BALANOGLOSSUS. 



255 



* 



erable extent (Fig. 120), increases greatly in size, and 

 assumes its definite position and proportions as the cavity 

 of the praeoral lobe (i.e. proboscis), thus replacing the 

 original blastocoelic space, 

 while the water-pore remains 

 as the proboscis-pore. 



As described in the previ- 

 ous chapter, the cavity of 

 the praeoral lobe (fixing 

 stolon) of the Ascidian tad- 

 pole is of the nature of a 

 blastocoel or primary body- 

 cavity, containing loose mes- 

 enchyme-cells, and it is 

 therefore of great impor- of J 

 tance to note that whether side. (After G. w. FIELD.) 



By a fusion of the two praeoral loops 



the Cavity of the praeoral O f the ciliated band across the apex of the 

 , , // 7 praeoral lobe, followed by a separation in 



lobe IS a blastOCCel Or an f he transvers ' e dire ction, the originally 



eiltcrOCCel, the morphological single circumoral band (cf. Figs. 119 and 



121) has become divided into two bands, 



value Of the Structure itself a preeoral ciliated band p.c.b. and a post- 

 rpmainQ thp came oral longitudinal ciliated band /.c.. The 



posterior transverse portion of the prae- 

 oral ciliated band has undergone a fusion 



Apical Plate of Tornaria. with the front end of the originally dis- 

 tinct adoral band (cf. Fig. 119). />./. Prae- 



At the anterior end of oral lobe, into which the enterocoal has 



extended, m. Mouth, r.e. and I.e. Right 



the body, Or, in Other words, and left enteroccelic cavities, st. Stomach. 

 ... . a. Anus. 



at the apex of the praeoral 



lobe, in Tornaria, there is an ectodermic thickening in 

 which nerve-cells and nerve-fibres and a pair of simple 

 eyes have become differentiated. This is the so-called 

 apical plate, and it constitutes the central nervous system 

 of the larva. It can be recognised for some time after the 

 metamorphosis at the tip of the proboscis, but eventually 

 disappears completely. A similar apical plate occurs in 



