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are only diiferent modes of execution of one and the same plan, by 

 showing that they are convertible one into the other by means of such 

 changes as are only diflferent modes of carrying out the same plan ; 

 or by showing that one formula which should represent the general 

 plan of Radiation can successively be so transformed, that it will be 

 the formula for a Polyp, an Acaleph, or an Echinoderm, and no longer 

 the general formula for the plan of Radiation ; or, that the formula 

 of a Polyp, an Acaleph, or an Echinoderm, can be so changed as to 

 reproduce the formula of the other two classes. 



As has been shown by Baer, there are four different modes of de- 

 velopment, corresponding to the four types which Cuvier was the first 

 to point out. If, then, classes are simply the different modes of exe- 

 cution of the plan, the mode of development of each of these classes 

 must be a particular way in which the mode of development peculiar 

 to Radiata is carried out. The egg in Radiata is wholly transformed 

 into what is to become the future animal, which is not formed by the 

 gradual development of one portion of the yolk, as in the other types, 

 but by the direct transformation of the cells of the yolk into the new 

 animal. In Polyps a main cavity becomes hollowed out from the cells 

 of the embryo by the liquefaction of the central portion of the yolk. 

 Vibratile cilia cover the whole surface of the yolk, enabling the young 

 embryo to move freely about. Little by little there is a slight de- 

 pression formed, which eventually becomes the mouth, around which 

 , are formed small tentacles. About this time the small Polyp, instead 

 of continuing to move freely about, becomes attached, and we have 

 an animal with radiating partitions little developed, and with a 

 small number of chambers. The new Polyp once fully developed in 

 this way will in its turn lay eggs which will undergo the same 

 changes. But, besides the development by eggs, these animals can 

 increase by budding. A small knob on the outside wall of one ot 

 these Polyps first makes its appearance, a digestive cavity is grad- 

 ually scooped out of the solid substance, and tentacles become 

 formed in a manner similar to that observed in the development from 

 the egg. To the multiplication of individual Polyps by division I 

 need not allude here. 



Neither the eggs nor the embryos of Acalephs are at first different 

 from those of Pdyps. Like the Polyp, the young Acaleph rarely 

 remains free; generally it soon becomes fastened to the ground, 

 lengthens, a cavity is formed, and tentacles are seen round the 

 mouth ; but as it lengthens, slight bands are noticed across the Polyp- 

 like animal ; these gradually become deeper and deeper until there 

 finally separates from the upper part of this fixed Polyp an animal 

 entirely different from the parent stock ; this is a young Medusa. 

 The Acalephian mode of development passes quickly through the 



