OVUM. 



15 



testine, and anus, and moves actively through 

 the water. Sars who had observed this body 

 in 1835, was the first to suggest in 1844 that 

 it might be the early condition of a star-fish*, 

 and this view was confirmed by the admirable 

 researches of J. Miiller f, and by observations 

 of Koren and DanielsonJ, who have shown 

 that the Asterias is gradually formed out of a 

 small granular mass which surrounds the 

 stomach of the Bipinnaria, and becomes se- 

 parated from the stock when in a compara- 

 tively early state of advancement. The larva 

 stock moves about afterwards for a few days, 



Fig. 8. 



Pluteus puradoxus {from Midler). 



A, Fluteus before the commencement of the 

 formation of the Ophiura. 



u, Ophiura formed on the side of the gullet. 



* Wiegmann's Archiv. 1844, part i. p. 176. 

 f Mem. of the Berlin Acad. 1846 and 1848. 

 1 Ann. des Sc. Nat. 1847, p. 348. 



and then appears to die without giving rise to 

 any farther progeny. 



The gemmiparous larva of some other kinds 

 of the Echinodermata was first described 

 by J. Miiller as a distinct animal, under the 

 name of Pluteus, before he was acquainted 

 with the phenomena of its subsequent de- 

 velopment : in 1846 he traced the relation 

 between one kind of this body which he had 

 called Pluteus paradoxus, and the Ophiura, 

 and between another kind of Pluteus and 

 Echinus, ascertaining it to be the same that 

 has just been stated to exist between the 

 Bipinnaria and the Asterias. The Plntens 

 presents the form of a quadrangular pyramidal 

 frame, with four large ciliated limbs at the 

 angles, and four smaller ones suspended from 

 the middle below, while the upper part is 

 surmounted by ;i sort of dome. It bears some 

 resemblance to a Beroe, and might be de- 

 scribed as the ciliograde larva of an Echino- 

 clerm. The form differs, however, somewhat 

 for various species of Ophiura and Echinus. 



In the centre of the dome and round the 

 mouth of the Pluteus a granular mass is de- 

 scribed, and from the side of this, non-sym- 

 metrically, the gemmation of the new indi- 

 vidual proceeds. The Pluteus moves at first 

 with great activity through the water, pro- 

 pelled by its ciliated limbs and cirrhi; but as 

 the new Ophiura or Echinus buds from it 

 and spreads more and more over its dome, 

 the Pluteus shrinks, becomes less active, and 

 at last disappears.* 



Various other forms of the Pluteus-like 

 animal have been described by Midler, and 

 the process of gemmation has been traced 

 by which the new Echinoderm takes its rise 

 within them. The result of these discoveries 

 is already to throw an entirely new light on 

 the nature and organisation of this class of ani- 

 mals ; but the species of all of those observed 

 is not yet determined, and something still 

 remains to be learned of the exact mode of 

 origin of the new animal. By some-j- the 

 process has been looked upon merely as a 

 secondary development from the remains of 

 the yolk attached to the parts first formed ; 

 but the researches of Miiller do not appear 

 to give support to such a view ; and would 

 rather appear to show (as in Auricularia, 

 fig. 9.), that the new animal is formed from a 

 minute germ in a determinate part of the 

 parent animal without that germ being traced 

 to the yolk of the egg. 



In a farther series of researches on the 

 larvae and metamorphoses of the Echino- 

 dermata J, J. Miiller has pointed out that the 

 Holothuridae are formed from a larva body 

 somewhat analogous to the Pluteus, but that, 

 instead of a process of new formation, the 

 whole of the larva is converted by a very 

 remarkable metamorphosis into the Holo- 

 thuria ; and he has been enabled, from his 



* J. Miiller, in Mem. of Berlin Acad. 1846 and 

 1848 ; Derbes, in Ann. des Sc. Nat. 1847. 



t As Carpenter, loc. cit. p. ( J39. 



J Memoirs of the Acad. of Scicn. of Berlin, Nov. 

 184D, and April 1850, published in 1851, p. 35. 



