ON THE STRUCTURE OF ECHINOIDS. 169 



13. The Anterior Ambulacrum o£ Ecldnocardium eordatum 

 Penn.,and the Origin oF Compound Plates in Echinoids. 

 By Herbert L. Hawkins, M.Sc, F.G.S., Lecturer in 

 Greology, University College, Reading.* 



[Received November 8, 1912 : Read February 4, 1913.] 

 (Plate XXYI.t and Text-figures 39-41.) 



Index. 



Page 



Structure of ambulacrum III. in Spatangidae 171 



„ of petals in CJi/peaster 178 



„ of perignathic girdle 178 



Physiologj' : use of podia of amb. III. in JEcJiinocardium ... 176 

 Development of new coronal plates in Regular Echinoids 



and Spatangidae 177 



Variation in position of apical system in Echinocardiwii ... 174 



I. Introditction. 



During the course of research into the anatomy of the test of 

 the primitive Irregular Echinoids, I had occasion to re-examine 

 the corresponding structures in living forms, and naturally 

 included among these the common British " Heart -Urchin," 

 Echinocardium eordatum Penn. It might have been thought 

 that, in the case of a species so long known and so frequem;ly 

 examined, all the essential structural features had been deter- 

 mined. It was with considerable surprise, therefore, that I 

 found in it a character (from my point of view of first-rate 

 importance) that has not hitherto been certainly recognised, and 

 never adequately figured or described. 



Loven, with his customary fidelity, indicated some of the 

 complexity in ambulacrum III. of the species (Etudes sur les 

 Echinoi'dees, PL xii. fig. 107, and PI. xxxix. fig. 222), but he 

 gave no comment on the strvicture, and his drawings of it are 

 incomplete. Beyond this casual recognition, I have been unable 

 to find any account or figure of the very remarkable features 

 here described, and, indeed, most of the descriptions and drawings 

 of the area that I have seen are positively misleading. This is 

 undoubtedly due to the fact that a surface examination, however 

 careful, shows little or no trace of the elaborate structure pi-esent. 

 It was not until I accidentally discovered a new method of dis- 

 playing sutures that I had any suspicion of the remarkable 

 character of the area. 



The examination of some 150 prepared specimens of various 

 sizes has led to considerations which have an important bearing 

 on the question of the origin of " plate-crushing," and a discussion 



* Communicated by Dr. F. A. Bather, F.R.S., F.Z.S. 

 t For explanation of the Plate see p, 181. 



Proc. Zool. Soc— 1913, No. XII. 12 



