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 Ammocoetes. In other words, on my view, a conversion of the prosomatic appendages 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, however, totally different since Miss Alcock's discovery of the rudimentary 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 {305}transformation, but throughout the Ammocoetes stage remain in almost as rudimentary a condition as the tentacular muscles.