DISCUSSION OF METHODS OF STUDY. 231 



panying figures interambulacral plate 1' is the equivalent of Professor 

 Loven's plate 1, and plates 1 and 2 are the equivalents of his plates 2. He 

 does not figure in his " Etudes " any genera with more than two columns 

 of plates, and therefore no further comparison with his notation needs 

 to be made. 



The later or postembryonic stages in growth in animals are most impor- 

 tant to study, from the light they throw on morphology and the ontogeny 

 of the individual, as well as the phylogeny of the group. This line of 

 investigation has yielded most fruitful results and needs no excuse. 

 There are two methods of studying stages in growth. One, and obvi- 

 ously the best, is to get actual young in progressively advancing stages 

 in growth. The other method, which has been employed in some groups, 

 is to ascertain the stages through which an animal has passed by study- 

 ing the form and lines of growth of relatively older or adult individuals. 

 This method of study has been employed in moUusks, brachiopods and 

 somewhat in corals. Good examples in which the condition of the 

 young may be ascertained by studying lines of growth are Nautilus, Ver- 

 metus, Hinnites and Lingula. In Crustacea which shed the hard cuta- 

 neous covering with advancing age, actual young must be had for a study 

 of the development. 



In echinoderms the conditions are somewhat complex, and vary in 

 different parts of the skeleton. Purely internal hard parts, as teeth, per- 

 pendicular supports between the upper and lower sides of the test, as in 

 Meoma, etcetera, must increase in size by a combined process of resorp- 

 tion and growth, such as we get ordinarily in the bones of a vertebrate, 

 and probably could not by any means be made to yield stages when 

 studied in an adult. The plates of the corona of an echinoderm are quite 

 a different matter, however. These plates, especially the interambulacral, 

 are found at quite definite positions in the corona, as shown in Melonites 

 (plate 2, figure 2) and other genera. While the actual form of the plates 

 may be modified by growth and sometimes by resorption, the existence 

 of the plate in itself is an important factor, even if it has sometimes suf- 

 fered alterations. Professor Loven has shown (27) from a study of actual 

 young, that certain plates of the corona, which make up the two inter- 

 ambulacral columns of modern Echini, occur and apparently originated 

 at stated intervals in the development of the individual,* and he com- 

 pares these young plates with plates found at the ventral border of adult 

 types, the Clypeastroids and Spatangoids, which have similar plates. It 



f *See Loven's figures in the " Echinologica" of Goniocidaris, Strongylocentrotus {reTprodnced in 

 this paper, figures 3 and 5, page 234, and plate 3, figures 8 and 9) ; also Echinus, and especially a 

 young Eehinoid on page 11 of his paper. In this last figure Lov6n shows that the corona at an 

 early stage consists of the first row of 10 ambulaeral plates, succeeded by the 5 initial plates of the 

 interambulacral areas, no other coronal plates apparently having yet appeared. 



