378 



TUNICATA. 



primary slits (Fig. 174, tr). These folds are the rudiments of the 

 transverse bars; prominences (p) form on them both anteriorly 



Pig, 174. — Two young stages of Phollnsin (Ascltliclln) sea I in rides (after van Beneden 

 and Julin, somewhat altered). 1-6 in A, the six primary gill-clefts, in B, the series 

 of gill-clefts derived from them. el, atrial cavity ; d, intestinal loop ; e, atrial aper- 

 1 are ; es, endostyle ; /', ciliated pit ; fb, ciliated arch ; <j, ganglion ; tjs, genital strand ; 

 tjs', genital vesicle; /, inhalent orifice; br, internal longitudinal bars; or, paired 

 rudiment of the atrial aperture ; j>, papilla-like rudiment of the longitudinal ribs ; 

 rs, renal vesicles; t, ring of tentacles; tr, transverse ribs of the branchial network. 



and posteriorly and give rise to papillae. The papillae on the con- 

 secutive branchial arches lie so near each other as to come into 



