CHAP. VI EXTERNAL CHARACTERS Teel 
but in the Hag-Fishes (J/yaine and Ldellostoma), where a buceal 
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 JMyxine 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, 1S a series of small 
and almost circular bran- 
chial clefts (Petromyzon, 
Bdellostoma). 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- oes.cl.d 
: o Fic. 92.—Head of Myxine glutinosa (A), and of 
o 
hind the head (Fig. Bdellostoma forsteri (B), from beneath. br.ap, 
92, A). At the Junc- Left external branchial aperture ; dr.c/.1, first 
: =D branchial cleft ; mth, mouth; na.ap, nasal aper- 
tion of the trunk with ture ; oes.ct.d, oesophageo-cutaneous duct. The 
Ghe tak isi the anus, smaller openings in A are those of mucous glands. 
(From Parker and Haswell, after W. K. Parker.) 
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 Mywine a caudal fin only is present, surround- 
ing the extremity of the tail. 
