ORAL AND APICAL SYSTEMS OF THE 12CHIN0DERMS 195 



larger proportion of the test of the Cidaridce than the plates 

 on the aetinal membrane do in the other Urchins. For the 

 number of coronal plates is small, especially in young 

 specimens, the test of which seems to consist almost 

 entirely of an aetinal and abactinal system, separated by a 

 narroAv band of coronal plates. Further, these imbricated 

 buccal plates are arranged radially in rows made up of 

 more than two plates, with the plates lapping in opposite 

 directions in the ambulacra and interambulacra. These 

 peculiarities are also found in the plates not only of the 

 buccal membrane, but also of the corona of the Echinothu- 

 ridce, and they are characteristic of the PalachinidcE. 

 Hence, Agassiz^ has suggested that the test of these last 

 was made up of plates homologous with the buccal plates 

 of Cidaris. V/ere the narroAV Isand of coronal plates in a 

 young Cidaris to disappear entirely, and the buccal plates 

 to take a correspondingly great development, we should 

 have a spherical Urchin agreeing in every respect with the 

 typical PcdcBchinus. The test would be reduced to the 

 aetinal and abactinal systems, and be entirely made up of 

 small ambulacral and interambulacral plates consisting of 

 several rows, and homologous with those of Asthenosoma and 

 Cidaris. Instead of regarding the test of a Palaeclwius as 

 consisting only of an abactinal, together with an enlarged 

 aetinal system, Agassiz~ has pointed out that the latter may 

 be also considered as a corona, reaching almost to the jaws, 

 the aetinal membrane being reduced to an insignificant 

 member, as in the Clypeastroids. This view is essentially 

 similar to that put forward by Loven. It appears to me that 

 it is the more correct of the two, and I imagine that the 

 Pal(Bchinid(E exhibit to us a condition of the Echinoderm 

 skeleton, closely similar to, and yet different from that which 

 is found in many Crinoids. 



It is worth notice that there are many Holothurians, the 

 condition of which is in some measure comparable to that 

 of the Palachinida and EchinotJucrida. Thus, in Psolus, 

 Ocrus, Colochirus, and Echinocucumis, there is a flexible 

 test, i. e. a thick leathery membrane, in which large cal- 

 careous plates are imbedded, and in Colochirus they are 

 pierced for the tubular feet. The homology of these plates 

 with the test of Echini has been already pointed out by 

 Semper." In Psolus there is also an oral system, but no 

 distinct apical system has been traced, while in the Perisso 



1 ' Ptevisiou,' pp. 257, 646. 



2 'Revision,' p. 646. 



' " Reisen im Arcliipel der Pbilippinen," ii, 1. ' Holothurien,' p. 58. 



