OTHER UNSEGMENTED WORMS 187 



thin shells and develop parthenogenetically. The winter eggs have thick 

 shells, and development follows fertilization. The winter eggs require 

 a considerable degree of acidity of the water to soften the shells so that 

 they may hatch, and if this does not occur they are capable of living 

 for many years and of still developing when placed under suitable 

 conditions. 



A marked peculiarity of certain rotifers is their power to undergo 

 drying and when again under favorable conditions to resume life activi- 

 ties. This ability and their small size contribute to the ease with which 

 they may be dispersed by the wind. This ease of dispersal by wind, 

 added to the possibility of their being carried on the feet of water birds, 

 has resulted in a distribution which is of more cosmopolitan character 

 than that exhibited by any other group of 

 animals. Rotifers which produce fertilized eggs 

 cannot resist desiccation. 



220. Phylum Bryozoa. — The animals included in 

 the phylum Bryozoa (bri o zo' a; G., hryon, moss, 

 and zoon, animal) are known in a general way as 

 moss animals and sea mats. They are colonial 



forms and their manner of growth reminds one of yig. 95. A colony 



the colonial hydroids, but their structure distinguishes o^ marine bryozoans, 



, , , T -1 1 1 1 • 1 1 1 Bugula turhinata Alder. 



them very clearly. Like the hydroids they have a British species; an 

 planthke characteristics which often cause them to American species, 



,.11,. 11 ,. r 1 • 1 11 1 J Bugula turrita (Desor) 



be mcluded m collections of dried seaweeds brought ig somewhat similar, 

 as souvenirs from a seaside trip. The bryozoans (P'rom Harmcr, "Cam- 



orxdoG N^ atuToJ H xstovu 



are mostly marine, though a few live in fresh water, ^^z the courtesy of The 

 They are all very small, but a colony consisting of Macmiiian Company.) 



1 1 -.,.., 1 ,, . Natural size. 



a large number oi individuals may attain con- 

 siderable size, projecting in some cases several inches from the 

 surface to which it is attached. Some of these colonies show a 

 treelike manner of growth and suggest the name sea moss (Fig. 95). 

 Other colonies form matlike masses and are quite appropriately termed 

 sea mats. They may be free, like the fronds of seaweeds, or encrust the 

 surface of stones or other objects. 



The individual animal lives in a cup-shaped or tubular chitinous 

 shell, known as a zooecium, which is open at the outer end (Fig. 96). 

 The surface of the body lines the inner wall of the shell and the animal is 

 capable of withdrawing itself into a body cavity which in these forms is a 

 true coelom. The chitinous shells of the individuals form together a 

 support for the colony. Sometimes lime is added. Because of their 

 superficial resemblance to corals such forms are known as coralline 

 bryozoans. A mouth at the outer end is surrounded by a crown of 

 ciHated tentacles termed the lophophore, which exhibits the form of a 

 horseshoe when it is expanded. The U-shaped alimentary canal opens 



