4i 8 NATURAL SCIENCE. j UN e, 



many stalked crinoids present an expanded base of attachment, often 

 involving more than one columnal. Such a structure, inherited from 

 ancestors, still of functional importance, and specially necessary in all 

 cases during larval life, would surely not be developed in precisely the 

 same way as an ordinary columnal. Such considerations, it is true, 

 would not be enough to upset the theory of a dorsocentral, were any 

 evidence in its favour forthcoming ; but, in the absence of such 

 evidence, they are enough to suggest caution in the acceptance of far- 

 fetched homologies. The foregoing conclusions were based, as 

 stated, chiefly on palaeontological grounds, and were opposed to the 

 views of those who laid more stress on embryology. But now even 

 an embryologist has struck a severe blow at the generally received 

 homologies. Mr. MacBride states (Proc. Roy. Soc, vol. liv., p. 433) 

 that in Asterina as in Antedon, the prae-oral lobe of the larva becomes 

 a fixing organ or stalk, but the relations of this stalk to the body of 

 the adult are totally different in the two cases. " It follows that the 

 abactinal poles of Asterina and Comatula (Antedon) are not comparable 

 with each other, and that all conclusions based on the supposed 

 homology of the dorsocentral in Echinids and Asterids, and that in 

 Crinoids, are incorrect." Of course if this be so, not merely are the 

 conclusions to which Mr. MacBride refers untenable, but the last 

 support of the dorsocentral itself is also demolished. The dorso- 

 central, considered as a morphological element of the Echinoderm 

 type, has got to go. 



To return to Dr. Lang, a similar error in the use of terms occurs 

 in his treatment of the perisomatic skeleton. On p. 923 this is 

 defined as composed of all those skeletal pieces which protect the 

 body between the apical and oral systems. Further, in the Crinoidea 

 (p. 949) the perisomatic skeleton is said to consist of (1) the periso- 

 matic skeleton of the calyx ; (2) the skeleton of the arms and pinnules ; 

 (3) the skeleton of the stem. And in the perisomatic skeleton of the 

 calyx are included all skeletal pieces other than the dorsocentral, 

 infrabasals, basals, radials, and orals. 



Now the term perisomatic skeleton appears to have been first 

 employed by Sir Wyville Thomson in his paper, " On the Embryogeny 

 of Antedon rosaceus" (Phil. Trans. 1865, p. 540) in these words: "The 

 skeleton of the pentacrinoid is composed of two systems of plates, 

 which I shall term respectively the radial and the perisomatic system, 

 thoroughly distinct in their structure and mode of growth. The radial 

 system consists of the joints of the stem, the centrodorsal plate, the 

 radial plates, and the joints of the arms (and subsequently of the 

 pinnules). The perisomatic system includes the basal and oral 

 plates, the anal plate, the interradial plates, and any other plates or 

 spicula which may be developed in the perisom of the cup or disk." 

 The lateral and covering plates of the ambulacral grooves were 

 likewise included by Sir Wyville in his perisomatic system. Precisely 

 the same significance was attached to the term by subsequent writers 

 on the Crinoidea, at least down to and including the " Challenger " 

 reports of P. H. Carpenter. In their important paper, "The 

 Perisomic Plates of the Crinoids" (Proc. Ac. Nat. Sci., Philadelphia, 

 1890, p. 345), Messrs. Wachsmuth & Springer seem to have used the 

 term in a different sense, or rather they substituted for the old division 

 one that is simpler and, to my mind, more logical. They divide the 

 skeletal elements of a crinoid into primary and secondary. Primary 

 elements include : (a) The abactinal plates, developed on the right 

 antimere and connected with the axial nerve-cords, viz., columnals, 

 infrabasals, basals, radials, and all brachials ; and (b) the actinal 



