ALIMENTARY SYSTEM 



303 



to close the mouth, and while the animal is feeding the oral cirri 

 are directed inwards and make a filter to exclude sand particles ; 

 the cilia in and around the mouth assist in producing an inwardly 

 directed current, but there are sense cells both on the oral cirri 

 and on the velum, and the chief function of these appears to be 

 to test the water. 



The pharynx runs for rather more than half the length of the 

 body. Its wall, originally complete and containing part of the 

 coelom, is early in development pierced laterally by a number of 

 visceral clefts or gill slits, and more of these are added as the 



,c.c. 



^.f>. 



Fig. 227. — Branchiostoma. The forepart of the body cut in half longitudinally. 



ai., Atrium ; at.fl., atrial floor ; c.c, central canal of nerve cord ; c.v., cerebral vesicle ; d.f.r., dorsal fin rays ; 

 est., endostyle ; n.c, nerve cord ; nch., notochord ; or.c, oral cirri ; or.h., oral hood ; p.ph.b., peri- 

 pharyngeal band ; pg., anterior pigment spot ; ph., pharynx ; sk.c, skeleton of cirri ; sk.r., skeleton 

 ring in oral hood ; v.t., velar tentacles ; vm., velum ; w.o., part of wheel organ. 



animal grows, until there are nearly two hundred of them. They 

 are oblique, the lower ends being posterior to the upper, so that 

 several are cut in one transverse section. The first gill slits corre- 

 spond to the myomeres, but later they become more numerous. 

 The strips of body-wall left between the gill slits are called primary 

 gill bars, and each contains a part of the coelom and a skeletal 

 rod. Each original gill sUt is divided into two by the down-growth 

 of a secondary or tongue bar, which contains a skeletal rod but 

 no coelomic space. Each slit is further subdivided by two or three 

 horizontal bars or synapticulae, also with skeletal rods. The inner 

 (frontal) and anterior and posterior surfaces of the gill bars are 

 covered with ciha ; the last two groups of these, which project 

 into the gill slit and are usually called lateral, although they are 



