350 PROCEEDINGS OP THE AMERICAN ACADEMY. 



of some ancestral Annelid worm. The cylindrical rowing appendages 

 of the Nauplius appear to be secondary and adaptive characters, fitting 

 it for its free-swimming surface life. 



The Zoea larva of the Decapoda, with its body composed of head and 

 abdomen alone, without thoracic segments and appendages, is also an 

 adaptive stage, a differentiated or farther advanced Nauplius. 



The Megalops stage of the Brachyura, or crabs, induced at the end 

 of the free-swimming life of the zoea, and intermediate between the 

 zoea and crab stage, the thorax and thoracic appendages being present, 

 is likewise an adaptation to the transition period connecting the free- 

 swimming or surface life and the creeping and bottom life of the 

 adult. 



These different stages, the result of adaptation, are signal examples 

 of the inheritance of characters at corresponding periods of life, and 

 would appear to have beeu originally the result of the inheritance of 

 characters originated or acquired during the life of the individual ; 

 i. e. the ancestor of the existing decapodous Crustacea. 



On the other hand, in groups where a metamorphosis is the rule, 

 there are exceptional forms in which development is abbreviated or 

 direct. Such cases are the direct development of the starfish, Lepty- 

 chaster kerguelenensis Smith, while Pteraster militaris is viviparous ; and 

 the direct development of Anochanus sinensis and of Hemiaster caver- 

 nosas among Echinoids. The lobster and crayfish are exceptions to 

 other macrurous Crustacea, in which there is a complicated metamor- 

 phosis, their development being condensed or abbreviated, and limited 

 to embryonic life. So with the crabs of several species living in the 

 Black Sea, whose direct development was traced by Rathke. 



In such exceptional cases as these, the phenomenon of direct de- 

 velopment will undoubtedly be found to be caused by some change in 

 the conditions of existence. 



Attention should also be here drawn to the fact that the term con- 

 genital is an elastic one. Mammals, at least the placental ones, are only 

 born after a long uterine life, but fish and tadpoles, as well as the higher 

 nesting birds, are born in a more premature condition, and congenital 

 characters in these animals have quite a different significance from 

 those of mammals. So it is in a less degree with the larvae of insects, 

 the degree of inequality in the perfection of the larva being very great 

 in different groups, especially in the parasitic Hymenoptera and 

 Coleoptera. 



