ECHINODERMATA. 225 



secreting calcareous matter to a greater or less extent. The 

 integument is, therefore, either composed of calcareous plates 

 articulated together, or is coriaceous, and has granules or spic- 

 ules of lime developed in it. The water-vascular system usually 

 communicates with the exterior, and generally subserves locomo- 

 tion. The adult animal exhibits more or less distinctly a "radial 

 symmetry',' or star-like arrangement of its parts, but the young 

 animal is more or less bilaterally symmetrical. 



The Echinodermata are divided into the following seven 

 orders : 



1. Echinoidea. Ex. Heart-urchin (Spatangus). J >i/Vt ^~~ 



2. Asteroidea.Ex. Star-fish (Uraster). (//' / 



3. Ophiuroidea. Ex. Brittle-star (Ophiura). 

 ^ Crinoidea. Ex. Stone-lily (Encrinus). 



5. Cystoidea. Ex. Hemicosmites. 

 . Blastoidea.Ex. Pentremites. 

 7. Holothuroidea. Ex. Trepang (Holothuria). 



The above is not a true or natural arrangement of the 

 orders of the Echinodermata, but it is convenient for many 

 reasons to consider them in this sequence. As regards the 

 general distribution of the class, the Echinodermata are rep- 

 resented more or less abundantly in all the great formations 

 from the Upper Cambrian to the present day. The orders 

 Cystoidea and Blastoidea are not only extinct, but are ex- 

 clusively Palaeozoic ; while in the Crinoidea we have an order 

 which has passed its prime, and appears to be verging on 

 extinction. On the other hand, the orders Echinoidea, Aster- 

 oidea, Ophiuroidea, and Holothuroidea appear to have attained 

 their maximum of development at the present day. The 

 Asteroidea and Ophiuroidea commence in the Silurian period. 

 The Echinoids commence in the Upper Silurian, but reach no 

 marked development till we enter upon Mesozoic deposits. 

 Lastly, the Holothurians, as might be expected from the soft 

 nature of their integuments, are hardly known as fossils, 

 though they seem to have existed at any rate as early as 

 the Carboniferous period. 



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