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 Bathke, 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 Ammocoetes. 

 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 w T e 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 



