GENEEA AND GEOUPS OP THE ECHINOIDEA. 37 



absence from England of Sir Wj. Thomson prevented him from 

 giving more time to the study of the specimens, so that some 

 errors were published. It is evident, however, that the mistake 

 made about tbe direction of the imbrication of the plates in ' The 

 Depths of the Sea ' was corrected in the Phil. Trans. 1874. But 

 the description and drawing of the continuation of interradial 

 overlapping plates beyond the peristome to the true mouth were 

 unfortunate and so was the failure to recognize the external 

 branchiaD. Some of the internal structures were described, such 

 as the series of longitudinal muscles running up the sides of 

 the ambulacra, and having to do with the positive motions of 

 the plates one over the other; but, incomprehensibly enough, 

 the huge internal branchise were not recognized, and their dis- 

 covery has fallen to the Drs. Sarasin in 1888 ! A. Agassiz, in his 

 Eeport on the Challenger Echini, 1881, considered the question 

 of the amount of the imbrication of the plates of the Echino- 

 thuridse — its cause, nature, and its relation to bevelling in thicker 

 Palseechinoidea. These phenomena were exhaustively and most 

 judiciously explained. 



Kothing can be more definite than the description of A. Agassiz 

 of the construction of the test of Asthenosoma pellucidum, A. Ag. 

 He remarks that the test is remarkably thin, and that even in a test 

 of the diameter of 64 millim. the plates do not give the test any 

 degree of solidity. The examination of a specimen of PJiormo- 

 soma luculentum, A. Ag., in the British Museum proves the com- 

 paratively large size of some of the interradial plates, but they are 

 excessively thin, consist of very open reticulate carbonate of lime, 

 and they thin off at the edges, the calcareous structure being lost; 

 in the membranous part of the plate. This membranous part is 

 continuous between plates, and the soft edge of one plate merges 

 into the corresponding membranous part of the neighbouring 

 plate. 



In Asthenosoma coriaceum, A. Ag. ('Challenger' Eeport, 

 pi. xvii. a. figs. 5-7), the amount of soft interplate tissue is con- 

 siderable; in some places the plates show an extremely small 

 calcareous part, and the excess of soft tissue is great. The semi- 

 transparency of some species, when kept in alcohol, is remarkable, 

 and the calcareous part of the plates is seen to be surrounded by 

 a greater or less amount of soft movable, but probably not 

 extensible, connective tissue — that is, of uncalcified plate-area. 

 The flexibility of such tests is considerable in large parts of them, 



