THE PROSOMATIC SEGMENTS OF AMMOCCETES 307 



the chelicerge and endognaths had dwindled do.wn to mere tentacles, 

 the ectognath was the large swimming appendage, and the metas- 

 toma was on the upward grade from the two insignificant chilaria of 

 Linmlus. 



The transformation of these foremost appendages into a suctorial 

 apparatus is very common among the arthropods, as is seen in the 

 transformation of the caterpillar into the butterfly, and it is in 

 accordance with the evidence that the main mass of that suctorial 

 apparatus should be formed from appendages corresponding to the 

 ectognath and metastoma rather than from the four endognaths. In 

 all probability the nucleus masticatorius of the trigeminal nerve with 

 its innervation of the great muscles of mastication is evidence of the 

 continued development of the musculature of these two last pro- 

 somatic appendages, just as the descending root of the Vth demon- 

 strates the further disappearance of all that belongs to the foremost 

 prosomatic appendages. As yet, however, as far as I know, the 

 musculature of the head-region of Petromyzon has not been brought 

 into line with that of other vertebrates, and until that comparative 

 study has been completed it is premature to discuss the exact posi- 

 tion of the masticating muscles of the higher vertebrates. 



The analysis of these tentacular segments belonging to the 

 trigeminal nerve presents greater difficulties than that of any of the 

 other cranial segments, owing to the deficiency of our knowledge 

 of what occurs at transformation. Light is required not only on the 

 origin of the new muscles but also on the origin of the new and 

 elaborate cartilages which are newly formed at this time. 



Miss Alcock has not yet worked out the origin of all these carti- 

 lages and muscles, so that we are not yet in a position to analyze 

 the trigeminal supply in Petromyzon into its component appendage 

 elements, an analysis which ought ultimately to enable us to deter- 

 mine from which appendage-muscles the masticating muscles in the 

 higher vertebrates have arisen. As far as the muscles are con- 

 cerned, she gives me the following information : — 



The tongue-nerve supplies in Ammoccetes the rudimentary 

 muscles which pass laterally from the base of the large ventral 

 tentacle to the wall of the throat, and even in Ammoccetes must 

 possess some power of moving that tentacle. 



At transformation these muscles proliferate and develop enor- 

 mously, and form the bulk of the large basilar muscle which 



