304 THE ORIGIN OF VERTEBRATES 
into a separate category from the rest of the tentacles. On the other 
hand, the innervation of the rest of the tentacles by a single nerve 
which sends off a branch as it passes each one, together with the 
concentration of their skeletal elements into a single bar, with pro- 
jections into each tentacle, points directly to the conclusion that these 
tentacles must be considered as a group, and not singly. 
I suggest that these tentacles are the remains of the ectognaths 
and endognaths; the tongue representing the two ectognaths, and 
the four tentacles on each side the four pairs of endognaths. 
As we see, this method of interpretation attributes segmental 
value to the tentacles, a conclusion which is opposed to the general 
opinion of morphologists, who regard them as having no special 
morphological importance, and certainly no segmental value. On 
the other hand, the importance of the pair of ventral tentacles, the 
‘tongue’ of Rathke, which lie in the mid-line of the lower lip, has 
been shown by Kaensche, Bujor, and others, all of whom are 
unanimous in asserting that at transformation they are converted 
into that large and important organ the piston or tongue of the adult 
Petromyzon. It is supposed that the rest of the tentacles vanish 
at transformation, being absorbed; they appear to me rather to take 
part in the formation of the sucking-disc, so that I am strongly 
inclined to believe that the whole of the remarkable suctorial 
apparatus of Petromyzon is derived from the tentacles of Ammoccetes. 
In other words, on my view, a conversion of the prosomatic appen- 
dages into a suctorial apparatus takes place at transformation, just 
as is frequently the case among the Arthropoda. 
It is to the arrangement of the muscles that we look for evidence 
of segmental value. As long as it was possible to look upon these 
tentacles as mere sensory feelers round the mouth entrance, it was 
natural to deny segmental value to-them. Matters are now, how- 
ever, totally different since Miss Alcock’s discovery of the rudimen- 
tary muscles at the base of the tentacles and their development at 
transformation. If these muscles represent some of the appendage 
muscles belonging to the foremost prosomatic segments just as the 
ocular muscles represent the dorso-ventral somatic muscles of those 
same segments, then we may expect ultimately to be able to give 
as good evidence of segmentation in their case as I have been able 
to give in the case of these latter muscles ; for the two sets of muscles 
are curiously alike, seeing that the eye-muscles do not develop until 
