ECHINOIDEA. 103 



1. Echinoidea. Ex. Heart-urchin (Spatangus). 



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



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



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

 ( 5. Cystoidea. Ex. Hemicosmites. 



?** { 6. Blasloidea Ex. Pentremites. 



7. Holothuroidea. Ex. Trepang (Holothuria). 



The above is not a true or natural arrangement of the orders 

 of the EchinocUrmata, but it is convenient for many reasons to 

 consider them in this sequence. As regards the general dis- 

 tribution of the class, the Echinodermata are represented more 

 or less abundantly in all the great formations from the Upper 

 Cambrian to the present day. The orders Cystoidea and Blas- 

 toidea are not only extinct, but are exclusively Palaeozoic ; in the 

 Crinoidea we have an order which has passed its prime, and ap- 

 pears to be verging on extinction. On the other hand, the orders 

 Echinoidea, Asteroidea, Ophiuroidea, and Holothuroidea appear 

 to have attained their maximum of development at the present 

 day. The Asteroidea and Ophiuroidea commence in the Silu- 

 rian 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 in the Mesozoic period. 



ORDER I. ECHINOIDEA. 



The members of this order commonly known as Sea- 

 urchins are characterised by the possession of a more or less 

 globular, heart-shaped, discoidal or depressed body, encased in a 

 " test" or shell, which is composed of numerous calcareous plates, 

 immovably connected together. The intestine is convoluted, and 

 there is a distinct anus. The mouth is usually armed with cal- 

 careous teeth, and is always situated on the inferior aspect of the 

 body, but the position of the vent varies. 



As a matter of course, the palaeontological student has to 

 deal with nothing except the test of the Echinoids and its 

 appendages, and these must be described in some detail. 

 The " test " of the Echinoidea may be regarded as essentially 

 composed of the so-called " corona" and of the " apical disc/' 

 though other less important elements are present as well. The 

 " corona " is the main element of the test, and includes all 

 the calcareous covering of the animal except the scattered 

 plates round the mouth and anus and the "apical disc." The 

 test is composed of numerous calcareous plates, firmly united 



