ing down around the ventral surface of the animal, form 

 the atrium (Figure 14-1). The atrial cavity expands up 

 around the pharynx to either side as the folds meet below. 

 The adult mouth is formed during metamorphosis by 

 appearance of a buccal depression (perhaps the stomo- 

 deum), extending anteriorly from the margin of the primi- 

 tive mouth, and by the outgrowth of the posterior margin 

 of the mouth to form the membranous ventral and lateral 

 walls bearing the marginal cirrae. The buccal funnel comes 

 to lie directly below the rostrum and leads back to the an- 

 teriorly facing mouth set in a muscular velum. Languets 

 grow out from the velar margin. The ciliated pit of Hatschek 

 (Figure 1-3) now lies in the roof of the funnel and from it 

 extends a ciliated tract, the wheel organ. The pit also gives 

 rise to a band of solenocytes (Figures 1-2, 1-3), lying just to 

 the left of the notochord, whose duct opens into the pharynx 



just behind the velum. At the end of the metamorphosis, 

 there are eleven pairs of pharyngeal fenestrae. The branchial 

 openings increase in number, and each is subdivided into 

 anterior and posterior halves by the downgrowth of a 

 languet from the dorsal margin. There are about ninety pairs 

 of openings in the adult, each subdivided vertically by 

 a languet and horizontally by a number of bars, the 

 synapticulae. 



The liver arises as a diverticulum from the gut behind the 

 pharynx and grows forward on the right side. The anus opens 

 far back to the right of the caudal fin. The atrium opens 

 midventrally about two-thirds of the way back. The noto- 

 chord extends nearly to the tip of the rostrum. In the early 

 stages of its development — cleavages, gastrulation, and neu- 

 rula formation, Amphioxus resembles the tunicate very 

 closely (Figure 7-5). 



polar body 



follicular wall 



perivitelline cells 



EGG 



notochord neural plate 



B 



2 CELL 



entral 



4 CELL 



entode 



notochord 



neural tube 



ectoderm 



BLASTULA 

 (SAGITTAL SECTION) 



ckers 



GASTRULA (LONGI SECTION) 

 ^notochord 



mesoderma 



entoderm 



F EARLY NEURULA 



neural tube 



endostyle 



openings between 



atrium and pharynx 



epicardium 

 heart. 



entoderm 



stomach 

 opening of gut into atrial sac 

 entodermal tract in tail 



muscle ce 



EARLY NEURULA 



H 



LATE NEURULA 



notochord 

 LARVA AT HATCHING 



Figure 7-5. Early stages of the tunicate, C/ave//no. A, egg with follicular wall and envelope of 

 perivitelline cells; B, 2-cell stage; C, 8-cell stage; D, blastulo with arrow indicating axis and future 

 dorsal and ventral aspects; E, sagittal section of gastrula with anterior end to left and dorsal side 

 up; F, cross section through E; G, sagittal section of an early neurulo; H, cross section of G; I, larva 

 at hatching. (After von Beneden and Julin) 



CLEAVAGE, GASTRULATION, ORGANOGENESIS • 197 



