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 



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 (mt 3 ) 

 and (mti). As the section shows, there 

 is clearly a group of tubular muscle- 

 fibres belonging to the hyoid segment 

 (////• 2 ), and also another group belonging 

 to the segment in front of the hyoid 

 (mti); 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 



Fig. 119.— Ventual View op 

 Head-Region of Ammoccetes. 



Th., thyroid gland; M., lower 

 lip, with its muscles. 



