122 CEPHALOCHORDATA CHAP. 
(Fig. 75, brep), but the external, wider, non-ciliated cells (Fig. 
75, at.ep) are ectodermal cells lining the atrium. The gelatinous 
skeletal rods in the primary bars are forked ventrally, while those 
in the secondary bars are simple ; and there are other points of de- 
tail in which the two kinds of bar differ. These bars are obviously 
more numerous in the adult than the myotomes, but in the 
young larva the first formed gill-clefts are metamerically arranged, 
and then later they 
increase greatly in 
number. It is the 
cilia covering the 
pharyngeal — epithe- 
lum on the branchial 
bars, possibly aided 
by the ciliated tracts 
of - the ‘orales hood? 
which cause the cur- 
rent of water already 
alluded to. 
Transverse bran- 
chial junctions (syn- 
apticula) run across 
the branchial bars, 
“atep connecting them at 
Fic. 75.—Transverse sections through primary (A) and frequent intervals, 
secondary (B) branchial bars of Amphioxus. at.ep, and these transverse 
Atrial epithelium; 6/.s, blood spaces or ‘vessels ” ; : 2 
br.ep, branchial epithelium ; coed, coelomic cavity in CONNeXIONS, like the 
nee hae sk, skeletal rods. (From Willey, after branchial bars, are 
supported by skeletal 
rods. Along the ventral median line of the pharynx runs a 
groove, the endostyle or hypopharyngeal groove, comparable 
with the similar structure in the branchial sac of Tunicata. 
This longitudinal groove (Fig. 76, g/) is lined by ciliated epithe- 
lium containing four tracts of gland cells (compare endostyle 
in Ascidians, Fig. 20, p. 46). There is reason to believe 
that this organ is the homologue of the thyroid gland of Verte- 
brata. As in the case of Tunicata the endostyle secretes mucus, 
which is carried forwards by the cilia to constitute a train with 
entangled food particles which pass back dorsally to the stomach. 
At the anterior end the ciliated lips of the endostyle diverge to 
Minti 
La 
q “al 
