406 



TUNICATA. 



on either side with the wall of the peribranchial sacs, and the 

 perforation then taking place in the base of the outgrowth. The 

 entodermal lamella thus takes a more active part in the development 

 of the gill-slits than does the ectodermal wall of the peribranchial 

 sac. The vertical bars between the adjacent gill-slits which, in 

 cross-section, are almost quadrangular, are not merely covered on 

 their inner surfaces with entoderm, but their lateral surfaces also 

 which are turned towards the slits belong to the entoderm. Only 

 the covering of the external surface is derived from the ectodermal 



Fig. 200. — Diagrammatic views of an Ascidiozooid at the stage depicted in Pig. 194 

 (following Salensky). A, viewed from above ; B, from below, cl, atrium ; d, 

 pharynx (branchial sac) ; ed, rectum ; el elaeoblast ; es, rudiment of endostyle ; fl, 

 ciliated pit ; i, inhalent or branchial aperture ; ks, gill-slits ; m, stomach ; n, 

 nervous system ; oe, oesophagus ; p, peribranchial sacs ; jjc, pericardial vesicle ; m. 

 rudiment of the lateral nerves. 



wall of the peribranchial cavities. The gill-slits in Pyrosoma, accord- 

 ing to Salensky, appear in order from before backward, the most 

 anterior slit forming first. After the gill-slits have broken through, 

 they very soon lengthen ; those of Pyrosoma, indeed, are distinguished 

 for their length. The internal longitudinal bars, which cross the 

 slits at right angles and give rise to the characteristic lattice-like 

 appearance of the branchial wall, develop later as independent in- 

 growths from the vertical bars which become secondarily connected. 

 The gill-slits, as Seeliger has pointed out, seem always to lie 

 at right angles to the endostyle (Fig. 201, ks and es). Since the 

 endostyle of the Ascidiozooids originally runs horizontally, as may 

 be seen in the diagram Fig. 201 A, and then later adopts a vertical 

 position (Fig. 201 C) the gill-slits pass gradually from a vertical to 



