632 - Heredity and Evolution 



TRIGGER 

 PRONG 



NEMA 



EPITHELIO-MUSCLE 

 CELL 



NEMATOBLASTS, BEFORE 

 AND AFTER DISCHARGE 



PROTONEURON 



CELL TYPES - 

 CLOSELY ASSOCIATED 



SENSORY 

 CELL 



Fig. 32-8. Hydra. Diagram of cellular structure. Note the different types of cells and the 

 absence of clearly organized tissues and organs. 1, nematoblast; 2, interstitial cell; 3, ecto- 

 dermal cell; 4, mesoglea; 5, endodermal cell; 6, gland cell. 



recognizable and there are no other well- 

 developed organs. Sometimes the body is 

 covered by a horny, but delicately transpar- 

 ent envelope as in the case of Obelia (Fig. 

 32-9). Also many Coelenterata, especially the 

 coral animals, secrete supporting structures 

 at the basal (attached) end of the body. Usu- 

 ally these are composed of calcareous mate- 

 rial, but sometimes they have a protein com- 

 position. 



(9) All Coelenterata are aquatic, chiefly 

 marine. 



Classes of the Phylum Coelenterata. Three 

 distinct classes have been evolved among 



coelenterate animals. These may be listed as 

 follows: 



Class 1. The Hydrozoa (Hydra, Obelia, and 

 Related Forms). At some stage in the life cycle, 

 typical Hydrozoa tend to resemble Hydra. 

 Many are colonial, however. Also many dis- 

 play an alternation of generations, which is 

 a phenomenon cpiite rare in the animal king- 

 dom. In the case of Obelia (Fig. 32-9), for 

 example, the asexual hydroid generation is 

 represented by the colonial form of the or- 

 ganism, in which the individual members of 

 the colony display a hydralike form; whereas 

 the sexual medusoid generation takes the 



