458 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



The oldest fishes had a bony covering of extreme hardness." But 

 " the animal which is shut in a fortress or in a coat of mail is con- 

 demned to an existence of half-sleep. It is in this torpor that the 

 Echinoderms and even the mollusks are living to-day. The Arthro- 

 pods and Vertebrates escaped from it, and on this happy circum- 

 stance depends the present development of the highest forms of life. 



"In two directions, indeed, do we see the impulse of active life 

 regaining the upper hand. The fishes exchange their ganoid armor 

 for scales. Long before them the insects had made their appearance, 

 having also rid themselves of the armor that once protected their 

 ancestors. In both groups the inefficiency of the protective envelop 

 was compensated for by a nimbleness that enabled them to escape 

 their enemies and also to take the offensive and to select the place 

 and time of the encounter." 



These remarks rest on a solid foundation, but they should be 

 modified in one particular which is of paramount importance in the 

 explanation of the structure and the special psychology of the Ar- 

 ticulates. These animals have never lost the chitinous armor that 

 protected their primitive ancestors. They have preserved it in its 

 entirety and with greater or less thickness. Coleoptera, crabs, scor- 

 pions, and thousand-legs of our times are by no means inferior in 

 this regard to the ancient forms from which they are descended. 



As a matter of fact they are covered to-day as in times of yore, 

 with an external skeleton of chitin. That is why Edmund Perrier, 

 in his desire to emphasize their dominant character, has called them 

 Chitinophores. To escape imprisonment within their protective en- 

 velope, to acquire the flexibility and mobility necessary to their evo- 

 lution, they underwent certain superficial modifications. These con- 

 sisted in the division of the armor into several pieces by means of 

 articular lines, along which the chitin is less thick than elsewhere, 

 thus allowing the pieces to move one upon the other. This is the 

 very way in which they became Articulates, at once acquiring agility 

 without losing their protective cover. Naturally such joints were 

 formed wherever the several segments, arranged in a row and con- 

 stituting the body of the animal, came together. As a result, these 

 segments acquire a certain independence and their uniformity is to 

 a certain extent preserved. Indeed, we see that many Articulates 

 possess a pair of appendages on each segment (Myriapods and the 

 majority of Crustaceans) and that the insects most remote in this 

 regard from the primitive types are still provided with seven pairs 

 of appendages (one pair of antennae, three pairs of buccal append- 

 ages and three pairs of legs) not to speak of the modified or rudi- 

 mentary organs to be seen on the different parts of the abdomen. 

 And the chitinous envelope of these appendages has broken into 



