THE SPONGES AND THE TWO-LAYERED ANIMALS 



159 



fundamentally sound. He thought the pe- 

 culiar formations started out as a fringing 

 reef around an island, but due to shifts in 

 the earth's surface the island gradually be- 

 gan to sink. The rate at which it submerged 

 was about as fast as the corals were able to 

 secrete lime and keep themselves in the 

 tidal zone. By keeping pace with the sink- 

 ing island, the corals built the fringing reefs 

 sufficiently high to catch vegetation and 

 support growth of plants while the central 

 portion gradually became submerged be- 

 low the water's surface. This then produced 

 a rim of coral, inscribins; the outline of the 

 old island and producing the strangely- 

 shaped atolls seen in tropical seas today. 



RELATIONSHIP AMONG 

 COELENTERATES 



A possible explanation for the many dif- 

 ferent life cycles among the coelenterates 

 might be that the original primitive coelen- 

 terate was much like the phinula larva, 

 which is found rather consistently in all 

 of the groups, with the notable exception 

 of hydra. The planula is a ball of cells that 

 might be compared to Volvox except that 

 the former is solid and the latter is hollow. 

 It would not be difficult to understand how 

 the early planula-like coelenterate might 

 become temporarily attached to the bot- 

 tom of the ocean and invaginate, to form 

 the hydra-like polyp (Fig. 8-22: 1 to 5). 

 The formation of the sex organs would be 

 similar to those found in hydra (Fig. 8-22: 

 6), and the life history would also be the 

 same, except that the zygote would develop 



into a planula which would eventually settle 

 down and grow into the polyp. 



From the hydra-like type could be pro- 

 duced a form resembling Htjdractinia ( Fig. 

 8-22: 7), where the sex organs develop into 

 separate bodies, although they are not de- 

 tached from the polyp. The next logical 

 step would be an obelia-like form in which 

 the medusae detach themselves from the 

 polyp and become the free-swimming sex- 

 ual stage of the animal (Fig. 8-22: 8). This 

 affords an opportunity for the species to 

 spread both by means of the planula and the 

 medusa stages. The next variation mis;ht be 

 a continued emphasis of the medusa and a 

 de-emphasis of the polyp stage, as is seen in 

 Aiirclia ( Fig. 8-22 : 9 ) . This can be carried 

 still further until the polyp stage disappears 

 altogether, which is the case with many of 

 the large jellyfish (Fig. 8-22: 10). This is 

 merely a possible explanation for the great 

 variety of forms found in this phylum of 

 animals. 



In review we have found the coelenter- 

 ates a widely diverse group of animals that 

 have explored many possibilities of form, 

 structure, and habitat, remaining all the 

 while within the limitations of two body 

 layers, ectoderm and endoderm. Animals 

 could have gone no further in complexity 

 had they remained within the limitations 

 imposed by these two body layers. The next 

 group surmounted this difficulty by the in- 

 troduction of a third body layer which 

 resulted in a modification in the entire body 

 plan of the group. Let us see how this was 

 accomplished. 



