CHAPTER X 



ECHINODERMATA 



EcHiNODERMATA (Gr. ecMnos, a sea hedgehog; derma, skin) Is a 

 group of marine animals representing the most highly specialized 

 of the radially symmetrical forms, and is further distinguished by the 

 presence of a calcareous skeleton, which is sometimes in the form of 

 scattered particles or spines, in other cases developed into plates. 

 A well-developed coelom points to a high degree of organization. 



Classification 



Class 1. Asteroidea (Gr. aster, a star; eidos, resemblance) pentamer- 

 ous; arms not sharply marked off from disc; ambulacral 

 groove present. (Starfishes.) 



Class 2. Ophiuroidea (Gr. ophis, a snake; oura, a tail; etdos, form) 

 pentamerous; arms sharply marked off from disc; no am- 

 bulacral groove. (Brittlestars.) 



Class 3. Echinoidea (Gr. echinos, hedge-hog; eidos, form) pen- 

 tamerous; without arms or free rays; test of calcareous plates 

 having movable spines. (Sea urchin, sand dollar, heart 

 urchin.) 



Class 4. Holothuroidea (Gr. helos, whole; thurois, rushing) long 

 ovoid; muscular body wall; tentacles around mouth. (Sea 

 cucumbers.) 



Class 5. Crinoidea (Gr. krinon, a lily; eidos, form). Arms gen- 

 erally branched and with pinnules; aboral pole usually with 

 cirri or sometimes with stalk for temporary or permanent 

 attachment. (Feather star, sea lily.) 



Class 6. Cystoidea (Gr. cystis, bladder; eidos, form) — extinct. 

 Confined to Paleozoic; extend from Cambrian to Permian 

 inclusive with maximum development in Ordovician and 

 Silurian. 



Calyx usually stemmed; mouth nearly or quite central 

 upon the upper (ventral) surface. From the mouth radiates 

 two to five or more simple or branching ambulacra along 

 which food particles pass to the mouth probably driven 



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