14 COMPARATIVE ANATOMY. 



The early condition of the embryo itself may determine 

 the position of the animal. Thus in contrasting the early 

 stage of the developing egg of an articulate with that of a 

 vertebrate animal, it can be ascertained before the appear- 

 ance of rings on one hand, or chorda dorsalis on the other, 

 to which type the animal belongs. In the first, the embryo 

 presents serrated borders, and is without a median groove; 

 in the second, there is a median grove, but no lateral ser- 

 rations. 



Apart from the application of the general law (p. 1 3), the 

 inferiority of an animal within its group can be determined 

 either (1) by the repetition of parts, or (2) by the permanency 

 of embryonic conditions. (1) The tentacles and partitions of 

 Actinia are indefinitely repeated throughout life; they are 

 reduced to a fixed number eight in Alcyonium: therefore 

 Actinia is a lower form of polyp than Alcyonium. In like 

 manner Gammarus (fresh-water shrimp), a crustacean articu- 

 late, from the repetition of its segments, is a type inferior 

 to Lupa dicantha (crab), where they are consolidated toward 

 ithe anterior extremity of the animal. (2) Upon observing 

 .among acaleph radiates that the genus Hydra corresponds 

 in form to the first larval change of the Aurelia (SQQ p. 126), 

 Hydra is said to be a lower form than Aurelia. In perceiving 

 that a canal of communication between an arterial and venous 

 .current (foramen of Botal) obtains in the systemic circulation 

 of the reptile, while such a condition is peculiar to the foetal 

 circulation of the mammal, the reptilian type is said to be 

 lower than the mammalian. 



When the relations of closely allied animals are sought 

 for, the characters are selected not so much from their phy- 

 siological importance as from their constancy. Indeed, the 

 most important vital organs with some groups of animals 

 may furnish no characters for classification as, for example, 

 the heart in insects; while others having no apparent sig- 

 nificance, such as minute differences in form and number of 

 joints of antennae, may be of the greatest value. 



Systems of classification change with increase of knowl- 

 edge, and, all things being equal, the latest is the best. But 



