ECHINOCYAMUS PUSILLUS. 57 



sents a pentagonal outline with the angles in the direction of the in- 

 terradia, PI. IX, fig. 109. The spinous teeth protrude through the oral 

 opening. In spite of repeated investigations, I have not succeeded in 

 detecting an}^ anal aperture, wherefore I suppose that the posterior ex- 

 tremity of the alimentary canal still remains closed. The dorsal region 

 is strengthened hy a large calcified perforated plate covering the greater 

 part of the back, PL IX, fig. 107. 



If we try to trace the origin of this dorsal plate, it appears, sin- 

 gularly enough, to arise in a way different from that in which the other 

 plates develop in the young sea-urchin. An examination of the figures, 

 PL VIII, fig. 100 — 104., shows clearly that it does not originate from 

 a new centre of calcification as is the case with the other plates, but 

 that it grows out from the central part of the odd dorsal rod which 

 characterizes the fully mature Pluteus, fig. 100 o. As may be remem- 

 bered, this rod consists of two anterior arms and a shorter posterior one 

 which all meet and join in a centre situated close to the place where 

 the madreporic canal opens externally. From an inspection of the figures 

 it is clear that even in the pluteus stage the rods send off branches 

 which by degrees unite and protrude new branches, so as to produce 

 a network with large irregular meshes round the water-pore. The 

 plate thus formed gradually increases in size and during the process of 

 growth the primary rods become more and more constituents of the 

 plate. When the plate has attained such a size as to cover a good deal 

 of the dorsal region, traces of the two anterior rods of the spicule still 

 remain distinguishable. Of course, I do not mean by this to deny that 

 some parts of the rods in question, especially their ends, have under- 

 gone resorption, which seems to be the case with most of the larval 

 skeleton. In order to understand the growth of the calcareous deposits 

 in general and all the changes which take place in connection with it, 

 we must note that a re-deposition of calcareous matters must occur side 

 by side with an absorption just as is the case during the formation and 

 growth of bone, or in other words, while a part of the wandering ceils 

 are employed in building up, other so-called phagocytes devour and 

 destroy such parts of the plates, rods etc., which are useless or impede 

 the advance of a normal development. 



