190 



METAZOAN PHYLA 



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 con- 

 ditions to resume life activities. 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 distribu- 

 tion which is of more cosmopolitan character 

 than that exhibited by any other group of 

 animals. 



222. Phylum Bryozoa. — The animals 



included in the phylum Bryozoa (briozo'a; 



G., bryon, moss, and zoon, animal) are known 



-c^ -.f^, rr.. ..^ rr , in a general way as moss animals and sea mats. 



Fig. 101.— The rotifer Hyda- „, *= , • i i- wi • 



Hnasenta Ehrenberg. A, mature i hey are colouial lorms and their manner of 

 female. B small degenerate male growth reminds ouc of the coloiiial hydroids, 



showing the large sperm sac but , 



c, but their structure distinguishes them very 



Like the hydroids they have plant- 



lacking a digestive system. 



parthenogenetic egg covered with dearlv 



a thin membrane and developing . . 



within a few hours into a female, hkc characteristics which often cause them to 

 D, smaller parthenogenetic egg be included in Collections of dried seaweeds 



covered with a thin membrane 



and developing within a few brought as souvcnirs from a Seaside trip, 

 hours into a degenerate male The biyozoaiis are mostly marine, though a 



which IS sexually mature at time . 



of hatching. E, fertilized egg few livc in fresh Water. They are all very 



small, but a colony consisting of a large num- 

 ber of individuals may attain considerable 

 size, projecting in some cases several inches 

 from the surface to which it is attached. 

 Some of these colonies show a treelike man- 



mentai Zoology.) 



covered with a thick horny shell; 

 it will survive desiccation and the 

 extreme temperatures of 100°C. 

 or — 180°C. Some have remained 

 viable in stale culture water for 

 22 years. If unfertilized, this 

 egg would have formed a small 

 parthenogenetic egg which would 



have developed into a male, ner of growth and suggest the name sea moss 

 {From Whitney, Journal of Experi- (Fig_ ^02). Other colonies form mathke 



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. 103). 

 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 



