IV PHARYNX AND GILL-CLEFTS I21I 
pharynx, but are in a sense portions of the body-wall as well, 
and correspond in nature, though not in number, to the visceral 
arches in a Vertebrate lying between the visceral clefts which 
open on the exterior. In the adult Amphioxus the clefts in 
the wall of the pharynx do not open directly to the exterior, 
but into the peribranchial cavity or atrium, which, however, is 
only formed at a late larval period as an invagination or enclosure 
Fic. 74.—Branchiostoma lanceolatum. A, transverse section of the pharyngeal region. 
a, Dorsal aorta; 6, atrium; ¢, notochord; co, coelom; e, endostyle; g, gonad 
(ovary); kb, branchial septa; kd, pharynx; 7, liver; my, myotome; 7, neph- 
ridium ; 7, spinal cord; sn, sn, dorsal and ventral spinal nerves. 5B, Transverse 
section of the intestinal region. atr, Atrium; coe/, coelom ; d.ao, dorsal aorta ; 
int, intestine ; myom, myotome ; nch, notochord ; new, spinal cord; s.int.v, sub- 
intestinal vein. (From Parker and Haswell’s Zoology. A, From Hertwig, after 
Lankester and Boveri ; B, partly after Rolph.) 
of ectoderm. Previous to that the first formed gill-slits opened 
to the exterior in Amphioxus (see larva, Fig. 86, p. 134), just as 
they do in a fish or a young tadpole. The atrial cavity is there- 
fore, from its origin, lined by ectoderm, and the outer surface of 
a branchial bar is virtually a part of the outer surface of the 
body. It is only natural then to find that each bar contains a 
small section of the coelom in its interior, communicating dorsally 
and ventrally with other parts of that cavity (see Figs. 75 and 
76). There are also blood-vessels which run in the branchial 
bars and their junctions. The greater part of the epithelium 
covering a branchial bar is pharyngeal epithelium or endoderm 
