138 MARINE ANIMALS OF MASSACHUSETTS BAY. 



kingdom, although the growth of the young Echinoderm on 

 the Brachiolaria may at first sight remind us of the budding of 

 the little Medusa on the Hydroid stock, or even of the passage of 

 the insect larva into the chrysalis. But in both these instances, 

 the difierent phases of the development are entirely distinct ; the 

 Hydroid stock is permanent, continuing to live and grow and per- 

 form its share in the cycle of existence to which it belongs, after 

 the Medusa has parted from it to lead a separate life, or if the 

 latter remains attached to the parent stock, after it has entered 

 upon its own proper functions. The life of the caterpillar, 

 chrysalis and butterfly, is also distinct and definitely marked ; the 

 moment when the animal passes from one into the other cannot 

 be mistaken, although the difierent phases are carried on suc- 

 cessively and not simultaneously, as in the case of the Acalephs. 

 But in the Echinoderms, on the contrary, though the aspect of the 

 Brachiolaria, or plutean stage, is so difierent from that of the adult 

 form, that no one would suppose them to belong to the same ani- 

 mal, yet these two stages of growth pass so gradually into one 

 another, that one cannot say when the life of the larva ceases, and 

 that of the Echinoderm begins. 



The bearing of embryology upon classification is becoming 

 every day more important, rendering the processes of develop- 

 ment among animals one of the most interesting and instruc- 

 tive studies to which the naturalist can devote himself, in the 

 present state of his science. The accuracy of this test, not only 

 as explaining the relations between animals now living, but as 

 giving the clew to their connection with those of past times, can- 

 not but astonish any one who makes it the basis of his investiga- 

 tions. The comparison of embryo forms with fossil types is of 

 course difiicult, and must in many instances be incomplete, for 

 while, in the one case, death and decay have often half destroyed 

 the specimen, in the other, life has scarcely stamped itself in 

 legible characters on the new being. Yet, whenever such com- 

 parisons have been successfully carried out, the result is always 

 the same ; the present representatives of the fossil types recall in 

 their embryonic condition the ancient forms, and often explain 

 their true position in the animal kingdom. One of the most re- 

 markable examples of this in the type we are now considering, 



