134 



On the Cidaridae of the Oolites, with a description of some new 

 species of that family. By Thomas Wright, M.D. &c. 



Read 24th June 1851. 



The Echinoderms form the highest class of the radiated animals ; 

 it includes organisms which are either fixed or free, composed of 

 a regular but very complicated skeleton, secreted by and inclosed 

 within organized membranes, and often preserved in admirable 

 perfection in the fossiliferous strata of all periods of the earth's 

 history. The study of this class, although hitherto much neg- 

 lected by geologists, presents many points of importance to the 

 progress of their science, for the test of Echinoderms exhibits 

 characters of more import and significance than those afforded 

 by the shells of Mollusca. Unlike the testaceous covering of that 

 class, the test of Echinoderms constitutes an internal and inte- 

 gral part of the animal, participating in its life, intimately con- 

 nected with the organs of digestion, respiration and generation, 

 as well as with those of locomotion and vision, and having in 

 consequence many of the distinctive characters of the organism 

 impressed upon it. 



In all Echinoderms, the external parts of the body, with the 

 organs of locomotion, are disposed around a common centre ; in 

 the spherical forms they are arranged in rows like the lines of 

 longitude on a terrestrial globe, and the mouth and the anus are 

 situated at the opposite poles : the elements of the body are re- 

 peated several times in the composition of the skeleton. 



It has been shown by M. Agassiz* that the radiated type of 

 structure observable in this class can be resolved into a modi- 

 fication of the bilateral symmetry seen in the higher groups of 

 the animal kingdom. The elements of the skeleton are arranged 

 on two sides of a median line. If we take for example the Spa- 

 tangus purpureus, we observe that the test is elongated in the 

 direction of the line which connects the mouth with the anus ; the 

 mouth being situated at the base and nearer the anterior border 

 of the test, whilst the anus occupies an elevated position on the 

 posterior border. Were we to make a transverse section of the 

 Spatangus, we should have an oral or anterior half, and an anal 



* Prodrome d'une Monogr. des Echin , Mem. Soc. de Neuchatel, torn. i. 

 p. 168. 



