ZOOLOGY 



SECT. 





it 



ill 



3j| 



O tt><3 



connective-tissue sheath of the noto- 

 chord and nervous system arises, and 

 perhaps also the fin-rays. From the 

 parietal layer of the protovertebrse is 

 formed 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. 754, c). As 

 growth proceeds, new segments are 

 added behind those already formed, 

 the notochord grows forwards to the 

 anterior end of the snout, and the eye- 

 spot (au.) and olfactory pit appear, the 

 latter as an ectodermal pit which com- 

 municates with the neurocosle by the 

 still open neuropore (np.). The mouth 

 (m.) attains a relatively immense size, 

 stnH-emaining on the left side. 

 /^Additional gill-slits arise behind the 

 one already mentioned : they all make 

 their appearance near the middle 

 ventral line, and gradually 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 produced 

 in a single longitudinal series. Above, 

 i.e. dorsal to them, a second longitudinal 

 series makes its appearance, containing 

 eight clefts, so that at this stage there 

 are two parallel rows of gill-slits on 

 the right side of the body, and none 

 on the left. But as growth goes on, 

 the first or ventral series gradually 

 travels over to the left side, producing 

 a symmetrical arrangement, and at the 

 same time the first slit and the last 

 five of the first or definitively left series 

 close up and disappear, so that the 

 numbers are equalised on the two sides s 

 At first each gill-slit is simple, but 

 before long a fold grows down from its 

 dorsal edge, and, extending ventrallyy 

 divides the single aperture into two : 

 this fold is the secondary or tongue- 



