LARVAL DEVELOPMENT. 135 



cells, the pigment-ccUs, which arise as modified epithelial 

 cells of the central canal. These cells send out several 

 branching processes, which lose themselves in the fibrous 

 tract of the spinal cord. 



Already in the youngest larva — namely, that shown in 

 Fig. 64 — the prasoral pit had become subdivided into two 

 portions, which, however, retained a free communication 

 with one another. 



In the course of the changes u'hich the left head-cavity 

 had to undergo in its conversion into the prseoral pit it 

 had come to lie transversely below the notochord. Sub- 

 sequently it extended itself, in the form of an offshoot, 

 dorsally to the right of the base of the notochord. 



This offshoot from the prasoral pit appears to serve as a 

 special sense-organ lying ultimately, as mentioned above, 

 in the roof of the oral liood, whose function is possibly to 

 test the water as it enters the mouth (Figs. ^6 A and B, 

 and Fig. 74, etc.). 



Formation of Secondary Gill-slits. 



Above the primary gill-slits in Fig. 74, and like them, on 

 the right side of the body, is to be observed a longitudinal 

 ridge provided with a series of nodal enlargements which 

 alternate with the primar\' gill-openings, the first of them 

 lying above and between the third and fourth primary slits. 

 Each of these enlargements represents a thickening in the 

 wall of the pharynx, which has undergone fusion with the 

 body-wall beneath the right metapleural fold, in the angle 

 formed by the latter with the bodj'-wall. 



These metameric fusions of the pharyngeal wall with the 

 body-wall are the forecast of a second row of gill-slits, whose 

 relation to the primarv row will become clear as we pro- 



