HOMOLOGIES. 209 



the present Comatulae and Star-Fishes with stems. 

 So is it often with the study of Nature ; many 

 scattered links are collected before the man comes 

 who sees the connection between them and 

 speaks the word that reconstructs the broken 

 chain. 



I will begin my comparison of all Echinoderms 

 with an analysis of the Star-Fishes and Sea- 

 Urchins, because I think I can best show the 

 identity of parts between them, notwithstanding 

 the difference in their external form ; the Sea- 

 Urchins having always a spherical body, while 

 the Star-Fishes are always star-shaped, though in 

 some the star is only hinted at, sketched out, as 

 it were, in a simply pentagonal outline, while in 

 others the indentations between the rays are very 

 deep, and the rays themselves so intricate in their 

 ramifications as to be broken up into a complete 

 net-work of branches. But under all this vari- 

 ety of outline, our problem remains always the 

 same : to build with the same number of pieces 

 a star and a sphere, having the liberty, however, 

 of cutting the pieces differently and changing 

 their relative proportions. Let us take first the 

 Sea-Urchin and examine in detail all parts of its 

 external structure. I shall say nothing of the 

 internal structure of any of these animals, be- 

 cause it does not affect the comparison of their 

 different forms and the external arrangement 



