210 TOWARDS THE HUMAN FORM 



have previously enumerated their various metamorphoses, and 

 presaged their great future. 



The evolution of the Arthropods can be outlined very 

 simply. We have already called attention to Peripatus, 

 that ambiguous creature tossed from Worms back to 

 Arthropods, which even to-day still appears to be 

 distributed along the edge of the old Gondwana continent. 

 All the segments of its body are alike, except three : the 

 first, bearing the tactile appendages comparable to antennae ; 

 the second, bearing the mouth ; and the third a pair of 

 appendages directed towards the mouth, and provided with claws 

 that serve as mandibles, incorporated with the cephalic region 

 through the formation of a kind of lip rising behind them 

 and united to the corners of the mouth, thus enclosing them in 

 a kind of buccal cavity. Similarly among the Arthropods, 

 the locomotor appendages are successively put to the use of 

 mastication in varying numbers and different ways. The first 

 stages in this adaptation are still unknown, but the Algonkian, 

 which has already revealed so many traces of the Arthropods, 

 may perhaps some day give us the desired information. 



In the Cambrian Period we find ourselves in the presence 

 of fairly advanced adaptations. In one group the five 

 (Eurypteridae) or six first feet (Limulus) differ but little from 

 the locomotor appendages, or, at times, even preserve this 

 character, but they surround the mouth and from the base arises 

 a laminated process which assists in mastication. The first 

 pair in Pterygotus resembled pincers, and this creature during 

 the Devonian Period acquired a length of two and a half metres. 1 

 This first pair resembled the appendages of Eurypterus, and in 

 both these genera the fifth pair, very large and flattened, have 

 become a swimming organ. In Limulus, which already existed 

 in Silurian times, and to-day are to be found in Mouccan 

 and Japanese waters and along the two sides of the Isthmus 

 of Panama, the first five pairs of legs terminate as pincers, the 

 first being smaller than the others, and the extremity of the 

 last pair is furnished with complicated appendages that, how- 

 ever, hardly modify their appearance. All the segments 

 provided with appendages are united in a large cuirass bearing 

 the eyes, which, strictly speaking, might be considered as a kind 



1 Pterygotus anglicus. 



