SUBPHYLUM I : CNIDARIA 



115 



CLASS III—ACTINOZOA 



A medusa is never present ; the tentacles are hollow ; the 

 enteron has vertical partitions (mesenteries) ; the gonads are formed 

 from the endoderm ; and there may or may not be a skeleton. 



The typical members of this class are the sea-anemones, 

 illustrated in Figs. 79, 80. These are solitary forms, usually 

 capable of many muscular movements both of feeding and 

 locomotion. They have a ciliated ectoderm, and in the plankton 

 feeders, such as Metridium, this is important in carrying food 

 to the mouth. Others, such as the 

 common Actinia equina, kill and eat 

 relatively large animals which ac- 

 cidentally touch the tentacles, and 

 experimentally can swallow pieces of 

 flesh almost as large as themselves. 

 The larva is a planula, which develops 



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i^^y^^^ 



\-T7UiA 



pOTi. 



Tnu»! 



Fig. 79. — External appearance of 

 Tealia felina, a sea anemone. — 

 From Thomson. 



Fig. 80. — A diagram of a vertical 

 section of a sea-anemone. 



ect., ectoderm of tentacle ; ect.g., ectoderm 

 lining gullet : end., endoderm ; ent., 

 enteron ; g., gullet ; gon., gonad ; w./., 

 mesenterial filament ; mus., longitud- 

 inal or ' retractor ' muscle ; mus'., 

 oblique or ' parietal ' muscle ; st.l., 

 mesoglcea ; ten., tentacle ; i°mes., 

 primary mesentery. 



directly into a new polyp. Many Actinozoa are colonial, and 

 amongst such forms are most of the corals. These include the red 

 coral, which forms its calcareous skeleton in the mesogloea, and the 

 reef-building species in which it is external. The coral reef, which 

 may be thousands of feet thick, consists of a large mass of calcium 

 carbonate built up by past generations of animals. The living 

 individuals protrude from tubes of this material only in the 

 narrow zone from the surface to a depth of twenty or thirty 

 fathoms. 



