482 RECORD OF SCIENCE FOR 1886. 



II. The external plate or epitheca. "This is a coutiuuatiou of the basal 

 phite, aud yet more or less distiuctly separable from it. It incloses the 

 lateral body wall, but does not otherwise come into contact with the 

 substratum." 



III. The internal plate or thica. "This rises from the basal plate in 

 the form of a circular ridge, usually iiarallel to the external plate, and 

 generally eusheathed in an intruding fold of the body wall. There may 

 be more than one internal plate." 



IV. The radial plates or septa. "These are represented by numerous 

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

 radial folds of the body wall, which alternate with the parietes (mes- 

 enteries)." 



In further explanation it is also noted : 



(a) " The body wall always lies between the external and internal 

 plates." 



{h) " The parietes always lie between two radial plates, and the latter 

 are, for some distance upwards, always separated by the internal plate 

 into a peripheral and central portion." 



(c) " The external plate is only clothed with tissue on its inner surface, 

 but the internal plate on both surfaces." 



{d) " Where the radial plates come into secondary contact with the 

 external plate they penetrate the body wall." 



With these postulates, "taken along with the fact that the skeleton 

 is excreted by the ectoderm, a number of important conclusions may 

 be deduced as to the origin, growth, and structure of the 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 thick- 

 ening 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, cov- 

 ered by successive strata, but the growth on either side may be dispro- 

 portionate." 



(5) " In the radial jdates, the oldest portion is in the middle, and the 

 subsequent depositions are symmetrically laid down on either side." 

 (Morphol. Jahrb., Xiil, pp. 154-lGO, 1 pi; J. E. M. S. (2), vi, pp. 

 805-80G.) 



Acalephs. 



Neic form of fresh-icater hydroids. — Fresh- water hydroids are so few 

 in number that any new species is noteworthy, and one has been de- 

 scribed lately which is further interesting on account of some phases of 



