THE SPONGES AND THE TWO-LAYERED ANIMALS 



141 



group which has much more significance in 

 our story of the rise of animal Hfe on earth. 



PHYLUM COELENTERATA 



The first true metazoan animals to have 

 their somatic cells organized into definite 

 tissues are the two-layered, sac-like coelen- 

 terates. Greater differentiation of somatic 

 cells and well-established division of labor 

 make this form considerably more complex 

 than the sponges. 



Coelenterates possess special epithelial 

 cells, called cnidoblasts, which produce the 

 nematocysts used for offense and defense. 

 In many coelenterates there are two types 

 of individuals, the polyp, representing the 

 asexual phase of the life cycle, and the 

 medusa, the sexual phase. In their life his- 

 tories each generation successively gives 

 rise to the alternate type, a phenomenon 

 called metagenesis. The asexual polyp is 

 tubular in shape, with a mouth at one end, 

 surrounded by tentacles richly supplied 

 with nematocysts. The other end of the 

 tube is closed and forms an attachment 

 organ, the foot. These animals are usually 

 sessile or nearly so. The free-swimming 

 sexual medusa is a delicate transparent 

 animal, shaped like an umbrella. Around 

 the periphery or edge of the umbrella are 

 tentacles, which are also heavily fortified 

 with nematocysts. Both the polyp and the 

 medusa have primitive muscle fibers which 

 make movement possible. 



The coelenterates are water-inhabiting 

 animals, most of them marine. One form, 

 hydra, has invaded fresh water and is very 

 successful, being found in nearly all the 

 ponds and streams of the world. The large 

 marine jellyfishes and the sea anemones 

 are members of this group in which the 

 asexual generation has been greatly re- 

 duced. In both the sea anemones and their 

 close relatives, the corals, the asexual forms 

 have been completely lost. 



With the exception of some jellyfish, 

 which may annoy bathers by the stings of 



their nematocysts, and the coral animals, 

 which are used for jewelry, the group as a 

 whole has little direct significance to man. 

 There are three classes of coelenterates 

 (Hydrozoa, Scyphozoa, Anthozoa) whose 

 characteristics can best be understood by 

 studying two representatives. Hydra and 

 Obelia, of the class Hydrozoa. Members of 

 the other two classes are discussed briefly 

 later in the chapter. 



Hydra 



This tiny animal may be found cling- 

 ing to underwater vegetation in nearly any 

 fresh-water pond, lake, or stream ( Fig. 8-6 ) . 

 At some periods of the year they may be so 

 numerous in swift-moving streams as to im- 

 part a gray color to the rocks to which they 

 are attached. Hydra moves about very little, 

 hence it must seek its food by means of 

 its long tentacles. Though the column of the 

 body may be less than half an inch in 

 length, the tentacles in some species may 

 reach 10 to 12 inches. The cylindrical body 

 may extend so that it is many times longer 

 than its diameter or it may contract into 

 a pear-shaped, compact body with the 

 tentacles resembling tiny stumps. The hol- 

 low tentacles surround a raised conical hy- 

 postorne, in the center of which is the 

 mouth. The mouth opens into the digestive 

 cavity, the coelenteron or gastrovascular 

 cavity, which continues into the tentacles 

 ( Fig. 8-7 ) . At certain times of the year small 

 swellings, the sex organs, appear on the 

 external walls. At other times or, in some 

 cases, at the same time, small buds form 

 on the sides which grow into tiny hydras, 

 and finally detach themselves to grow into 

 adults. Ciliate Protozoa are found crawling 

 over the surface of the body of most hydras. 

 Just what their relationship is to the hydra 

 is not known. Perhaps they play the same 

 part for hydra that a flea does for a dog. 

 It might be expected that in the evolu- 

 tion of Metazoa the primitive body organ- 

 ization would be very much as it is in 

 hydra. When cells group themselves to- 



