16 



OVUM. 



own researches and the comparison of some 

 others, to bring the whole of the Echino- 

 dermata under a general view, the result of 

 which is the determination of the three fol- 



Fig. 9. 



B 



Auricularia, or larva of Echinoderm (from Miiller). 



A, young larva of Auricularia. I, alimentary 

 canal ; a, Echinoderm beginning to be formed. 



B, larger larva [of the same kind, a Echiuodenn 

 farther advanced. " 



lowing varieties of metamorphoses and pro- 

 duction among them. In all of them the 

 embryo, immediately developed from the 

 ovum, has a bilateral symmetrical form, and 

 passes by the subsequent metamorphosis into 

 the radiated type. This change is, however, 

 more or less direct, or by intermediate forms. 



1. In the first variety the change of the 

 bilateral larva, or embryo, into an Echino- 

 derm takes place at its earliest period, when 

 the embryo has a general covering of cilia, 

 but not the special ciliated borders or limbs 

 of the Pluteus. A part of the body of the 

 embryo takes the form of the Echinoderm ; 

 the rest of it is absorbed into the body of 

 the new animal. This occurs in a part of 

 the Asteriadse, as in Echinaster, Asteracan- 

 thion, and others, described by Sars, Agassiz, 

 Desor, and Miiller. 



2. In the second variety the change occurs 

 when the larva is fully organised, that is, 

 when it possesses digestive organs and a spe- 

 cial motor apparatus of ciliated borders or 

 limbs. The Echinoderm is placed upon the 



Pluteus somewhat in the manner of a picture 

 on an easel, or a piece of embroidery in its 

 frame and stand, and incorporates a part of 

 the digestive cavity with itself. The remains 

 of the larva gradually disappear, as in Ophiura 

 and Echinus ; or are broken off and die, as in 

 Bipinnaria. 



3. In the third variety the change of the 

 larva takes place twice. First, it passes 

 from the bilateral type with ciliated borders 

 into the radiate type, and having taken some- 

 thing of the shape of a barrel, it acquires a 

 larval locomotive apparatus consisting of ci- 

 liated hoops ; and then from this state the 

 Echinoderm is developed without any part 

 of the larva being separated. Either the 

 Echinoderm is formed of a part of the Vermi- 

 form larva, and the rest of the larva is ab- 

 sorbed into the Echinoderm, as in Tornaria ; 

 or the whole larva is simultaneously trans- 

 formed into the Echinoderm, as in Holothuria. 



From Busch's observations it appears that 

 the Comatula passes very rapidly through 

 the stage of the bilateral form into that which 

 Miiller has called pupa with ciliated crowns. 

 It is also an interesting fact in connection 

 with the history of animal metamorphoses, 

 that the early condition of the Comatula is 

 that of a pedunculated Crinoid. 



Miiller has remarked that these pheno- 

 mena partake in part of the nature of meta- 

 morphosis, and in part of that of the non-sexual 

 gemmation of the alternate generations. As 

 the Echinoderm arises like a bud in the larva, 

 there is alternate generation ; but as the es- 

 sential internal organs (that is, the alimentary 

 canal from the stomach to the anus, but not 

 the mouth and gullet) are taken into the new 

 animal, there is also true metamorphosis. 

 " I understand," says he, " by alternate ge- 

 nerations nothing more than the succession of 

 two forms of organism, of which the one 

 arises in or upon the other as a minimum, 

 or as a bud ; the second, that is, the deve- 

 loped bud, is destined for sexual generation, 

 producing from its ova the non-sexual larva, 

 which again is destined for gemmation."* 



Adopting the view that the Echinodermata 

 present an example of alternate generation, it 

 is to be observed that the product is single in 

 all the instances known : but in all the other 

 forms of intermediate or alternate generation 

 hereafter to be noticed, the product of non- 

 sexual gemmation is multiple. 



Polypina. The animals usually compre- 

 hended in the general denomination of Polypes 

 or Polypina present very various kinds of 

 structure and degrees of complication in their 

 organisation ; and recent researches, as to 

 their mode of development, which point out 

 that some of them are subject to a process of 

 alternate or dissimilar generation, would ap- 

 pear to indicate a very different distribution 



* Loc. cit. p. 106. The researches of J. Miiller 

 on this subject have been published in a separate 

 form, as well as in the Mem. of the Berlin Acad. 

 These Memoirs, and others ou the same subject, will 

 be found also in Miillers Archiv. 184G,' p. 108 ; 

 1X47, p.l GO; 1848, p. 113; 1849, pp. 79. 84. oG4. 

 400 ; and 1850, p 452. 



