THE ORDER AMONG ANIMAL TYPES 103 



represents only about i per cent of the original weight of the 

 animal. 



Class III. ACTiNozoA (Fig. 35). Attached forms. The digestive 

 cavity is always divided into compartments by membranous parti- 

 tions. Some are single individuals, as the Sea Anemone; others are 

 composite, as the Corals. The last named are in large measure re- 

 sponsible for the formation of coral islands and the enormous Great 

 Barrier Reef of the Pacific Ocean. Countless millions of minute 

 Actinozoa have died, each leaving a deposit of calcareous material. 

 The accumulation of this material from enormous numbers of ani- 

 mals over years of time has resulted in the formation of islands and 

 reefs. 



Phylum Ctenophora 



Includes forms which are commonly called Comb Jellies (Fig. 

 36) or Sea Walnuts. They do not possess cnidoblasts as do the 

 Ccelenterata. The body wall is composed of three layers, a third 

 layer, the mesoderm, appearing between the ectoderm and endo- 

 derm. The symmetry is biradial, that is, the parts are radially ar- 

 ranged, but the animal may be divided by a plane of bilateral 

 symmetry into right and left halves. This group is of considerable 

 interest to the taxonomist because of its possible relation to the 

 Ccelenterata and of the possibility of its being the ancestor of the 

 next higher phylum. 



Phylum Platyhelminthes 



This phylum includes the Flatworms, the simplest of the truly 

 bilaterally symmetrical animals. The nerve cells are arranged in 

 longitudinal tracts which are near the ventral surface of the ani- 

 mal. Derivatives of three layers — ectoderm, mesoderm, and endo- 

 derm — make up the various tissues. The digestive cavity has but 

 a single opening, the mouth. In some of the parasitic types the 

 entire digestive system is lacking. There are three classes: 



Class I. TURBELLARiA (Fig. 37). The ectoderm (surface layer of 

 cells) is covered with cilia. This class includes the free living 



