542 The Animal Kingdom 



Characteristics of the Phylum. — The radial symmetry is the 

 most noticeable feature of the members of this phylum. Actually this 

 symmetry is somewhat marred by the presence of a special sieve plate, 

 the madreporite, on the upper surface. Most of the echinoderms show 

 a pentamerous arrangement of parts ; that is, most starfish have five 

 arms, the body of the sea urchin is divisible into five parts, and there 

 are five well-marked areas on the upper surface of the sand dollar. 



The independent development of the echinoderms is further evi- 

 denced by their complete lack of segmentation or cephalization. In spite 

 of these presumed handicaps, the protective device of their skeleton 

 and their special habits have enabled them to be successful. 



Like many other animals with an organ grade of construction 

 the echinoderms have three germ layers and a true coelom. The meso- 

 derm, however, arises as outpocketings from the roof of the archen- 

 teron in a manner identical with that of the vertebrates ; thus is revealed 

 the relationship of these forms to the vertebrate stock. As a result, the 

 coelom is formed between the layers of the mesoderm and is a true 

 enterocoel rather than a schizocoel. In the adult echinoderm, the vis- 

 ceral organs are contained within the coelom. Again like the vertebrates, 

 the mouth arises as a new structure, while the anus develops from the 

 embryonic blastopore. Further cleavage is indeterminate, that is, 

 isolated blastomeres have the potentialities of developing into any part 

 of the future organism. 



The echinoderm body is protected by a skeleton made up of a series 

 of calcareous plates. These individual plates allow for great flexibility. 

 Actually it is an endoskeleton derived from mesoderm and is covered by 

 a thin ciliated epithelium. The cilia are of service in preventing sessile 

 animals such as bryozoans, sponges, or coelenterates from settling on 

 the surface and making an additional load for the echinoderm to carry 

 about. In addition, the starfishes and sea urchins possess small pincerlike 

 structures, pedicellariae, which are scattered over the upper surface. 

 These remove small grains of sand and other tiny pieces of debris. 



All classes but the brittle stars have a complete digestive tract 

 with the anus opening on the surface opposite to the mouth. Inasmuch 

 as the orientation of the echinoderm body varies from that of other 

 animals, the use of dorsal and ventral does not apply. The surface 

 containing the mouth is known as the oral and the opposite one is the 

 aboral. In most forms, the digestive system lacks glands but possesses 

 large diverticula. 



