THE ORDER AMONG ANIMAL TYPES 



lOI 



ing cell termed a cnidoblast. The cnidoblast contains a coiled barbed 

 organ which is employed in defense but more especially in the 

 capture of its food. The ectoderm contains cells which exaggerate 

 the property of irritability and function as a rudimentary nervous 

 system. In this and in other respects the phylum marks a distinct 

 advance over the phylum Porifera. Three classes are recognized: 



Class I. HYDROZOA (Fig. 33A and B). Attached or free swimming 

 forms. The digestive cavity is undivided. Examples are the fresh 



COELENTERATA 



ECTODERM 

 ENDODERM 



TENTACLE 

 NEMATOCYST 



CASTROVASCULAfi 

 CAVITY 



FIG. 33^. HYDROZOA-HVDRA (WALL CUTAWAY) 



water Hydra, the composite attached Hydroids, mostly marine, 

 found usually attached to rocks or debris, and very small free swim- 

 ming forms shaped like umbrellas, known as Medusae, which are 

 in reality the free swimming stages of the Hydroids. 



Class II. scYPHozoA (Fig. 34). The Jelly Fishes. Free swimming 

 forms shaped like umbrellas. Some have stalks and attach to plants 

 or other objects. These animals contain a very high proportion of 

 water and if stranded in the sun soon evaporate to a residue that 



