XIII 



PHYLUM CHORDATA 



adjacent (anterior and posterior) walls. 

 In this way the cavities of the entire 

 series of ventral plates, right and left, 

 unite to form the single unsegmeiited 

 coalome of the adult, their walls giving 

 rise to the coslomic epithelium. 



At the same time the cells of the 

 splanchnic layer of the protovertebrse 

 become converted into muscular fibres, 

 which nearly fill the myocoele, and give 

 rise to the myomeres : the myocommas 

 arise from the adjacent anterior and 

 posterior walls of the protovertebrse. 

 An outpushing of the splanchnic layer, 

 at about the level of the ventral sur- 

 face of the notochord, grows upwards 

 between the n.yomere externally and 

 the notochord and nerve-tube intern- 

 ally: from the cells lining this pouch 

 the connective-tissue sheath of the 

 notochord and nervous system arises, 

 and perhaps also the fin-rays. From 

 the parietal layer of the protovertebra3 

 arises the derm or connective tissue 

 layer of the skin. 



The larva increases in size, and be- 

 comes very long and narrow, with a 

 pointed anterior end and a provisional 

 caudal fin posteriorly (Fig. 709, c). As 

 growth proceeds, new segments are 

 added behind those already formed, the 

 notochord grows forwards to the an- 

 terior end of the snout, and the eye- 

 spot (au.) and olfactory pit appear, the 

 latter as an ectodermal pit which com- 

 municates with the neurocoele by the 

 still open neuropore (np.). The mouth 

 (m.) attains a relatively immense size, 

 still remaining on the left side. 



Additional gill-slits appear behind 

 the one already mentioned : they all 

 make their appearance near the middle 

 ventral line, an3~gractually shift over 

 to the right-side : at first they corre- 

 spond with the myomeres, so that the 

 segmentation of the pharynx is part 

 of the general metamerism of the 

 body. Altogether fourteen clefts are 



