CHAPTER XXVIII 

 MOSS ANIMALCULES (BRYOZOA) 



By CHARLES B. DAVENPORT 



Director of the Station for Experimental Evolution, Cold Spring Harbor, Long Island, N.Y. 



Prominent among the animals commonly discovered in fresh 

 water are the Bryozoa, or moss animalcules, also called Polyzoa. 

 They are forms of exceedingly dehcate and attractive appearance, 

 often so transparent that under favorable circumstances the entire 

 structure may be made out under the microscope. Almost all 

 species form colonies composed of many individual animals (zooids) 

 united together and the whole mass is not only easily visible but 

 often conspicuous, whereas the separate zooids are so minute that 

 they can ordinarily be detected only with a hand lens. These 

 colonies take the form of branching threads spread on the surface 

 of stones, sticks, submerged plants or other objects in the water. 

 Others produce a thick crust, while still others form soHd jelly-like 

 masses which in one species reach the size of the closed fist and not 

 infrequently surpass that (Fig. 1401). The bulk of this mass con- 

 sists of transparent or faintly tinged gelatinous material from 

 which the individual zooids project into the water as they also 

 do from the filamentous colonies previously mentioned. The 

 expanded "head" (lophophore) of the zooid with its crown of 

 tentacles is difficult to see since the animals are exceedingly timid 

 and respond to the slightest disturbance by retreating instantly 

 within their protective covers where 

 they remain even long after the water 

 has become quiet again. Continuous 



study of the colony in a dish of fresh Fig. 1393. CristalcUa mucedo; colony, 



. natural size. (After Allman.) 



water is rewarded by the appearance of 



the spreading disks or heads, until the surface of the colony blossoms 



with abundant groups of delicate tentacles. (Fig. 1393.) 



All are essentially sessile, but a few, like Cristatella and Pectina- 

 tella, have the capacity for a sHght movement of the colony on 



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