10' ON THE EMBRYOLOGY OF ECHINODERMS. 



Echinoid larvae in which the young Echinus has absorbed more or less of the plutean 

 frame. From what I have observed on several of these larvae, the Pluteus is as com- 

 pletely resorbed as is the case in the Brachiolaria observed by me. Not a single part 

 of the framework is thrown off; this process of resorption begins at the base of the 

 arms ; they are thus gradually shortened, the rods apparently melt away before our 

 eyes, the extremity of the arms is the last to disappear, and immediately before the 

 time when the young Echinus is freed from the plutean appendages, the extremities of 

 all the arms are still there, as perfect as when these appendages stretched symmetrically 

 on both sides of the longitudinal axis. From many of the figures of MUller himself it 

 is evident that, in the larvae he has observed, the young Echinus resorbs the whole of 

 the framework, and does not separate from it by losing the arms, as he has stated. 

 See Plates III., IV., V., VI. of the first, and Plate VIII. of his seventh Memoir. The 

 larva represented in fig. 20 of this paper was kept in confinement from the 1st of 

 October to the 20th of November before eveiy trace of the arms had disappeared, and 

 the young Sea-urchin had assumed the appearance of fig. 24. Fig. 24 was drawn from 

 a specimen found floating on the surface in the middle of June. This young Sea- 

 urchin bears a striking resemblance to a young Echinocidaris figured by Miiller on 

 Plate IV. of his seventh Memoir. The development of the separate parts is very dif- 

 ferent in the two. The number of spines is much greater in our Toxopneustes, and 

 they are of an entirely different shape. Pedicellaria are likewise present in Echino- 

 cidaris ; * these do not make their appearance till a much later period in our young 

 Sea-urchins. (See fig. 28, p.) What is particularly characteristic of these earlier stages 

 of the young Sea-urchins is the great size and small number of the spines. Their 

 position is also peculiar ; they are all placed on the edge of the test, which is exceed- 

 ingly flat. (Compare this with Podophora.) Five of the tentacles are strikingly 

 prominent, equalling in length the diameter of the shell ; they are also I'emarkable 

 for their great thickness, and the presence of a calcareous ring in the sucker, which is 

 entirely wanting in young Starfishes. A similar calcareous ring is figured by Miiller, 



* The function of pedicellaria has long been a puzzle to naturalists. While watching a Sea-urchin which 

 was discharging its excrements, I noticed that the pellets always moved in definite paths, down the interambu- 

 lacral spaces, till they were pushed off from the test. On examining this with a magnifying-glass, I could 

 distinctly see the innumerable pedicellaria liard at work seizing in their tiny prongs the pellets which tiiey 

 pushed down the interambulacral areas. If by chance a pellet found its way into the ambulacral zone and 

 came in contact with the suckers, it was immediately seized by the ambulacral pedicellaria and thrown back 

 into the interambulacral zone to move on in its accustomed path. The pedicellaria are more numerous in the 

 interambulacral zone than in the ambulacral. This will explain the function of these pedicellaria, at least for 

 Sea-urchins, as that of scavengers. ^ 



