£.-ll.-l.VOGLOSSi'S. 



^55 



/<=--l-.. 



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

 assumes its definite position and proportions as tlie cavity 

 of the prasoral lobe (/.<■. proboscis), thus replacing the 

 original blastoccelic space, 

 while the water-pore remains 

 as the proboscis-pore. 



As described in the previ- 

 ous chapter, the cavit)- of 

 the prajoral lobe (fixing 

 stolon) of the Ascidian tad- 

 pole is of the nature of a ^c.i 

 blastocct'l or primary body- 

 cavity, containing loose mes- 

 enchvme-cells, and it is 



.If L- ,. ■ Fie. 122. — Older lar\'a 1 Bipinn.iri.i) 



therefore of great minor- , ,= , , \/ , , 



& i ot . ls/^-/\\is :'.v.;'\:r;.\-, troni the ventral 



tance to note that whether side. i.After g. w. field.) 



B\' a fusion oi the two pr;vornl loops 

 the cavity of the pr^roral of the ciliated hand aeross the ape.x of the 

 1 ,u . :., ^ ;,/,.^,.^./ .^,. r^^ pra^oral lobe. follo\\ed by a separation in 



lobe IS a oiastocai or an ^ . j- ,■ ■ ., ■ ,, 



the transverse direction, the originalty 

 Olft'lVar/, the morphological single eircumoral band (cf. Figs, iig and 



I2i) has become divided into two bands, 

 a pr.voral ciliated band /.t'.^. and .i post- 

 oral longitudinal ciliated band /.t..'-. The 

 posterior transverse portion of the pr.v- 

 oral ciliated band lias undergone a fusion 

 with the Iront end of the originally dis- 

 tinct adoral band (cf. Fig. no). /./. Pra?- 



At the anterior end of ^^"' ^f'' '"'•> ^^hi'-'h <he enterocofl has 



extended, m. Mouth, r.tr. and /.f. Right 

 the botlv, or, in other words, and left cnteroCLvlic cavities, ji'. Stomach. 

 , ' , , T '!■ Anus. 



at the apex of the praxiral 



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 ner\'ous system 

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

 metamorphosis at the tip of the proboscis, but evontuall\' 

 disappears completel)'. A similar apical plate occurs in 



value of the structure itself 

 remains the same. 



Apical Plate of Tornaria. 



