IX 



EXTEROPNEUSTA ALIMEXTARY CANAL 



567 



Ill certain sections, the tissue of the proboscidal diverticulum, especially that of 

 its head, has a vesicular appearance which recalls that of the noto-chord of Verte- 

 brates. The proboscidal diverticulum of the Enteropneusta has even been called the 

 chorda, and has been directly homologised with this structure in Vertebrates. More 

 recent researches have, however, shown that the tissue of the diverticulum is an epi- 

 thelium in direct continuation with the intestinal epithelium of the buccal cavity. 

 This epithelium seems to consist of thread-like cells, which at one point swell up to 

 form vesicles or vacuoles containing a fluid clear as water. The vacuoles of adjacent 

 cells have not room enough to lie in the same level of the epithelium. They have 

 to make room for one another, and thus come to lie in very different levels of the 

 thickened epithelium. This makes the proboscidal diverticulum appear to have a 

 vesicular structure, especially in tangential sections. 



C. The branchial intestine. At the posterior end of the collar, 

 the buccal cavity passes on into the branchial intestine, which lies in the 

 anterior portion of the branchio- 

 genital region of the body. 



Here, as already mentioned, 

 the intestine is in communication 

 with the exterior through two 

 rows of pouch-like canals, which 

 will be described more in detail 

 later. 



The numerous pouches of each 

 longitudinal row follow one an- 

 other closely, and mutually flatten 

 one another, so that their lumina 

 become slit -like, and lie trans- 

 versely in the body at right angles 

 to its longitudinal axis (cf. Figs. 

 457, 458, 459). 



From each of these flat, 

 vertical, and transverse gill- 

 pouches, an inner aperture the 



* process ; 6, branchial septum ; 7, lowest tip of the 



gill-Slit, leads into the alimentary branchial tongue ; S, wall of the alimentary canal ; 

 Canal, and an OUter aperture, the 9 > ventral vessel ; 10, dorsal mesentery ; 11, ventral 



branchial pore, leads to the ^m. 12f gi 

 exterior. 



The inner aperture, the gill -slit, is as long as the gill -pouch 

 itself, and would have the shape of a very long were it not com- 

 plicated by the formation of the tongue. The intestinal wall projects 

 from the upper end of the gill-slit in the form of a hollow process 

 down into the slit, changing the into a very long vertical TJ 

 (Fig. 457, 12). This hollow process is the tongue. Its cavity is in 

 open communication with the coelom of the trunk. It either hangs 

 freely down into the gill-slit (Balanoglossus, Glandiceps), or is attached 

 to the wall of the gill-pouch by means of rods or buds, the so-called 

 synaptieulse which run across the limbs of the U-shaped gill-slit 

 transversely, making the latter fenestrated. 



9 8 11 2 



FIG. 457. Portion of the branchial region of 

 an Enteropneustan, cut down through the 

 middle line, and seen from the cut surface, 

 diagrammatic. 1, Dorsal vessel ; 2, body wall ; 

 3, cavity of the trunk ; 4, branchial pore ; 5, con- 



