490 



TUNICATA. 



the first rudiment of the endostyle (Fig. 259, es). The latter is com- 

 pleted later through the rise of two lateral folds, the median groove 

 becoming the base of the endostyle-furrow. The condition here, 

 according to JOLIET and SEELIGEB, corresponds perfectly with that 

 described for the four, primary Ascidiozooids (p. 406). The rudiment 

 of the endostyle originally occupies the upper or neural wall of the 

 branchial sac, its proximal end corresponding to the later anterior end. 

 The endostyle-rudiment, according to JOLIET (as described on p. 413), 

 here passes over into the remarkable provisional structure known as 

 the diapharyngeal band, which runs towards the ganglion below the 

 branchial aperture. 



The two lower outgrowths of the branchial sac, as seen in trans- 

 verse section, are the rudiment of the alimentary canal proper (?'.#.,, 



FIG. 257. Two transverse sections through the stolon of Pyrosonw .(after SEELIGER). 

 ec, ectoderm ; en, entoderm ; g, genital strand ; I, body-cavity ; m, mesoderm ; it , 

 neural tube ; n, young egg-cell ; p, peribrancliial tubes ; sn, rudiment of the lateral 

 nerves. 



oesophagus, stomach and intestine). These two outgrowths com- 

 municate at the distal end of the bud. This rudiment, which soon 

 becomes abstricted from the branchial sac, is thus horse-shoe-shaped. 

 The right portion, which retains a connection with the branchial sac, 

 gives rise to the oesophagus and stomach, while the left portion be- 

 comes the intestine, which ends blindly at first and later opens into 

 the atrial cavity (Fig. 253). SEELIGER'S account agrees fairly well 

 with SALENSKY'S description given above (p. 408). The digestive 

 gland, which originates as an outgrowth, arises at the boundary 

 between the stomach and the intestine. 



The lateral walls of the branchial sac are occupied by the gill-clefts. 

 These are at first (Fig. 256 (7, ks) more or less round perforations of 



