352 THÉEL, SKELETON OF THE ECHINOUERMS. 



The process of ahsorbing calcareous deposits commences when 

 the skeleton becomes unsiiited tor further evolution of the larva 

 and when the sea-urchin in process of growth is in need of cal- 

 careous salts in great quantities, that is to say when the 

 pluteus has attained its mature state. At this period of the 

 development not only plates and spines but even pedicellaria, 

 pedicels, sphseridia and teeth with their alveoli begin to arise 

 almost simultaneousiy, all these organs requiring calcareous salts 

 in abundance. Under such circumstances it must be very suit- 

 able if not a necessity to have a supply of calcareous substances 

 in reserve within the body. Therefore, the calcareous substances 

 in the larvse of Echinoids, accumulated from the blastula or 

 gastrula stages up to the mature pluteus, may be looked on as 

 reserve materials for building up the future complex skeleton 

 and other organs in the young sea-urchin. One might suppose 

 such a continous accumulation of reserve materials would exer- 

 cise a pernicious influence on the larva itself, but this is so 

 far from being the case. that on the contrary it has been an 

 essential condition of its life, the materials in question having 

 taken the most suitable form possible for facilitating the float- 

 ing about of the larva in the sea. 



As I have mentioned above, the absorbent cells swallow 

 small particles of calcareous substances, but they also attack 

 the large rods which support the pluteus larva. One naturally 

 concludes that by the agency of several cooperating cells the 

 degenerative changes would occur simultaneousiy ajong the whole 

 calcareous rod. Though this very probably may happen if nature 

 requires, I never had an opportunity of observing it. It was 

 always the hinder part of the rod that was first afFected which 

 is quite natural, seeing that just this part would be the first 

 hindrance to the growth of the sea-urchin, while the remaining 

 part continues for some tirae to be of great advantage for 

 the larva in its fioating life. Thus, even in this case the 

 cell strives to detach a piece from the rod large enough for it 

 to be easily enveloped by the plasm, and this is evidently 



