THE ASCIDIANS. 



the oesophageal aperture, as a low ridge, which joins the 

 posterior extremity of the endostyle.* 



Visceral Anatomy. 



Except in its most anterior region, the dorsal border of 

 the pharynx lies freely in the atrial chamber. On the 

 contrary, along its ventral border, throughout the whole 



length of the endo- 

 style, it is attached to 

 the muscular mantle. 

 In other words, the 

 right and left halves 

 of the atrial cavity are 

 continuous round the 

 dorsal side of the 

 pharynx, but are 

 separated from one 

 another ventrally by 

 the concrescence of 

 the endostyle with 

 the mantle. (Cf. Fig. 

 95.) In Amphioxus, 

 as we have seen, the 

 opposite condition ob- 

 tains. There, the dor- 

 sal wall of the phar)'nx 



-r.o 



Fig- 95. — Diagrammatic transverse section is closely applied tO 

 througii the middle of the body ol Asctdia. (After 

 Herijman.) The muscular mantle is indicated 

 by the blaclc shading. 



a. Peribrancbial cavity traversed bv numeious 

 vascular trabeculne, through which the blood flows 

 into the branchial bars. br.s. Branchial sac. 

 b.v. " Blood-vessels." d.l. Dorsal lamina, e. Endo- 

 style. ec. Ectoderm, g. Gonad, gd. Double 

 genital duct. /. Intestine, with typhlosole. r. Rec- 

 tum, r.o. Renal vesicles, t. Test. 



the notochord, while 

 the endostylar tract 



' Compare the above with 

 the description of the course 

 of the ciliated tracts in the 

 pharynx of Animoccetes, 

 given on p. 16S. 



