10 



Invertebrate* 



FIG. 5. 



the octopus or sea-spider, a closely allied form, has a 

 similar groove which vanishes, leaving no trace behind. 

 Instances of the kind might be multiplied, as there is 

 scarcely an individual form in the higher sub-kingdoms 

 which does not in its life history exhibit instances of 

 provisional and rudimental organs. 



Characters Essential and Adaptive. In each 

 animal we can divide the characters into two groups, 



essential and adaptive; 

 the former of these are 

 those whereby we can 

 learn the relations of 

 animals among them- 

 selves, and these are of 

 primary importance in 

 classification ; the latter 

 show the relations of 

 the animal to its sur- 

 roundings, but these in 

 the adult often so overlie 

 the essential characters 

 as to obscure them. A study of the embryogeny of 

 the animal will enable us to understand its relation- 

 ship, for the adaptive characters are of later origin 

 than the essential and may be traced as they are 

 becoming superinduced. Thus among the parasitic 

 mites of the genus Pentastoma, we could not know 

 the true relations of the worm-like adults if we were 

 not acquainted with the limb-bearing larva. 



Classification and Nomenclature. The animal 

 kingdom is a vast assemblage of individuals, and 

 we require to arrange these in larger categories for 



Adult form of Balanus porcatus. 



