THE DEVELOP.MENT OF S YMBRANCHUS MARMOllATUS. 19 
pressed, merges eventually iuto the tail. The ventral ends of 
the myotonies have now met ventrally and form a complete 
investment of muscles to the trunk. Chromatophores are 
still confined to the dorsal and lateral parts, the ventral 
surface being quite colourless. 
The length is 26 mm. The true tail is distinguished by the 
continuous median fin, which gradually rises from the dorsal 
surface in front of the anus, is continued round the tip of 
the tail, and dies away on the ventral side at a point midway 
between the extreme hind end and the anus. 
Although this continuous fin is only conspicuous in the tail, 
there is, however, a low-lying dorsal fin-like structure extend- 
ing as far forwards as the pericardium. A similar structure 
continues forwards — the ventral portion of the fin — this being 
the last remains of the transparent tissues traversed by the 
sub-intestinal vein of the earlier stages. 
The tail fin is still transparent, though branching chromato- 
phores are now more copiously present. The anus is about 
9 mm. from the tip of the tail. 
ddie extreme front end of the snout is squarish, the upper 
jaw longer than the lower, so that the mouth is slightly 
ventral. The gape is not great, and is well protected by the 
thick fleshy upper and lower lips. Just inside the mouth- 
cavity on its dorsal surface there are some forwardly directed 
blind pockets, the floors of which form valvular flaps for the 
more efficient closing of the mouth aperture. Both anterior 
and posterior nares are visible, the latter, which first appear 
in Stage 30, being situated just in front of the eyes, the 
former at the very tip of the snout on either side of the middle 
line. The posterior nares do not arise by a division into two 
of one single olfactory opening, but by a hollowing out of a 
channel to the exterior. 
Sensory canals, which first make their appearance in Stage 
34 just posterior to the olfactory organ, are present in 
the head, opening to the exterior at intervals of about '08 mm. 
The epithelium of the snout is richly glandular, as, indeed, is 
the epithelium of the trunk. 
