vni ECHINODERMATAENDOSKELETON, ETC. 405 



Holothurioidea is no longer maintained. The two structures are altogether differently 

 related to the nervous and water vascular systems. 



The homology of the calcareous ring with the perignathous apophysial ring of 

 the Ecliinoidea is equally doubtful. The radials of the calcareous ring were in this 

 case compared with the auriculae (ambulacral apophyses). But each auricule is 

 paired and consists of two processes, or folds, of the edge of the peristome, which 

 may or may not be connected together by an arch ; the radials, however, are 

 from the first unpaired. Only the arches of the auriculae could be compared with 

 the radials ; the arch, however, is not a single plate, but is formed by apposition 

 of the two neighbouring ambulacral apophyses of one and the same ambulacrum. 



The comparison of the calcareous ring with the masticatory apparatus or 

 Aristotle's lantern of the Echinoidea still remains. The five radials have been 

 compared with the five fork pieces, and the five interradials with the five arches 

 of the pairs of jaws (pyramids) of the lantern. This comparison is in many ways 

 plausible, but here, as before, many difficulties appear when the subject is carefully 

 investigated. The arches of the jaws are paired structures, and cannot therefore be 

 compared with the interradials, which are from the first unpaired. Moreover, it 

 is very doubtful whether they represent independent skeletal pieces ; they appear 

 rather to be merely muscular processes of the halves of the jaws. Further, the 

 sinews which proceed from the forks of the radial fork pieces are attached to the 

 perignathous apophysial ring interradially (i.e. to the interambulacral apophyses), 

 while the muscles which are attached to the radials of the calcareous ring of the 

 Holothurioidea run strictly radially. 



G. Further Deposits of Calcareous Matter. 



Deposits of calcareous corpuscles and masses may occur in the 

 connective tissue of the walls of various internal and external organs, 

 especially in the ambulacral and alimentary systems. These will be 

 considered in connection with the systems to which they belong. 



We shall here only mention certain calcareous deposits in the 

 Clypeastroida. An endoskeleton is here formed. On the oral, as well 

 as on the apical, inner surface of the test, needles, pillars, lamellae, 

 etc. rise, sometimes only at the edge, sometimes over large areas. 

 These may traverse the whole depth of the test, connecting its oppo- 

 site walls. They more or less completely separate the ambulacral 

 structures from the other internal organs, such as the intestine, the 

 genital organs, etc., and may in some cases attain such great develop- 

 ment that, as in Encope, they form a sponge-like or cellular calcareous 

 framework throughout the whole interior of the test, in which larger 

 spaces are left for the masticatory apparatus, the intestine, the 

 ambulacra, etc. Not infrequently, the ambulacral vessels are completely 

 vaulted over by deposits of calcareous matter. 



H. Concluding Remarks on the Section on the Skeletal System. 



In the above section, I have adopted the views of those investigators whose wide 

 and for the most part difficult researches have convinced them that at least the 

 plates [of the apical and oral systems (the central, infrabasals, basals, radials, and 

 orals) are homologous throughout the whole group of the Eehinodermata. These 

 plates therefore must be ascribed to the common racial form. But these plates are 



