298 THE ORIGIN OF VERTEBRATES 
eyes. In Fig. 117 the dorsal part is seen cut across on its way to its 
dorsal attachment. Such an origin is reminiscent of the tergo-coxal 
group of muscles, arising, as they do, from the primordial cranium 
and the tergal carapace, and suggests at once that when the chilarial 
Fig. 119.—Vrenrran Vinw oF 
Heap-REGION OF AMMOCG@TES, 
Th., thyroid gland; M., lower 
lip, with its muscles. 
appendages expanded to form a meta- 
stoma, their tergo-coxal muscles formed 
a sheet of muscles similar to those of 
the lower lip of Ammoccetes, 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). 
Besides the striated visceral muscles, 
each branchial segment possesses its own 
tubular muscles, shown in Fig. 116 (ats) 
and (mt,). As the section shows, there 
is clearly a group of tubular muscle- 
fibres belonging to the hyoid segment 
(mtg), and also another group belonging 
to the segment in front of the hyoid 
(mt); 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 seg- 
ments, 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 Ammoccetes the velar folds on each side can be seen 
