3IO THE ORIGIN OF VERTEBRATES 



— the striated constrictor and the tubular constrictor. Of these 

 muscles, both the muscles possessing ordinary striation are attached 

 to the branchial cartilaginous skeleton, whereas the tubular con- 

 strictors have nothing to do with the cartilaginous basket-work, but 

 are attached ventrally in the neighbourhood of the ventral aorta. 



These segmental tubular muscles are found also in the velar folds 

 — the remains of the septum or velum which originally separated 

 the oral from the respiratory chamber. In the branchial region they 

 act with the other constrictors as expiratory muscles, forcing the 

 water out of the respiratory chamber. In the living Ammoecetes, 

 the velar folds on each side can be seen to move synchronously with 

 the movements of respiration, contracting at each expiration ; they 

 thus close the slit by which the oral and respiratory chambers com- 

 municate, and therefore, in conjunction with the respiratory muscles, 

 force the water of respiration to flow out through the gill-slits, as 

 described by Schneider. 



These tubular muscles thus form a dorso- ventral system of 

 muscles essentially connected with respiration ; they belong to each 

 one of the respiratory segments, and are also found in the velum ; 

 anterior to this limit they are not to be found. What, then, are these 

 tubular muscles in the velar folds ? Miss Alcock has worked out 

 their topography by means of serial sections, and, as already fully 

 explained, has shown that they form exactly similar dorso-ventral 

 groups, which belong to the two segments anterior to the purely 

 branchial segments, i.e. to the facial or hyoid segments and the lower 

 lip-segment of the trigeminal nerve. If the velar folds could be put 

 back into their original position as a septum, then the hyoid or facial 

 group of tubular muscles would take up exactly the same position as 

 those belonsdnu; to each branchial segment. 



The presence of these two so clearly segmental groups of muscles 

 in the velum — the one belonging to the region of the trigeminal, the 

 other to the region of the facial — is strong confirmation of my con- 

 tention that this septum between the oral and respiratory chambers 

 was caused by the fusion of the last prosomatic and the first meso- 

 soinatic appendages, represented in Limulus by the chilaria and the 

 operculum. 



Yet another clue to the meaning of these muscles is to be found 

 in their innervation, which is very extraordinary and unexpected. 

 Throughout the branchial region the striated muscles of each segment 



