264 PRINCIPLES OF ANIMAL BIOLOGY 



kinds of individuals reproduce by budding (asexually), another kind by 

 eggs and spermatozoa. 



In some groups (Scyphozoa, Fig. 219B) only the medusoid generation 

 exists, and in them its structure is different (see table of characterizations 

 below). 



Corals are the skeletons of two kinds of coelenterates, the Hydrocoral- 

 linae and the Madreporaria (see below), the latter being the more com- 

 mon. Aside from their use as ornaments, corals are of interest because of 

 the long debate concerning the origin of coral reefs and atolls. The theo- 

 ries of their origin differ largely in whether the sea bottom on which they 

 grew was assumed to be subsiding, stationary, or rising. 



Class I. Hydrozoa. Coelenterates without stomodaeum and mesenteries; sexual 

 cells discharged to the exterior; life history including hydroid form, or medusa (with 

 velum), or both hydroid and medusa in same species. Polyps (including Hydra), a 

 few corals, small jellyfishes. (Figs. 58, 59, 65A, 142, 144, 145, 146, 218, 219^, 220.) 



Class II. Scyphozoa. Coelenterates with only the jellyfish, not hydroid form; 

 velum lacking; notches at margin of umbrella. Larger jellyfishes. (Fig. 219-B.) 



Order 1. Stauromedusae Order 3. Cubomedusae 



Order 2. Peromedusae Order 4. Discomedusae 



Class III. Anthozoa. Coelenterates without medusoid forms, with well-developed 

 stomodaeum and mesenteries. Sea anemones, most corals. (Figs. 65A, B, 219C, D.) 



Phylum 4. Platyhelminthes. — This phylum includes the planarians 

 (Fig. 221), the flukes (Fig. 222), and the tapeworms (Fig. 223). The 

 name of the phylum comes from the generally flat form of the bod}'-, and 

 its members are commonly called flatworms even when the body is not 

 flat. The body is bilaterally symmetrical, the only phylum so far men- 

 tioned to possess this form. The animals are triploblastic, the third 

 layer being mesenchyme (page 82), which makes up the bulk of the body. 

 The digestive tract is a coelenteron (page 80), opening only at the mouth, 

 and there is no other body cavity. Parasitic forms may, however, lack 

 the digestive tract completely. The free-living species have cilia 

 on the epidermis, but the parasitic ones lack them. The excretory sys- 

 tem is of the protonephridial type (page 134) ending in flame cells. 



