176 MR. H. L. HAWKINS ON THE 



developed region. A further support to tins view is found in the 

 fact that in very young specimens of E. cordatum (where " plate- 

 crushing " is very slightly developed), the anterior groove is 

 scarcely perceptible. 



It appears, then, that after an early stage, the development of 

 new coronal plates from the oculars in E. cordatimi is arrested, 

 save in the case of ocular III., where the issuing ambulacrum is 

 continually gi-owing by the addition of- new plates. The rest of 

 the test grows by the increase in size of the plates already 

 developed. Little or no resorption of coronal plates occurs in 

 Spatangids, and ambulacrum III. of E. cordatum is no exception 

 to the rule. Indeed, the influence of the continuous growth of 

 the area seems confined by the anterior part of the internal 

 fasciole. In consequence, all the i-e-adjustment due to the con- 

 tinuous growth of new plates has to be carried on in the 

 "petaloid" region of the area. The results are: firstly, an 

 inward sagging of that part of the ambulacrum to form the 

 anterior sulcus, and secondly, mutual resorption and rearrange- 

 ment of the plates, causing " plate-crushing." The additional 

 feature enters, as described above, that when the area is expanded 

 laterally the intensity of the " plate-crushing " is proportionately 

 reduced. 



The physiological advantages of the character of the area in 

 E. cordattom are obvious". A far greater number of podial pores 

 can be compressed into the short space available than would be 

 possible by other means, and this results in a greater number of 

 prehensile podia being developed. Again, the absence of the 

 complexity in E. Jiavescens seems explicable on the grounds of 

 habitat. E. jiavescens does not occur between tide-marks, and 

 seems to be rarely, if ever, deeply buried in the sand, but 

 E. cordatum is often more than six inches from the surface, and 

 must therefore find greater difficulty in collecting food. Hence 

 the development of a larger number of podia in the latter 

 species. 



The present paper is concerned more with the origin and 

 development of the structure than with its physiological meaning, 

 a,nd I pass to a consideration of the views that have been expressed 

 to explain "plate-crushing" in other groups of Echinoids. 



V. Previous views on " Plate- crushing J^ 



Most of the work that has been done on ambulacral structures 

 has had reference to the Regular Echinoids, among which the 

 various characters have been largely used for purposes of classi- 

 fication since the publication of Duncan's paper in 1885 (Quart. 

 Journ. Geo]. Soc. vol. xli. pp. 419-452). Duncan expressed the 

 view that " growth-pressure" was the cause of "plate-crushing," 

 and he included as a subsidiary cause of the pressure the growth 

 of tubercles on the plates. Many of his figures, however, show 

 that the sutures separating the elements of a compound plate 



