STRUCTURAL MODIFICATIONS 135 



well-being, which to-day we tend to replace by what we call 

 tacticism, it disposes its anus as far as possible from its mouth ; 

 it will thus reach that stage of coiling into a helix, with whose 

 phases we are familiar in its embryogeny. Except that they are 

 fixed, certain Crinoids of the Primary Period, e.g. Agelacrinus, 

 seem to have been arrested at this stage of their development. 

 The causes limiting budding to the posterior region of the 

 body in mobile animals, and operative in the case of the Annelid 

 Worms, are here absent. Any segment of the body can produce 

 a series of buds that first share the linear arrangement of the 

 parent, but subsequently can also become ramified [Astrophyton 

 and other Ophiuroids that attach themselves to polyparies, also 

 Pentacrinoids and Comatulides) . It is in this manner that Starfish 

 originated, which go back to the most remote antiquity, and it is 

 easy to derive all the other Echinoderms from them by reference 

 to simple embryogenetic considerations. I have shown 1 that 

 among the Starfish, Brisinga still has its arms regularly 

 segmented ; that the new segments form directly in front of 

 the oldest segment, represented by the radial disc plate of the 

 embryo as in the case of Annelid Worms, and that all the 

 transitions between this type that therefore appears primitive 

 and the pentagonal Starfish, so far removed from it, such as 

 the Culcites and Pent agon aster, can be followed. After the 

 metamorphosis resulting from their pleuronectean attitude, 

 all the Echinoderms pass through a common embryonic phase. 

 What was formerly the right side, and has become the dorsal 

 surface, takes on a radiate structure characterized by the 

 presence of a central calcareous plate, surrounded by five 

 similar plates called basals, followed by five others alternating 

 with them, called radials. The former left side, now the ventral 

 surface, is rayed in the same way, but each ray is essentially 

 composed of a double series of plates, called ambulacra! plates, 

 connected with tentacles or tube-feet. According to the 

 special fashion in which the calcareous plates multiply, 

 starting from this common embryonic form, all the various 

 classes of the Echinodermata have been derived. This 

 multiplication is almost non-existent in the Blastoids which 

 have to-day disappeared. Among the Starfish and the 

 Ophiuroids new plates are formed between all the dorsal 

 plates, particularly between the basal and radial ones. They 



1 XLI. 



