THE PROSOMATIC SEGMENTS OF AMMOCCETES 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 (mt 2 ) 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 (mt. 2 ), we see (Fig. 117) that 

 they pass between the auditory capsule and the opercular bar (sk 3 ) 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 2 ) ; 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 



