378 



TUNICATA. 



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

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



FIG. 174. Two young stages of Plwllusia (Ascidiella) scabroides (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, cl, atrial cavity ; d, intestinal loop ; e, atrial aper- 

 ture ; es, endostyle ; f, ciliated pit ; fb, ciliated arch ; g, ganglion ; gs, genital strand ; 

 gs', genital vesicle; i, inhalent orifice; Ir, internal longitudinal bars; or, paired 

 rudiment of the atrial aperture ; p, 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 



