MUSEUM OF COMPAKATIVE ZOOLOGY. 23 



take place quite independently of the former's. The lophophoric ridges 

 have now become elongated folds lying upon the right and left of the 

 polypide, which at this stage has a very compressed appearance (Plate IV. 

 Fig. 41). The folds occupy the position of the ridges, and therefore do 

 not lie throughout their whole extent in one plane, but oralwards are 

 nearly parallel to the body wall (Plate III. Fig. 25), analwards trend 

 nearly at right angles to it. It results from this fact, that one cannot 

 see the anal tentacles when looking at the polypide from the side of 

 the body wall to which it is attached. Figure 41 (Plate IV.) shows also 

 that no tentacles have yet made their appearance at the oral ends of the 

 two lophophoric ridges.-^ The tentacles are here seen to be arising in 

 two long rows, and so that those of one row are placed opposite the in- 

 tertentacular spaces of the other. There are six tentacles in each row. 

 The rows are not continuous with each other oralwards or analwards. 



The separation of the atrial and oral cavities, begun by the first 

 formation of the lophophore, is, now that the tentacles have arisen, much 

 more pronounced. Other changes now occur in this region, which pro- 

 duce an extensive modification in the form of the polypide. 



One of the first of these changes is the close approximation and 

 finally fusion of the anal extremities of the lophophoric ridges oralward 

 of the anus. A stage in this is shown in Figures 43 and 44 (Plate V.), 

 which are sections in the position of the lines Jf.8, Ii.Jf, of Figure 25 

 (Plate III.), but through a slightly older polypide than that represented 

 by Figure 25. The section shown in Figure 43 passes across the rec- 

 tum, grazes the outer lip of the ring groove of the anal tentacles, and 

 finally cuts, nearly longitudinally, one of the middle tentacles of the 

 row. The two lophophores are not yet completely fused in front of the 

 rectum. In Figure 44 (compare Plate III. Fig. 25, 44) this break in 

 the continuity of the lophophore is more prominent. 



By the completion of the union of the lophophores in front of the 

 anus, the rectum is quite cut off from communication with the inter- 

 tacular space. It now opens only into the thin-walled, funnel-shaped 

 depression of the atrial cavity. 



Pari passu with this operation the stomach and rectum are being 

 more completely separated from the pharyngeal cavity by the penetra- 

 tion of a double layer of mesoderm between these regions from each side, 

 and a fusion of the cox-responding layers of the two sides. Finally, the 



1 Compare Plate IX. Figure 77, which is a superficial view of the young lopho- 

 phore from Flustrella, in which tlie process is similar to that in Paludicella, only 

 the down curving of the anal tentacles occurs later than in the latter case. 



