail. 



oes.cl.d 



FIG. 92. Head of Myxine, glutinosa (A), and of 

 Bdellostoma forsteri (B), from beneath, br.ap, 

 Left external branchial aperture ; br.cl.l, first 

 branchial cleft ; mth, month ; na.ap, nasal aper- 

 ture ; oes.ct.d, oesophageo- cutaneous duct. The 

 smaller openings in A are those of mucous glands. 

 (From Parker and Haswell, after W. K. Parker.) 



