372 ECHINOZOA. 



Though superficially conspicuously " radial " in its symmetry, the test 

 of the Sea-urchins can nevertheless be shown, with more or less clear- 

 ness, to have also a bilateral symmetry. This can be demonstrated by 

 the position of such an unpaired organ as the " madreporite," and is more 

 conspicuously exhibited in the " Irregular " Sea-urchins than in the 

 " Regular " forms, though recognisable even in the latter. Thus, if the 

 test of a Reg'ular Sea-urchin (figs. 237 and 243) be viewed from above 

 while held in such a position that the madreporite is placed on the side 

 farthest from the spectator and on his right hand, it will be seen that 

 facing the spectator is an unpaired ambulacral area (" radius "), while on 

 the side nearest him is an unpaired interambulacral area (" interradius "). 

 A line drawn through the centre of these two unpaired areas gives a 

 middle line to the body, the structures on either side being for the most 

 part symmetrically disposed. It will further be seen that three ambula- 

 cral areas (the " trivium ") are directed towards the side farthest from 

 the spectator (the " anterior" side) ; while two (the " bivium") are directed 

 " posteriorly," or towards the side facing the spectator. The unpaired 

 ambulacral area is therefore " anterior," and the unpaired interambula- 

 crum is " posterior." In the " Irregular Sea-urchins " the bilaterality is 

 still more marked, the unpaired anterior ambulacrum being usually dif- 

 ferent to the others in form or size ; while the anus is commonly placed 

 on the ventral side of the body, in the unpaired posterior interambulacrum. 



As regards classification, the class of the Echinoidea is divided 

 by Zittel into the two orders of the Palechinoidea, in which the test 

 consists of more or fewer than twenty meridional rows of plates, and 

 the Euechinoidea, in which there are constantly twenty rows of plates 

 in the shell. The Palechinoids comprise the three groups of the 

 Cystocidaridce, Bothriocidaridce, and Perischoechinidce, and the Eu- 

 echinoids are divided into the two groups of the Regulares and Irre- 

 gular -es, according as the test is of the " regular "or " irregular " type. 



As regards their distribution in time, all the Echinoids of the 

 Palaeozoic formations except Eocidaris belong to the order of the 

 Palechinoidea, and a single member of this division (Anaulocidaris) 

 is known to exist in strata of Triassic age, the division being other- 

 wise unrepresented in Mesozoic or Tertiary deposits. The earliest 

 types of the Palechinoids are found in the Ordovician rocks. On the 

 other hand, the Euechinoids make their first appearance, so far as 

 certainly known, in the Permian {Eocidaris), and attain a marvellous 

 development in the Jurassic and Cretaceous periods. In the Tertiary 

 rocks, on the contrary, the number of known fossil forms is reduced, 

 most of the Kainozoic types being characteristic of shallow water, 

 and many of the genera being still in existence. Upon the whole, 

 the Euechinoids may be considered as having attained their maxi- 

 mum development in the period of the Chalk. 



In the following condensed account of the characters and geolo- 

 gical distribution of the chief groups of the Echinoids, the less im- 

 portant types are necessarily omitted. The arrangement adopted by 

 Zittel has been followed in the main. 



