WATER SYSTEM. 703 



In the Clypeastroids we' have at an early age a fixed equatorial periph- 

 ery. The plates which compose the actinal side of the test at an early age 

 attain the number which they reach in the largest specimens, and then in- 

 crease only in length and breadth, while in the upper part of the test the 

 plates do not increase as rapidly, and small new plates are added to the test 

 at its apical portion. The distinction established by Miiller between the 

 (Cidaridae) Desmosticha and Clypeastridae. based upon the permanence of 

 the periphery (edge of the test), is valid only after a certain stage of growth, 

 and does not hold good in the very youngest, stages of some of the Clypeas- 

 troids I have had occasion to examine (Echinarachnius, Mellita, Clypeaster). 

 Additional plates are also added at the apical pole of the Petalosticha In 

 Spatangoids proper we have a still smaller number of interambulacral coronal 

 plates than in the Clypeastroids ; additional plates are only added in the 

 ambulacral regions, the interambulacral plates keeping pace with the growth 

 of the ambulacral zone by their increase on the edge of the coronal plates. 



The ambulacral suckers in the case of the Desmosticha and Clypeastroids 

 ride upon double pores ; on the abactinal part of the test of the Spatangoids 

 below the petals the pores gradually approach and finally frequently coalesce, 

 forming a single opening. In the Clypeastroids we have, in addition to the 

 large suckers of the ambulacral system, transverse rows of single minute 

 pores also belonging to the ambulacral system, piercing the plates of the 

 ambulacral system near the sutures. These pores were first traced by 

 Agassiz in Echinarachnius, and shown by him to be connected with the am- 

 bulacral system by direct injection. They increase (laterally) in number 

 with the age of the test, as can readily be seen when comparing Clypeas- 

 troids of different sizes. 



The coronal plates of the Desmosticha are made up, near the actinal sys- 

 tem, of an equal number of zones of ambulacral and interambulacral plates ; 

 while round the actinostome of the Clypeastroids the so-called buccal rosette 

 is formed, which consists entirely of ambulacral plates touching one another, 

 the interambulacral plates commencing only as a single plate on the second 

 and sometimes even only on the third row from the actinostome. 



