PHYLUM ECHINODERMA 167 



skeletal plates are so poorly developed that their presence in the 

 soft body wall is restricted to minute discs and anchor-shaped 

 bodies scattered through the tissues. 



Other Organ Systems. — The locomotor and skeletal systems 

 described above are the most distinctive of the systems found in 

 echinoderms. There are no highly specialized organs for excre- 

 tion, no definite circulatory system, and the nervous system 

 represents a very low order of specialization. Excretion is 

 accomplished through the action of amoebocytes in the coelomic 

 fluid, and excretory products are liberated through gills variously 

 located on or in the bodies of the several classes. 



Like all higher Metazoa, the nervous system has its origin 

 in the skin of the embryo, but in echinoderms it remains perma- 

 nently associated with the skin and never migrates to an internal 

 position affording greater protection. The nerve ring and its 

 branches are of lowly organization, for there is no concentration 

 of nerve cells to form a brain, and even ganglia are lacking. 

 Echinoderms are well supplied with sense organs but most of them 

 are of generalized type. Tactile organs are extremely diversified, 

 including both ordinary and modified tube-feet and certain 

 kinds of spines. Pedicellariae are highly sensory in addition to 

 their function of defense. Eyes are found commonly in only the 

 starfishes and the sea urchins. Other sensory organs will be 

 mentioned under the various classes. 



The tubular digestive system, with both mouth and anus, 

 displays great diversity in the various classes. 



The sexes are separate. The gonads are single in the sea 

 cucumbers, but in other classes there are five gonads, each with 

 its duct but with no other accessory organs, for the germ cells 

 are liberated into the water for fertilization and subsequent 

 development. 



Researches on Echinoderm Eggs. — An enumeration of the 

 researches that have been conducted upon the eggs of echino- 

 derms would call for practically a complete history of the develop- 

 ment of the fields of embryology and cytology. Starting with the 

 pioneer descriptive embryology by Alexander Agassiz, we find 

 some of the most detailed and most superbly illustrated studies on 

 cleavage and on the larval forms that have been made for any 

 group of animals. Hans Driesch used sea urchin eggs to test 

 the powers of isolated cells of the embryo in the formation of a 

 whole larva. Working on the eggs and sperms of starfish and 



