28 Dr Gaskell, The Origin of Vertebrates. [Nov. 25, 



strainer for the mixed food and mud which the Ammocoetes takes 

 in to its pharyngeal chamber. The velar appendages have been 

 further utilized in line with the respiratory appendages to assist 

 in respiration, their movements being synchronous with the respi- 

 ratory movements. 



The separate part of the Vth nerve which supplies the velum 

 passes within it from the dorsal to the ventral surface of the animal, 

 and then, as shown by Miss Alcock, turns abruptly forwards to supply 

 the large median tentacle, so that this extraordinary course leads 

 to the direct conclusion that this median tentacle which is really 

 double constitutes with the velum of each side the true velar 

 appendages, and that that separate part of the Vth nerve corre- 

 sponds to the nerve of the large oar appendage of Eurypterus. 

 Again on each side of the middle line there are in Ammocoetes four 

 large tentacles each of which possesses a system of muscles, muco- 

 cartilage and blood spaces precisely similar to the median ventral 

 tentacle already mentioned. Each of these is supplied, as Miss 

 Alcock has shown, by a separate branch of the motor part of the 

 Vth, and each branch is comparable to the branch supplying the 

 large velar appendage with its median ventral tentacular part. 



As might be supposed from what has already been said, all 

 these tentacles and the velar appendages vanish upon transforma- 

 tion, and in their place is found the remarkable tongue and suction 

 apparatus of the Petromyzon. 



The very process of transformation of the locomotor appendages 

 of the Limulus into the tactile and velar appendages of Ammo- 

 coetes is shown to us by the condition of affairs in Eurypterus, 

 where these appendages are no longer chelate, but have been 

 changed into short tactile appendages with the exception of the 

 last one which acted as an oar appendage. We see how their 

 basal parts are all hidden by the lower lip or metastoma which is 

 strictly comparable to the lower lip of Ammocoetes, and how all 

 the foremost ones are capable of being retracted into the large 

 hood which is strictly comparable with the large hood or upper lip 

 of Ammocoetes ; let the lower lip grow larger and let the last 

 appendage follow the example of the others, and we have imme- 

 diately no longer Eurypterus but Ammocoetes, with the separate 

 branches of the motor part of the Vth nerve representing the 

 nerves of the locomotor appendages of Limulus, and so bringing 

 this nerve into its true position as an appendage nerve with the 

 Vllth, IXth and Xth. 



