TO-J 



WATER SYSTEM. 



Spatangoids the sutures of the plates, both in the ambulacral and inter- 

 ambulacral system, are always readily traced, while in the Desmostieha the 

 sutures of the ambulacral system, frequently become obliterated, and can 

 only be traced with difficulty; when seen from the inner side of the test, 

 the sutures are often still apparent when they can no longer be traced 

 from the outer side of the test. In the Desmostieha the new plates are 

 formed between the apical system and the upper part of the ambulacral sys- 

 tem;* the position in which additional pores are found, one on each side of 

 the odd terminal tentacle, corresponds to the mode of addition of new 

 suckers traced in the starfish.! The same is the ease in the Clypeastroids. 

 as has been shown by Philippi and Miiller; hut. as 1 have shown in the ex- 

 amination of the young Echini from the Report of the Deep Sea Echini of 

 Florida, the growth of the ambulacra! and interambulacral systems takes 

 place independently. — the ambulacral plates increase very rapidly in num- 

 ber, while the coronal plates increase slowly in number, but gain in breadth 

 and in height. This is especially the case with the ambulacral plates of the 

 Clypeastroids, in which they assume such an immense preponderance over 

 the interambulacral plates, which in many genera are quite narrow belts on 

 the lower surface. 



The buccal imbricated plates of the actinal membrane also increase in- 

 dependently of the coronal plates. The new plates are added next to the 

 fixed actinal (.''\^H' of the coronal plates, as fast as the opening of the actinal 



system increases in size ; the edge of the coronal plates is denoted by the 

 position of the auricles which arise from the outer oral v>]^^ of the coronal 

 plates. This is especially well -ecu in the mode of growth of the buccal plates 

 in Cidaris, as first shown by Miiller. An apparently very important modifi- 

 cation of this mode of growth is found in the buccal system of Asthenosoma ; 

 when seen from the outer actinal side the plates of the actinal system seem 

 to be firmly soldered with the coronal plates, as if no new plates were inter- 

 calated between the coronal plates and those of the buccal membrane. Such, 

 however, is not found to lie the case; when the junction of the coronal and 

 buccal plates is examined from the inner part of the test, we find small new 

 plates which gradually force their way between the two systems below the 

 thin slight auricles found upon the outer edge of the coronal plates proper. 

 as in other Echini. 



* First shown by Agassiz, and subsequently mnfirmprl by Philippi .•iml Miiller. 

 t See A. Agassiz's Embryology of the Starfish. lst>4. 



