IX 



ENTEROPXEUSTA— ALIMENTARY CANAL 



569 



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

 eiliated. 



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-branehial 

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

 mrdium), 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 (Balnnoc/Iossus). In the last case the hypobranchial streak 



Fig. 459.— Vertical longitudinal section 

 through the anterior part of a row of 

 gills, and through a collar pore of Schizo- 

 cardlum hrasiliense (after Spengel). cpi, 

 Anterior aperture of the collar canal (into 

 the coelom of the collar) ; cp-?, posterior aper- 

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

 gill-pouch) ; hpi-hpQy first to sixth branchial 

 pores (outer apertures of the gills) ; bsrbs^, 

 first to fifth gill-pouches ; 6/4, ?j/g, fourth and 

 fifth gill-slits (apertures of tlie gill-pouches 

 into the branchial intestine) ; dvm^ dorsoven- 

 tral musculature ; cc, crelom of the collar ; 

 fc, ccfiloni of the trunlv ; r, blood vessels ; 

 Ic, continuation of the ccelom of tlie trunlc 

 into the branchial tongues ; I, branchial 

 tongues ; s, branchial septa ; S2i, first anterior 

 septal bar or prong ; Iz, tongue bars or 

 prongs ; crs, septum dividing the trunk 

 from tiie 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 Ptychodera (Fig. 458, 15, 16), inasmuch as they are 

 here separated by longitudinal ridge-like projections of the intestinal 

 wall, which 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 the gill-pouches, the branchial pores, has 

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

 Glandiceps and Sehtocardiuvi, with the submedian line, which is indicated by the 



