XVIIl] 



DIVISIONS OF THE COELOM 



395 



The lower portions of the somites fuse with one another and 

 form a continuous body cavity round the hinder part of the 

 alimentary canal which is called the splanchnocoel (<nr\<iyxvov, 

 entrail) (b.c. Fig. 190) with the anterior end of which the lower 

 portions of the collar cavities fuse. In front, owing to the presence 

 of gill-slits, there are formed a right and a left dorsal coelomic canal, 

 and a ventral coelomic tube, or 

 endostylar coelom, the dorsal 

 and ventral portions communi- 

 cating with one another by 

 spaces in the gill-bars, that is, 

 in the pieces of body-wall in- 

 tervening between the gill-slits. 

 These spaces are termed bran- 

 chial coelomic canals (Fig. 

 198). 



A ridge grows out on each 

 side of the body into a flap, 

 which meets its fellow beneath 

 the ventral wall of the body and 

 thus they enclose a space, the 

 so-called a trial cavity (Figs. 

 189, 190). This still communi- 

 cates with the exterior through 

 the atrial pore. The gill-slits, 

 which occur in the front part of 

 the trunk region, open into this 

 atrial cavity. The atrial flaps, 

 enclosing the atrial cavity, are 

 an obvious arrangement by 

 which the slits are saved from 

 being choked with the sand in 

 which the creature lives. As in 

 the case of Hemichorda, the 

 slits of Ampliioxus are divided 

 into two, by a tongue-bar 

 reaching from the upper margin 

 almost to the bottom of the 

 slit. Each slit thus becomes U-shaped (Figs. 188, 191). 



The atrial flaps are at first formed entirely from the splanchno- 



FIG. 190. Transverse section through 

 post-pharyngealregionofyoung^mp/i/- 

 oxus, to show groups of renal cells in 

 floor of atrium. After Lankester and 

 Willey. 



ao. Aorta, at. Atrial cavity, b.c. Body- 

 cavity (coelom). c.c. Central canal of 

 nerve-cord (n.c.). ch. Notochord. 



d.f.c. Fin-cavity. i.m : Intercoelic 

 membrane, int. Intestine. l.m. and 

 r.m. Left and right metapleural folds. 

 r.p. One of J. Miiller's ' ' renal papillae." 

 s.i.v. Subintestinal vein. 



