THE MAIN LINES OF ANIMAL EVOLUTION 

 ECTODERM tgma 



mesogleaHI 

 enoodermoMqs 



HYDRA 



SEA 

 ANEMONE 



MEDUSA 



OR 



JELLYFISH 



(Inverted) 



Figure 47. An Inverted Jellyfish Compared to Hydra and to a Sea Anemone, 

 showing their structural similarity. (From Hunter and Hunter, "College Zoology," 

 W. B. Saunders Co., 1949. ) 



the medusa is much the more prominent generation, with the polyp being 

 reduced or absent altogether. The final class, the Anthozoa, includes only 

 the polyp phase, the medusa being suppressed entirely. The class com- 

 prises the sea anemones, the corals, and their allies. Many of these, in- 

 cluding all of the corals, secrete a calcareous exoskeleton, because of 

 which they have left an excellent fossil record going back to the Ordo- 

 vician. All three classes are ancient, and it is probable that they diverged 

 in pre-Cambrian times from a primitive hydrozoan type not dissimilar to 

 the Trachylina. 



Phylum Ctenophora. The Ctenophora are a small phylum, only about 

 eighty species being known. All are small, marine animals. They are com- 

 monly known as "sea walnuts" or "comb jellies" because of the presence 

 of comb-like plates of cilia. They share some important characteristics 

 with the coelenterates. Thus they are radially or biradially symmetrical 

 in contrast to all higher phyla, including the Echinodermata, in which 

 bilateral symmetry is primary, and radial symmetry derived. They are at 

 the tissue grade of construction, with an abundant mesoglea separating 

 the epidermis from the gastrodermis. In both phyla, the gastrovascular 

 cavity is present, and its branches distribute food through the body. But 

 the ctenophores lack nematocysts, and there is no alternation of genera- 

 tions. Unlike the coelenterates, they are hermaphroditic. There seems to 

 be little doubt that the coelenterates gave rise to the Ctenophora, yet all 

 attempts to relate them to any of the classes of living coelenterates have 

 failed. It seems most probable that the Ctenophora were derived from the 

 same ancient, pre-Cambrian, trachyline stock that gave rise to the three 

 classes of the Coelenterata, and at about the same time. 



PRIMITIVE BILATERAL PHYLA 



Some of the ctenophores have become elongate and flattened, and it has 

 been suggested that they are related to the ancestors of the flatworms, the 



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