MOSS ANIMALCULES (BRYOZOA) 951 



To kill expanded it is necessary first to narcotize. Chloral 

 hydrate is used, added slowly in crystals until the polypides do not 

 react to touch. To preserve in the natural form, the animals may be 

 plunged directly into 4 per cent formaldehyde (formalin, 10 per cent). 



The classification of fresh- water Bryozoa has been in a state of 

 great confusion owing to the great variability in the form of the 

 colony. The form of the colony depends very largely upon external 

 factors, such as food supply, form of substratum, and crowding. 

 The statoblast has a form that is quite independent of external 

 factors, and upon it, consequently, great stress is laid in systematic 

 work. The form of the statoblast is, however, not wholly uncor- 

 related with that of the stock, so the form of the stock is to be 

 considered. In the following classification that proposed by 

 Kraepelin has been adopted entire. 



KEY TO NORTH AMERICAN FRESH-WATER BRYOZOA 



1 (2) Anus opening inside the tentacular corona; tentacles incapable of 



complete retraction Subclass Endoprocta. 



Only one species known in North America. 



Urnatella gracilis Leidy 1851. 



Stock consisting of a basal plate, from which 

 there usually arise two segmented stems termi- 

 nating in the polypide. More rarely one or 

 more than two stems arise from the disk. 

 Habit, running water. From Schuylkill River, 

 Pa.; Scioto River, O.; Illinois River, 111. 



FIG. 1394. Urnatella gracilis. (a) Colony from 



fl Ek 1H> Illinois River at Havana. X 13. (After Daven- 



, 111 P rt -) (6) Single polyp. '*'*-- T --' J ~^ 







2 (i) Anus opening outside the tentacular corona, which is capable of being 



retracted Subclass Ectoprocta . . 3 



3 (6) Zooecia sharply separated from each other; no epistome. 



Order Gymnolaemata . . 4 



4 (5) Stock consists of stolons from which at intervals an erect cylindrical, 



hyaline single zooid arises, having a terminal aperture. 

 Pottsiella erecta Potts 1884. 



Lophophore circular, 20 (19 to 21) tentacles. Habitat, 

 photophil; on upper surface of stones in rapids; sometimes 

 penetrating incrusting sponges (Ephydatia leidyi). From 

 Tacony Creek, Montgomery Co., Pennsylvania. 



FIG. 1395. Pottsiella erecta. X 25. (From Kraepelin 's figure of a 

 Pennsylvania specimen.) 



