310 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 Ammoccetes, 
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, 7.¢. 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 belonging 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- 
somatic 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 
