CHAP. XVI ECHINODEEMATA 457 



experiments on the eggs of those groups of animals (c/. Coelenterata, 

 Nemertinea, MoUusca, and Nematoda), which we have already 

 discussed. For this reason it is necessary that the normal course of 

 development in Echinodermata should be described in detail. 



Of the four groups of Eleutherozoa, the one which presents the 

 most primitive features in its development is the Asteroidea, and we 

 shall commence our study of the development of Echinodermata by a 

 consideration of this group. 



ASTEKOIDEA 



In a few cases the entire development of an Asteroid has been 

 1 worked out, and the manner in which the organs of the adult are 

 fashioned out of the body of the larva has been elucidated. The 

 species which have been thus studied are Asterina gihhosa, Gribrella 

 oculata, and Solaster endeca. Unfortunately in all these cases we 

 have to deal with an egg which contains a good deal of yolk, and 

 with a development which is much modified and hurried on. 

 Asterina gihbosa is the least modified of the three, for its larva 

 possesses a mouth and it can take food in the larval condition, 

 whereas in the case of the other two species the gut is delayed in its 

 development, and the larva possesses no mouth. 



In the more normal type of development, such as is met with in 

 the genera Asterias, Astrojoecten, Luidia, etc., the egg develops into a 

 larva which possesses a complete alimentary canal, and lives a free- 

 swimming and self-supporting life for a long period, often extending 

 over two months, until at last it metamorphoses into the adult form. 

 The eggs of several species of Asterias have been successfully reared 

 through the metamorphosis by feeding them with cultures of various 

 forms of diatom. 



A complete account of the development of Asterias rubens by Dr. 

 Gemmill is now in the press, and will shortly be available to all 

 zoologists ; by Dr. Gemmill's kindness, however, we are allowed to 

 make use of some of his results which are as yet unpublished. 

 Field (1894) has given an account of the segmentation of the egg 

 and of the early larval stages in the development of the American 

 species {Asterias vulgaris). We have also an account of the full-grown 

 larva, and of the metamorphosis of Asterias pallida, by Dr. Goto (1897). 

 Now Asterias pallida is a mere synonym for Asterias vulgaris, and this 

 starfish is considered by systematists to be very closely allied to 

 Asterias rubens, the common British starfish, of which some indeed 

 consider it to be a local variety. By piecing together, therefore. 

 Field's, Gemmill's, and Goto's observations a fairly complete story 

 can be made out, and it is important to notice that in most important 

 points Goto's results demonstrate that, in so far as the building 

 up of the adult organs is concerned, Asterias pallida agrees very 

 closely with Asterina gihbosa. As the organogeny of the latter 

 species has been very thoroughly worked out we can thus use the 



