CLYPEASTRINA. 11 



test and ultimately are closed in at the distal end, or as depressions on the oral 

 side of the test which by resorption ultimately perforate both the oral and aboral 

 plates and attain to the full size of the lunule. But whether the two processes 

 have developed simultaneously in phylogeny or whether one is more primitive 

 than the other, is still unsettled. There are some reasons, however, for believing 

 that the resorptive process was the first to occur and that the lunule of the pos- 

 terior interambulacrum is the oldest phylogenetically. Practically nothing is 

 known as yet as to the function of the lunules — granting that they have any. 

 In certain genera, notably Rotula, marginal slits, which never close to form 

 lunules, occur, particularly in the posterior interambulacrum and the adjoin- 

 ing ambulacra. Their use is as obscure as that of the lunules. 



Another remarkable feature of the clypeastroids is the extension of the 

 ambulacral tube-feet on to other portions of the test than the strictly "poriferous 

 areas" of the ambulacra. The modification of the aboral part of the ambulacra 

 to form the characteristic " petals" is so well known, no description of it is neces- 

 sary here, though the form of the petals and certain details of their structure are 

 of great systematic importance. But the occurrence of a multitude of very 

 minute ambulacral feet is not so generally known. Mr. Agassiz (1874, Rev. 

 Ech., pt. 4, p. 695) has described the distribution of these supernumerary pedicels 

 in several genera, and Miss Gregory (1911, Zool. Anz., 38, p. 323) has given a 

 detailed account of them, as seen from the interior of the test, in Echinarachnius. 

 There is, however, much still to be learned in regard to their distribution in the 

 different genera. The resemblance between the distribution of the pedicels in 

 Echinarachnius and in such holothurians, as Thyone, is a most striking example 

 of "parallelism," the genetic connection between the two groups being of course 

 very indirect. Associated with this multiplication of pedicels in the clypeastroids 

 is the development of what are called the "ambulacral furrows," grooves on the 

 outer surface of the oral side of the test, radiating out from the mouth and 

 extending in the median ambulacral area towards the margin. They may be 

 simple, short and indistinct, or simple (i. e. unbranched) and extending clearly 

 to the margin, or branched more or less extensively and some or most of the 

 branches reaching the margin. Their arrangement is a feature of some sys- 

 tematic importance. 



The anus, or better the periproct, lies near the margin of the test in the 

 posterior interradius. In young individuals it is more or less aboral in posi- 

 tion and in adult Arachnoides, it is more aboral than oral. In several cases 

 it is marginal, but most commonly it is distinctly on the oral surface. In extreme 



