CHAPTER XVI 



ECHINODEEMATA 



Classification adopted — 



Eleutherozoa 



Felmatozoa 



Asteroidea 

 Ophiuroidea 

 Echinoidea 

 Holothuroidea 

 Blastoidea (Extinct) 

 Cystoidea (Extinct) 

 Thecoidea (Extinct) 

 .Crinoidea 



The Echinodermata, as they at present exist, are divided into five 

 very distinct classes of animals, viz. Asteroidea, Ophiuroidea, 

 Echinoidea, Holothuroidea, and Orinoidea. The first four of these 

 classes agree in possessing the power of free locomotion during the 

 adult stage; and, when a fixed stage does occur in the course of 

 development, the fixing organ is attached to the oral surface of the 

 animal. For this reason the four classes mentioned are grouped 

 together in a sub-phylum termed Eleutherozoa. The members of 

 the fifth class, Orinoidea, possess at some time of their existence a 

 fixing organ situated at the aboral pole of the body, and this organ 

 is retained by many of them throughout life ; for this reason the 

 Orinoidea, together with some extinct classes of Echinodermata, are 

 grouped together as Pelmatozoa. 



In three of the four classes of Eleutherozoa the development of 

 one species has been worked out in great detail, and in the case of 

 the fourth class, although many points are still obscure, the general 

 course of the development is well known. The development of only 

 one Pelmatozoon is known, and in this case a large part of the 

 development is passed through within the egg-membrane ; whereas 

 the corresponding stages of development in Eleutherozoa are passed 

 through in the free larval condition. 



A great deal of experimental work has been done on the eggs and 

 larvae of Echinodermata,* and most important results have been 

 obtained which differ in many points from the results obtained from 



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