ECHINODEEMATA 



THE echinoderms, although their shapes are very unlike in 

 the different classes, have the same general internal struc- 

 ture, and also other features which place them together in one 

 group. They are radiates (page 113) of the highest type; they 

 have an exoskeleton ; and many of them are beset with spines, 

 from which the name is given. They have locomotor organs, the 

 ambulacra, and a water-vascular system peculiar to themselves. 

 Some have the very strange power of casting off, and developing 

 again, parts of the body. 



The development from the egg to the adult is remarkable, and 

 of unusual interest to naturalists. The larval stage of echino- 

 derms is so unlike the mature animal that for a long time the 

 larvas were thought to be another class of animals, and therefore 

 were given the names which they still retain. 



In observing echinoderms it will be seen that the dorsal part 

 is carried uppermost by some, such as the starfishes and sea- 

 urchins. In the former the back is broad and extended ; in the 

 latter it is curved and contracted. But in crinoids the back is 

 carried downward and is extended like a stalk ; in the sea-cucum- 

 bers (holothurians) the dorsal and ventral surfaces are parallel 

 with the long axis of the cylindrical body, instead of being on a 

 plane with the mouth and excretory opening. Owing to the 

 unusual positions of the dorsal and ventral surfaces, the terms oral 

 (mouth side) and aboral (side opposite the mouth) are generally 

 used in describing these species. 



The surface of the body is divided definitely by the ambulacra, 

 the ambulacral zones, and the interanibulacral spaces, but differ- 

 ently in the different classes. The movement of the animals is 



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