C(ELENTERATA. 



175 



192. The Hydrozoa: Polyp stage. The group of ccelenterates 

 includes, beside Hydra, many similar forms which are considerably 

 more complex. They are like Hydra in that they have only one 

 cavity in the body in which digestion and circulation take place. 

 The mouth opens directly into this. There are only two layers of 

 cells, the ectoderm and entoderm, but between these is a layer of 

 secreted substance which helps to give form to the body. Like 

 Hydra, they bud, but the new individuals remain attached to the old 



FIG. 47. Hydractina echinata, after Hincks. c, ccenosarc, forming an incrustation 

 over the object on which it lives. From this the polyps bud. n, nutritive polyps; r, 

 reproductive polyps, bearing buds in which are ova; t, tentacles. 



Questions on the Figure. How many types of individuals seem 

 to be represented? What evidence of reproduction by budding? 

 How does this colony compare with that in Fig. 48? 



in such a way as to produce colonies of these slender tentacled 

 individuals (Fig. 47). The colonies are attached. While containing 

 many animals these colonies do not become very large. The 

 ectoderm has the power of secreting a layer of horny matter which 

 forms the skeleton of the main stalk and branches, and protects the 

 soft parts. As is seen in the buds of Hydra (Fig. 46, b), the digestive 

 cavity is continuous between the bud and the mother. This brings 



