phyLogeny of star-fishes. 169 



develops which is utterly different from the fully developed 

 Star-fish, but very like the ciliated larvae of certain seg- 

 mented Worms (Star- worms and Ring- worms). This peculiar 

 animal-form is generally called the " larva/' but more cor- 

 rectly the " nurse " of these Star-fish. It is very small and 

 transparent, swims about by means of a fringe of cilia, 

 and is always composed of two equal symmetrical halves 

 or sides. The fully grown Echinoderm, however — which 

 is frequently more than a hundred times larger, and quite 

 opaque — creeps at the bottom of the sea, and is always 

 composed of at least five co-ordinate pieces, or antimera, in 

 the form of radii. Plate VIII. shows the development of the 

 " nurses " of the four Echinoderms represented on Plate IX. 

 The fully developed Echinoderm arises by a very remark- 

 able process of budding in the interior of the " nurse," of 

 which it retains little more than the stomach. The nurse, 

 erroneously called the " larva," of the Echinoderm, must 

 accordingly be regarded as a solitary worm, which by 

 internal budding produces a second generation, in the fonn 

 of a stock of star-shaped and connected worms. The whole 

 of this process is a genuine alternation of generations, or 

 metagenesis, not a " metamorphosis," as is generally though 

 erroneously stated. A similar alternation of generations 

 also occurs in many other worms, especially in some star 

 worms (Sipunculidse), and cord worms (Nemertin^e). 

 Now if, bearing in mind the fundamental law of biogeny, 

 we refer the ontogeny of Echinoderma to their phylogeny, 

 then the whole historical development of the Star-fishes 

 suddenly becomes clear and intelligible to us, whereas 

 without this hypothesis it remains an insoluble mystery. 

 (Compare Gen. Morph. ii. pp. 95-99.) 



