74 



Dr. R, P. Cowles on the Body-cavities and 



I believe that the cavity of the trunk is formed in the 

 following manner : — As the tentacles grow out and increase 

 in number the posterior region of the larva about the rectum 

 increases greatly in length. In doing the latter the meso- 

 dermal lining of the collar is drawn away from the somatic 



Fi?. 3. 



Fi- 2 



Vny. 2. — Longitudinal section through young larva, not quite sagittal, 

 X 704. c.c, collar-cavity ; g., ganglion ; m.^, mesentery be- 

 tween lobe- and collar-cavities ; n.p., nephridial pit ; p.o.c, cavity 

 of preoral lobe. 



Fig. .3. — Young larva of Phoronis architecta ; three pairs of tentacles and 

 beginnings of the fourth pair, X 225. c.c, collar-cavity ; ex.c, 

 excretory cells ; f/., ganglion ; -jh}, mesentery between lobe- and 

 collar-cavities ; ?«.", mesentery between collar- and trunk- 

 cavities ; t., tentacle ; t.c, cavity of trunk. 



wall in the region back of the tentacular band, and a cavity 

 is left containing the rectum, part of the stomach, and the 

 proximal part of the nephridial diverticula. At the same 

 time this is taking place certain cells, which may iiave their 

 origin from the base of the nephridial diverticula, give rise to 

 the lining of the cavity of the trunk. As to the manner of 

 the origin of these cells, 1 am still in doubt. I have not been 

 able to find two coelomic sacs which Hatschek has figured, 

 and I have hunted for them in larvae where the diverticula 



