L. Agassiz on the Echiuodermata. 41 



growth is completed. The only difference between this pro- 

 cess and the direct observation of any development is, that in 

 the one case we observe in one and the same individual that 

 succession of changes, which in the other we trace through a 

 series, as complete as possible, of several individuals. Such 

 is the course that I have taken with respect to the Echinoder- 

 mata. The young Echini have a small number of plates in each 

 of their vertical series; they appear to be slowly increased in size 

 by the deposition of calcareous matter at their circumference 

 until those which surround the mouth have completed their 

 growth and are entirely consolidated. The superior plates con- 

 tinuing to grow, increase, from the top downwards, the peri- 

 phery of the body, which remains depressed so long as the in- 

 ferior are the only plates consolidated ; but in proportion as a 

 greater number of plates become immoveable, and as there is 

 formed, in the upper region, a greater number of plates reach- 

 ing down to the circumference of the spheroid, the testa be- 

 comes rounded and finally assumes a spherical form. It is to 

 this cause that we are to ascribe the differences of contour ex- 

 hibited by the Echini at different ages. In some species there 

 are found individuals presenting even a pyramidal shape, and 

 this takes place when there is still formed a great number of 

 plates subsequently to the consolidation of those occupying 

 the greater diameter of the animal's body. These facts suffi- 

 ciently explain the gradual growth of beings which approximate 

 more or less nearly to the spherical form, and show how care- 

 fully w r e should guard against the introduction of nominal spe- 

 cies in consequence of a mere difference of form resulting from 

 age only. 



It would be interesting to trace the development of these 

 animals ab ovo. But no naturalist has yet observed the state 

 of the Echini on their first issuing from the egg. As to the 

 spines, it is evident, especially in the Cidarites, that those sur- 

 rounding the mouth are the first that attain their full growth, 

 while the largest are those in the upper tier of the disc ; and 

 those which have not yet completed their growth are found 

 around the oviducal plates on the outside. The correctness of 

 these observations will be demonstrated by comparing the dif- 

 ferences of development exhibited in this region by the spines 



