102 



THE ORIGIN OF VERTEBRATES 



the anterior portion of the velum is covered over with a scale-like 

 or tubercular pattern remarkably like the surface- ornamentation 



J>s.br__\__l 



Fig. 120. — Ammocoetes cut open in Mid- Ventral Line to show Position of 

 Velum ; Velar Folds removed on one side. 



tr., trabecule; vol., velum; B., anterior gnathic portion of velum; ps. br., pseudo- 

 branchial groove ; ra 2 , muscles of lower lip segment ; m 3 , muscles of thyro-hyoid 

 segment ; mt s , insertion of tubular muscles of velum near thyroid. 



seen in many of the members of the ancient group Eurypterida\ In 

 Fig. 121 I give a picture of this surface-marking of the velum. It 

 is striking to see that just as in the case of the invertebrate this 

 marking aud these serroe are formed simply by the cuticular surface 

 of the epithelial cells ; a surface which, according to Wolff, possibly 



contains chitin. The interpretation which I 

 would give of the velar folds is therefore 

 as follows : — 



They represent the fused basal parts of 

 the opercular and metastomal appendages, 

 the gnatho-bases of the latter still retaining 

 in a reduced degree their rasping surfaces, 

 because, owing to their position on each side 

 of the opening into the respiratory chamber 

 they were still able to manipulate the food as it passed by them 

 after the closure of the old mouth. 



The whole evidence points irresistibly to the conclusion that the 

 mandibular or velar nerve of the trigemiual does supply a splanchnic 



Fig. 121. — Surface View 

 of Anterior Surface 

 of Velum. 



