PYROSOMA THE FOUR PRIMARY ASCIDIOZOOIDS. 405 



become more markedly constricted from one another, these rud 

 ments also are cut up into sections corresponding to the different 

 individuals. The peribranchial tubes become completely dissevered 

 at the boundaries of the individuals (Fig. 193 J3), each Ascidiozooid 

 then containing a pair of lateral closed sacs, the peribranchial cavities 

 (p). The remains of the peribranchial tubes in the Cyathozooid, as 

 already mentioned (p. 400) then atrophy. The enteric rudiment, 

 however, does not at first undergo such complete constriction 

 between the individual Ascidiozooids, but a canal is retained in the 

 connecting trabeculae which establishes communication between the 

 consecutive individuals (Fig. 194 E, v). This canal does not dis- 

 integrate until the Ascidiozooids separate completely on attaining 

 their full development. A vestige of it plays an important part in 

 the later development of buds, changing into the so-called endostyle- 

 process or en toderm- process of the stolon (SEELIGER, p. 486). 



The rudiment of the endostyle- fur row was recognisable in the 

 primary enteric rudiment of the Cyathozooid even before the chain 

 of Ascidiozooids began to form (Figs. 189, 190, en). That part of 

 the alimentary canal of the Cyathozooid which was distinguished by 

 the presence of the endostyle-rudiment was the part which, by 

 lengthening, changed into the common enteric rudiment of the 

 four primary Ascidiozooids. Consequently, the endostyle-rudiment 

 originally runs through all the four Ascidiozooids (Fig. 193, en). At 

 a later stage, however, only the posterior part of it is retained in each 

 individual (Figs. 194, en, 200, es), becoming the definitive endostyle 

 of the zooid. In cross-section, the endostyle-rudiment originally has 

 the form of a broad fold projecting into the lumen of the intestine 

 (Fig. 203 A), the lateral part of which shows epithelial thickenings 

 which, in a surface view, appear as dark bands. The depression 

 between them flattens out later, but, on either side, the paired 

 endostyle-folds project inwards (Fig. 203 B, es). It is not yet 

 clearly understood in what way these lateral endostyle-folds together 

 with the middle part yield the endostyle of the adult. 



In front of the anterior end of the endostyle-rudiment there is, in 

 the developing Ascidiozooid, a pit-like ectodermal depression (Figs. 

 194, 200, i), the rudiment of the inhalent or branchial aperture. 

 The space in front of this is occupied by the rudiment of the central 

 nervous system (Figs. 194, 200, n). 



The lateral walls of the branchial sac of the Ascidiozooid are in 

 contact with the peribranchial tubes (Fig. 193, p). Here the gill-slits 

 (ks) break through (Fig. 200), a small entodermal outgrowth fusing 



