THE PROSOMATIC SEGMENTS OF AMMOCGTES 299 
to move synchronously with the movements of respiration, con- 
tracting at each expiration, and thus closing the slit by which the 
oral and respiratory chambers communicate, and so forcing the 
waters of respiration through the gill-slits, as described by Schneider. 
Such a fact is clear evidence that these tubular muscles of the velar 
folds belong to the same series as the tubular muscles of the branchial 
segments, so that if, as I have already suggested, the latter muscles 
were originally the veno-pericardial muscles of segments corre- 
sponding to the branchial appendages, then the former would represent 
the veno-pericardial muscles of the segments corresponding to the 
opercular and metastomal appendages. What, then, are these velar 
folds, and how is it that the tubular muscles of these two segments 
become the velar muscles? I will consider, in the first instance, the 
posterior group of muscles (mt2) in Fig. 116. 
It has already been pointed out that the tubular muscles of the 
branchial segments are dorso-ventral, but do not run with the 
ordinary constrictors, having separate attachments and running part 
of their course internally to and part externally to the ordinary con- 
strictors. At first sight, as is usually stated, the hyoid segment does 
not appear to possess tubular muscles at all. If, however, we follow 
the posterior group of velar muscles (mts), we see (Fig. 117) that 
they pass between the auditory capsule and the opercular bar (sks) of 
muco-cartilage to reach the region of the jugular vein (/.v.) posteriorly 
to the auditory capsule, so that their dorsal origin bears the same 
relation to the hyoid segment as the dorsal attachment of the rest of 
the tubular muscles to their respective segments. Further, these 
muscles run along the length of the velar fold, and are attached 
ventrally on each side of the thyroid gland, so that their ventral 
attachment also corresponds in position, as regards the hyoid segment, 
with the ventral attachment of the rest of the tubular muscles as regards 
their respective segments. 
This ventral attachment is shown in Fig. 119 on each side of the 
thyroid, and in Fig. 120 (mt,); while in Fig. 117 the fibres are seen 
converging to this ventral position. In other words, this large 
posterior muscle of the velar folds is a dorso-ventral muscle, and 
would actually take the same position in the hyoid segment as the 
dorso-ventral tubular muscles in the other branchial segments, if 
the velum were put back into its original position as the septum ter- 
minating the branchial chamber. Conversely, the presence of these 
