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stages which form the whole of the development of a Polyp, and 

 has in common with them only that which is characteristic of the 

 mode of development of Eadiata, the manner in which it is carried 

 out being Acalephian. In many Acalephs, Medusae are formed directly 

 by budding from the Polyp-like body. 



Let us now consider the Echinoderms ; — they pass still more quickly 

 through the earlier stages which remind us of the Polyps, and soon 

 assume a form which recalls that of the higher Acalephs, the Cteuo- 

 phorae, and then connnences the peculiar mode of execution of the type 

 of development belonging to Eadiata which is characteristic of Echino- 

 derms. The young, which begins simply as a coating upon the digest- 

 ive cavity, encroaches gradually more and more upon the parent stock, 

 until finally the greater part, if not the whole, becomes absorbed or 

 is cast off, and the young Echinoderm has been formed by absorbing 

 the parent stock. Yet these Echinoderms, although their mode of 

 development is so peculiar, have in common with Polj^ps and Aca- 

 lephs the same mode of development, as far as it is characteristic of 

 Radiates, but it is carried out in a peculiar way for Polyps, Acalephs, 

 and Echinoderms. It is not a little strange that we should have in 

 Echinoderms a mode of carrying out the develoiDment peculiar to that 

 class which starts from a form recalling the highest Acalephs ; and 

 that the mode of development which is peculiar to the Acalephs should 

 begin with a form which is eminently Polyp-like, while all three 

 classes should agree in having in common a particular mode of de- 

 velopment concordant with the plan of Radiation, which is then car- 

 ried out further in a peculiar way for each class. 



To appreciate truly the organic connection of the growing Echi- 

 noderm and its Acalephian nurse, it is only necessary to take into con- 

 sideration the relation of the twin individuals, suspended along the 

 pendant chain of a Diphyes^ in which couples the Polypoid form pro- 

 duces one which is Medusoid. And I believe it is a mistake to con- 

 sider these chains as made up of single animals (Einzelthiere) ; they 

 are really twins, one of which is Polypoid and the other Medusoid ; as 

 during their growth the young Echinoderms form twin couples, one 

 of which is Acalephian and the other Echinodermoid. In the Sipho- 

 nophorce the polypoid animal is mostly split open upon one side, like 

 the radiate flowers of Compositce, forming a sort of scale-like cover- 

 ing, from which the proboscis hangs free. But proboscis and scale 

 (Hydrophyllium or Dechsiiiclc) are but one animal in these commu- 

 nities. 



Having shown how the mode of development peculiar to Radiata 

 was carried out in a special manner for the three classes which con- 

 stitute the type of Radiata, let us now take representatives from 

 each of these classes, and see if we can transform the formula which 



