20 ECHTNOIDEA. 



seize it," he observed, " when it instantly turned all its spines in the direction of ray 

 hand, as if to defend itself. 



" Surprised at this manoeuvre, I made an attempt to seize it on the other side, when 

 immediately the spines were directed towards me. 



" I thought from this that the urchin saw me, and that the motion of the spines was 

 intended as an act of self-defence ; but, to prove whether the movement of the animal 

 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 Echinida?, and his 

 inference was the conclusion drawn from carefully made observations. 



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

 preserved in a fossil state. It has hitherto failed to attract that amount of atten- 

 tion from the palaeontologist which the importance of its study demands ; although in a 

 stratigraphical point of view this skeleton is not inferior to that 'of any other class of the 

 Animal Kingdom. The fact seems to have been almost entirely overlooked by palaeon- 

 tologists, that most of the generic characters of the different groups of Echiniclao arc 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 mem- 

 branes, it participates in the life of the organism, and certain parts of the skeleton are 

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

 with those of vision and locomotion. 



As it is intended to give an analysis of the test of the Echinoidea, with anatomical 

 details of the structure of the skeleton in the Echinodermata in general, in the General 

 Introduction to these Monographs, it is at present unnecessary to enter minutely into the 

 subject ; but, as many of our readers are doubtless unacquainted with the terminology 

 employed in the description of the test, and the characters on which a diagnosis of the 

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

 the same, illustrating the terminology by a reference to the plates for accurate figures of 

 different parts of the test, and magnified details of the anatomical characters thereof. 1 



1 In connection with the physiology of the Echinodermata, the following discovery, made by Dr. 

 Wallich, is most important : 



" Thirteen living star-fishes, differing in no important particular from a species common on our own 

 and most northern coasts, were brought up from a depth of 12G0 fathoms, or very nearly a mile and a 

 half, at a point midway between the southern extremity of Greenland and Rockall, and 250 miles distant 

 from the nearest land. These star-fishes, however, cannot be said to have been captured by the sounding- 



