CHAP. VI 



EXTERNAL CHARACTERS 



ISI 



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 



tnth 



br.cl.. 



head in Myxine and 

 its allies (Fig. 9 2). In 

 the Kving 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, 

 Bdellosto'rna'). 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. 



y 



-br.CLT. 



ocs.ci.dL 



Fig, 92. — Head of Myxine glutinosa (A), and of 

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

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

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

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

 smaller openings in A are tho.se of mucous glands. 

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



