muscles formed a sheet of muscles similar to those of the lower lip of Ammocoetes, by which the movements of the metastoma were effected. The posterior limit of these muscles ventrally marks out the junction of the segment of the lower lip with that of the thyroid; in other words, indicates where the metastoma had fused ventrally with the operculum (Fig. 117). [Illustration: FIG. 119.--VENTRAL VIEW OF HEAD-REGION OF AMMOCOETES. _Th._, thyroid gland; _M._, lower lip, with its muscles.] Besides the striated visceral muscles, each branchial segment possesses its own tubular muscles, shown in Fig. 116 (_mt_3_) and (_mt_4_). As the section shows, there is clearly a group of tubular muscle-fibres belonging to the hyoid segment (_mt_2_), and also another group belonging to the segment in front of the hyoid (_mt_1_); so that, judging from this section, each of these segments possesses its own tubular musculature just as do the branchial segments, the difference being that the tubular muscles are more separated from the striated visceral group than in the true branchial segments, owing to the size of the blood-spaces surrounding them. What, then, are these two groups of muscles? Tracing them in the series of sections, both groups are seen to belong to the system of velar muscles, forming an anterior and a posterior group respectively; and we see, further, that there is not the slightest trace of any tubular muscles anterior to these muscles of the velum. In the living Ammocoetes the velar folds on each side can be seen {299}to move synchronously with the movements of respiration, contracting 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 corresponding 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.