ENTEROPNE USTA—A LIMENTABY CANAL 



569 



The epithelial walls of the gill- pouches and of the tongues are 

 ciliated. 



The depth (measured dorsoventrally) of the area occupied by the 

 gill-slits on the lateral wall of the branchial intestine varies greatly. 



In all cases the gill-slits leave only a narrow strip of the intestinal 

 wall in the dorsal median line ; this strip is the epibranchial streak. 

 Ventrally they never extend so far towards the median line. They 

 either leave a narrow strip of the intestinal wall, the hypo-branchial 

 streak, which is at any rate wider than the epibranchial streak (Schizo- 

 cardium), or they only extend a very short way on to the ventral wall 

 {Glandiceps), or again they only reach about half way down the lateral 

 wall (Balanoglossiis). In the last case the hypobranchial streak 



Fio. 459.— Vertical longitudinal section 

 through the anterior part of a row of 

 gills, and through a collar pore of Schizo- 

 cardium brasiliense (after Spengel). rp^, 

 Anterior aperture of tlie collar canal (into 

 the ccelora of the collar) ; o/io, posterior aper- 

 ture (collar-pore) of the same (into the first 

 gill-pouch); bpi-bpg, first to sixth branchial 

 pores (outer apertures of tlie gills) ; hsybs^, 

 first to fiftli gill-pouches ; 6/4, ft/g, fourth and 

 fifth gill-slits (apertures of the gill-pouches 

 into the branchial intestine) ; dvm, dorsoven- 

 tral nui.sculature ; ec, ccelom of the collar ; 

 /v, coeloni of the trunk ; v, blood vessels ; 

 If, continuation of the crelom of the trunk 

 into the branchial tongues ; I, brancliial 

 tongues ; s, Ijrancliial septa ; szi, first anterior 

 septal bar or jjrong ; Iz, tongue bars or 

 prongs ; crs, septum dividing tlie trunk 

 from the collar. 



occupies the ventral or nutritive half of the branchial intestine, which 

 is thus more or less distinct from the dorsal or respiratory half, into 

 which the gill-slits open. The distinction between these two halves is 

 still more marked in Fti/chndera (Fig. 458, 15, 16), inasmuch as they are 

 here separated by longitudinal ridge-like projections of the intestinal 

 wall, Avhich run on each side along the boundary between the two 

 (13). The two ridges growing towards one another may even touch, 

 in which case open communication between the branchial intestine 

 above and the oesophagus below ceases. 



The form of the outer apertures of tlie gill-pouches, the branchial pores, has 

 been described above. The furrows in which they lie correspond, in Balanoglossv.s, 

 Glandiceps and Schizocardium, with the subniedian line, which is indicated by the 



