PHARYNX AND GILL-CLEFTS 



121 



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 



B 



FIG. 74. Branchiostoma lanceolatum. A, transverse section of the pharyngeal region. 

 a, Dorsal aorta ; b, atrium ; c, notochord ; co, coelom ; e, endostyle ; g, gonad 

 (ovary); kb, branchial septa; kd, pharynx; I, liver; my, myotome ; n, neph- 

 ridium ; r, spinal cord ; sn, sn, dorsal and ventral spinal nerves. B, Transverse 

 section of the intestinal region, atr, Atrium ; coel, coelom ; d.ao, dorsal aorta ; 

 int, intestine ; tnycnn, myotome ; nch, notochord ; neu, spinal cord ; s.int.v, sub- 

 intestinal vein. (From Parker and Haswell's Zoology. A, From Hertwig, after 

 Laukester 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 



