IO Invertebrata. 



like the cords into which the embryonic blood-vessels 

 in mammalia degenerate. Such organs are known as 

 provisional organs. In the development of some ani- 

 mals, parts appear which never perform any function 

 and which either vanish or remain permanently as 

 rudimental organs. They are always such as in some 

 kindred form discharge an important duty. Thus em- 

 bryo cuttle-fishes (fig. 53) have a dorsal groove which 

 closes in and forms the cavity wherein the internal 

 shell is secreted; but 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 the majority of the ani- 

 mals of the higher sub-kingdoms exhibit examples of 

 provisional and rudimental organs. 



Characters Essential and Adaptive. The cha- 

 racters presented by each animal may be divided into 

 such as have been inherited from its ancestors, and 

 such as have been produced in the course of its own 

 life by its surroundings. The former are essential, 

 the latter adaptive. If the environments remain the 

 same the adaptive characters of one generation be- 

 come essential in future races. Such induced cha- 

 racters may so modify the animal as to obscure its 

 affinities ; but these difficulties are removed by .a 

 study of its embryogeny. Thus among the parasitic 

 mites of the genus Pentastomum (p. 109), we could 

 not know the true relations of the worm-like adults 

 if we were not acquainted with the limb-beaiing 

 larvae. 



Classification and Nomenclature. The animal 

 kingdom is a vast assemblage of individuals, and 

 we require to arrange these in larger categories for 



