180 ON THE STRUCTURE OF ECHINOIDS. 



is the result of enlargement of the plates already developed, 

 rather than of the addition of new ones. In many genera a 

 considerable development of large tubercles occurs (though rarely 

 on the ambulacra), and no distortion of the sutures follows such 

 a secondary development. But in Echinocardium cordatum (and 

 probably in Heteraster ohlongus), ambulacrum III. continues to 

 develop in the more primitive Echinoid way throughout life. 

 And here, with an exceptional character in the manner of growth, 

 we find a corresponding exception in the structure. Practically 

 no tubercles, and no granules of considerable size, occur on the 

 area, but the " plate-crushing " is carried to a degree of complexity 

 surpassing that of any other Echinoid. 



It is important to remember that in the Regularia, Holecty- 

 poida, and Nucleolitidse, the " plate-crushing," when it exists, is 

 always most strongly developed towards the peristome. More- 

 over, in most members of these groups the arrangement of the 

 compound plates is founded on a regularly three-fold plan. In 

 the Clypeastroida and Spatangidse, when " plate-crushing " occurs, 

 it is restricted to some part of the adapical siu'face, and there 

 seems to be no definite pattern in its arrangement. 



It seems safe to conclude that the structui-es found in the 

 last-named sections have no phylogenetic connection with those 

 of the former groups, and a reason for their development must be 

 sought. From the discussion given above it seems clear that the 

 only feasible explanation lies in the assumption that the high 

 specialisation of the petaloid parts of Clypeastroid and Spatangid 

 ambulacra precludes the passage of plates from those regions to 

 lower positions. The '' plate-crushing " is confined to the petals, 

 and the remainder of the areas have, in large measure, lost their 

 distinctively ambulacral characters as regards the plating. 



For the Irregular Echinoids, mechanical compression due to 

 the growth of new plates seems the only available cause of the 

 development of compound plates. To my mind it affords the most 

 reasonable explanation of similar structures among the Regularia. 

 The development of a tubercle is a superficial phenomenon, 

 having no visible influence on the texture of the plate- substance 

 in the deeper layers. It seems likely that the lateral expansion 

 of a tubercle may cause local distortion of sutures at the outer 

 surface (although in many cases it obviously has no such influence), 

 but, at the most, this effect would be local and secondary. 



YII. Summary. 



A new method for exposing sutures in dried specimens of 

 Echinoids is described. The method combines the processes 

 of staining and etching ; depends on the presence of organic 

 tissue between the plates ; and gives a fairly lasting result easily 

 photographed. 



The complex structure of the anterior ambulacrum of Echino- 

 cardium cordatum is described in detail for the first time, and a 



