LATER LARVAL STAGES. 551 



extend so far vent rally. Iii the middle of this ectodermal thickening, 

 the larval oral aperture forms at first as a narrow perforation which, 

 however, soon widens. Consequently, the oral aperture is surrounded 

 by a thickened ectoderm- wall. 



The fit'*? [i ill-cleft forms soon after (Fig. 289, /*) in the ventral 

 region of the second body-segment. A small outgrowth of the ento- 

 derm here forms, round which the entoderm-cells become elongated 

 and less granular. An annular entodermal thickening is thus formed 

 in the middle of which is the above-mentioned depression, the cells 

 of which soon fuse with the 'ectoderm, and a perforation, the first 

 gill-slit, takes place. Round the first gill-cleft, the ectoderm is not 

 thickened, but the entoderm forms a circular wall representing the 

 inner edge of the gill-cleft. The first primary gill-cleft which arises 

 in this way soon shifts to the right side of the body (Fig. 289 A). 



After the mouth and the first gill-clefts have broken through, the ciliated 

 organ, the pre-oral pit (derived from the left enteric vesicle), and the club- 

 shaped gland also open externally. 



The anal u/^ffn/^ breaks through at the most posterior end of the 

 alimentary canal on the left side of the body. At the same time, 

 the communication between the intestine and the ventrally curved 

 end of the medullary tube (neurenteric canal) (Fig. 289 B] is lost. 



After all these apertures have formed, the larva is capable of 

 taking in food. 



D. Later Larval Stages. 



The further development of the larva falls into three periods : 



I. Behind the first gill-slit, a series of other so-called primary yill- 

 .///> (as many as fourteen, WILLEY) develop ; most of these shift to 

 the right side of the pharynx. The metapleural folds arise and the 

 atrium begins to form and to close from behind. The primitive seg- 

 ments increase in number till the condition of the adult is in this 

 respect reached (sixty-one segments forming in A-mphiotu* lanceolatns). 

 The unpaired fin of the adult with its cavity develops (Figs. 290, 291). 



II. On the right side, above the row of primary gill-clefts, a second 

 row forms (wondanj clefts of WILLEY). After the atrium has closed, 

 the primary gill-clefts shift to the left side, while the secondary 

 remain on the right side. The larval mouth becomes changed into 

 the velum, and the definitive mouth-aperture arises as a consequence 

 of the formation of paired folds, the oral hood, round the larval 

 mouth. The mouth shifts into the ventral median line. The oral 



