122 



CEPHALOCHORDATA 



(Fig. 75, br.ep~), but the external, wider, non-ciliated cells (Fig. 

 75, at.ep) are ectodernial 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- 

 lium on the branchial 

 bars, possibly aided 

 by the ciliated tracts 

 of the oral hood, 

 which cause the cur- 

 rent of water already 

 alluded to. 



Transverse bran- 

 chial junctions (syn- 

 coe apticula) run across 

 the branchial bars, 

 connecting them at 

 intervals, 



at.ep 



at. 



ep 



FIG. 75. Transverse sections through primary (A) and frequent 

 secondary (B) branchial bars of Amphioxus. at.ep, Q rirl 



.... *iiT 77 IT -i i > Clllvl 



Atnal epithelium ; ol.s, blood spaces or " vessels ; 



br.ep, branchial epithelium ; coel, coelomic cavity in Connexions, like the 



SSham ) mr 5 Sk ' Skdetal r dS ' (Fr m Willey> aftCT 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, gT) 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 



