8 . ECHINOIDEA. 



was produced by my approach, or merely by the agitation of the water, I repeated the 

 experiment very slowly, and even over the water with a stick : the urchin, whether in 

 the water or out of it, having always directed its defensive spines towards the object which 

 approached it. From these observations I arrived at the conclusion that these urchins see, 

 and that their spines serve them as defensive instruments." 



It is worthy of remark, that Captain Cande, at the time he watched this urchin, was 

 ignorant of the anatomical fact that eyes had been detected in the Echrnidae, and his 

 inference was simply the legitimate conclusion drawn from carefully-made observations. 



The calcareous test of the Echinoidea is the only part of the structure of these animals 

 which is preserved in a fossil state. It has hitherto failed to attract that amount of 

 attention from the palaeontologist which the importance of its study demands, although 

 in a stratigraphical point of view it is not inferior to the skeletons of any other class of the 

 Animal Kingdom. The fact seems to have been almost entirely overlooked, that most of 

 the generic characters of the different groups of the Echinoidea are more indelibly impressed 

 on the separate pieces of their test than in the skeletons of any other class of the Invertebrata. 



Unlike the shells of the Mollusca, the test of the Echinoidea constitutes an internal 

 and integral portion of the animal, being secreted by, and enclosed within, organized 

 membranes, and participating in the life of the organism ; portions of the skeleton are 

 likewise intimately connected with the organs of digestion, respiration, and generation, as 

 well as with those of vision and locomotion. 



As the analysis of the test of the Echinoidea, with full anatomical details of the struc- 

 ture of the skeleton in the Echinodermata in general, will be given in the Introduction to 

 this Monograph, it is unnecessary to enter at present minutely into the subject ; but, as 

 many of our readers are doubtless unacquainted with the terminology employed in the 

 description of the test of the Echinoidea, and the characters on which a diagnosis of the 

 species is made, it is desirable now to preface our description of the species with brief 

 explanations of the same, in this part of the work, illustrating the terminology by a refe- 

 rence to the plates for accurate figures of the different parts of the test, and the magnified 

 details of its anatomical characters. 



