MUSEUM OF COMPARATIVE ZOOLOGY. 35 



Summary. 



The segmented stem of Urnatella consists of a two-layered cuticula ; 

 an outer epithelium (ectoderm) consisting of cells flattened except at 

 the transverse septa and at the distal end of the stalk ; and an axial 

 portion consisting of elongated cells, many of which are vacuolated, and 

 surrounding which there is no intercellular substance. (Pages 4, 5.) 



The musculature of the stalk consists of radial sheets of fibrillse, sev- 

 eral of which develop in a single cell. (Pages 5, 6.) 



Many of the vacuolated cells of the stalk end in flame cells like the 

 •water or excretory tubules of Platyhelminthes. (Pages G, 7.) 



Yolk is developed in the cells at the base of the stalk, first as fine 

 intercellular granules, which later fuse, this process being accompanied 

 by cell degeneration. (Pages 7, 8.) 



The lip of the atrium contains a sphincter, and resembles in its rela- 

 tions the "margined thickening" of Ectoprocta. (Pages 9, 10.) 



The epithelium of the tentacles encloses a parenchymatous core. A 

 pair of muscles is present. (Page 15.) 



The alimentary tract resembles that of the Pedicellinidse, except that 

 the lower wall of the rectum and the upper wall of the intestine are in 

 close contact. (Pages 11, 12.) 



The nephridial tubules end blindly in flame cells, and open into a 

 cloaca, into which open also anus and the vas deferens. (Pages 14,15.) 



Two kinds of buds arise from the Urnatella stalk ; " branches," which 

 are typically median, and "stolons," typically lateral. (Page 16.) 



The segmentation of the stalk is probably an adaptation to the pro- 

 cess of budding, which is accompanied by a greater liability of the wall 

 of the stock to rupture, and therefore by a greater need of separation of 

 the stalk into compartments. (Page 20.) 



In all Endoprocta the oral aspect of the buds is turned towards the 

 centre of proliferation, and in all Bryozoa the aspect in which that end of 

 the alimentary tract which arises from the principal outpocketing of the 

 atrium lies is turned towards the gemmiparous zone. (Pages 22, 23.) 



The youngest stocks found consisted of a stolon bearing two indi- 

 viduals. This has been derived from the *' stolons " of the parent stalk, 

 which habitually become free for the purpose of founding new stocks. 

 (Page 26.) 



Urnatella is, structurally, one of the Pedicellinidse and most nearly 

 resembles Arthropodaria Benedeni. (Page 30.) 



The Bryozoa have probably been derived from Rotifer-like ancestors. 

 (Page 34.) 



