PcveJopment of Ilohthurians. 425 



of its branches, develops into a small lattice-work ])late. h\ 

 the course of this process it may be seen that a thickeiiin;^ of 

 the rods sinudtancously takes place by apposition, llerouard's 

 view, according to wliich only a single formative cell corre- 

 s])onds to each mesh of the latticed plate, is not supported by 

 my observations ; on the contrary, I observed as distinctly as 

 possible that usually several, i. e. two to six, formative cells 

 occur ill each mesh. The five foremost latticed plates are so 

 arranged that their longitudinal axes fall exactly in the direc- 

 tion of the radii. These five plates together form a penta- 

 gonal projecting slicath for the crown of tentacles. Each 

 tentacle corresponds in position to the line of contact of two 

 ydatcs. Further backwards these five oral latticed plates 

 ( = ])Scud-oral plates) are connected with others of similar 

 formation, which originally come into contact with each other 

 just as little as do the oral plates at their first appearance, 

 hr'oon, however, they become larger and more numerous, collect 

 close together, and then thrust their edges over one another 

 like the slates of a roof, so that the fore border of one plate 

 rests upon ihe hind border of the one next in front. In the 

 walls of the tentacles and feet, also, smaller latticed plates very 

 soon appear in large numbers. About the hundredth day a 

 second sort of calcareous body is seen to appear in the integu- 

 ment of the trunk, occupying a position nearer the surface 

 than the latticed plates which Lave hitherto alone been 

 present. It is distinguished by its remarkable smallness, 

 elegance, and richly-branched shape, and in form it is arched 

 in such a way that its concave side is turned outwards, its 

 convex side inwards. Further particulars as to the form, 

 origin, and arrangement of the calcareous bodies and their 

 relation to the calcareous bodies of the adult will be commu- 

 nicated by means of figures in my detailed memoir. There, 

 also, it will be proved that the calcareous ring is formed from 

 the bcdy-Avall, and shows remarkable relations between its 

 radial ossicles and the ambulacral ossicles of the skeleton of 

 the starfish. 



Integumerd and Mesenchyma. — The circumstance ajjpears 

 to me to be not without interest that after the complete disap- 

 pcaraixe of the ciliated bands of the larva it is not possible to 

 make a sharp distinction either between the ectoderm and 

 the gelatinous nucleus of the cephalic hump (so long as this 

 is still present in the neck of the young Cucumaria), or 

 between the ectoderm and the mesenclij ma of the wall of the 

 trunk. Ectoderm and mesenchyma in young Cucumarians 

 form a single tissue, which does not difiereniiate until later 



