806 SUMMARY OF CURRENT RESEARCHES RELATING TO 



I. The basal plate. — This is excreted between the aboral terminal 

 surfaco of tbo body, and tbo substratum to which the skeleton is 

 attached. 



II. Hie external plate or epitheca. — This is a continuation of the 

 basal plate, and yet more or less distinstly separable from it. It 

 incloses tho latoral body-wall, but does not, otherwise, come into 

 contact with the substratum. 



III. The internal plate or theca. — This rises from the basal plate 

 in the form of a circular ridge, usually parallel to the external plate, 

 and generally ensheathed in an intruding fold of the body-wall. 

 There may be more than one internal plate. 



IV. The radial plates or septa. — These aro represented by nume- 

 rous radial ridges, which ascend at right angles to the basal plate, and 

 lie in radial folds of the body-wall, which altcrnato with tho parietes 

 (mesenteries). 



These definitions are further explained, and it is also noted — 



(a) That the body- wall always lies between the external and 

 internal plates ; 



(b) That the parietes always lie between two radial plates, and 

 that the latter are, for some distance upwards, always separated by the 

 internal plate into a peripheral and central portion ; 



(c) That the extornal plate is only clothed with tissue on its inner 

 surface, but the internal plate on both surfaces ; 



(d) That where the radial plates come into secondary contact with 

 the external plate, they penetrate the body-wall. 



From the above definitions, taken along with the fact that the 

 skeleton is excreted by the ectoderm, a number of important con- 

 clusions may be deduced as to the origin, growth, and structure of 

 tho skeleton. 



1. All parts of the skeleton are laid down as plates, and their 

 growth is effected by the apposition of new particles on those already 

 formed. 



2. As to superficial increase, all parts of the skeleton are alike, 

 but they vary in the mode of their increase in thickness. The 

 thickening of the basal and external plates is only effected from one 

 side, while that of the internal and radial plates may take place from 

 both. 



3. In the basal and external plates the oldest portions are thus 

 obviously on the outer side, and the newer portions are inwards. 



4. In the internal plate the oldest portions are on either side 

 covered by successive strata, but the growth on either side may be 

 disproportionate. 



5. In the radial plates the oldest portion is in the middle, 

 and the subsequent depositions are symmetrically laid down on either 

 side. 



These definitions and conclusions, which are stated with the 

 greatest conciseness and clearness, are accompanied with several very 

 lucid figures, and are applied to a score or so of instances, in the 

 examination of which the author was able to avail himself of numerous 

 living forms. 



