HIGH SCHOOL ZOOLOGY, 



245 



>b 



Fig. 170. —Thread-cells of Hjdroid. I, undergoing 

 de\elopinent ; II, with the thread protruded. 



5. The Coelenterata also are "radiate," 

 but the parts are usually disposed in 

 twos, or fours, or sixes ; instead of the 

 complete intestine and complicated 

 coelom of the preceding group, there is 

 only one cavity which discharges the 

 functions of both ; peculiar modified 

 cells (thread-cells or nettling organs, 

 Fig. 176), take on a defensive function, 

 by pouring an acrid poison into wounds 

 inflicted by their microscopic barbs. 

 From the tendency to form colonies 



(lU 



f 



Fl?. 177. — n>idra viridix, at- 

 tached to a weed, with buds in 

 two stai^es of development. 



/ n. 1 iTi • r \ ,1 • I" o scaifes 01 ae\ eiopmeni. 

 (Otten plant-like m lorm) this gl'OUp, The expanded tentaclesapj.ear 

 1 • 1, • 1 i 1 • 1 • n-ranular owintr to accumula- 



which IS almost exclusively marine, tions of thread-ceiis. a, a 

 used to be called the Zoophytes. thread-ceii. 



Apart from some singular pelagic forms which have comb-like 

 ridges of cilia on the surface (Ctenophora), the Coelenterates 

 fall into two classes — the Hydrozoa and the Actinozoa. The 

 former name is derived from the fresh-water Polyp, Hydra 

 (Fig. 177), a cosmopolitan form very much simpler in its organi- 

 zation than its marine allies. It does not form colonies, but 

 buds oflf individuals which soon become detached from the 

 parent. Eggs are formed, during a short period of the year, in 

 the wall of the two-layered tube which constitutes the body. 



