NO. 10 



CRUSTACEAN METAMORPHOSES — SNODGRASS 



By a different type of specialization for locomotion, members of 

 another branch from the ancestral stock developed ventrolateral, 

 lobelike outgrowths of the body segments, and thus became walking 

 animals. These primitive legs eventually evolved into the jointed 

 appendages of modern arthropods, the lobelike origin of which is still 

 recapitulated in the embryo. At the lobopod stage of evolution 

 (fig.i A) the animals resembled a modern onychophoran, and are 



Fig. I. — Theoretical evolutionary stages of the arthropods. 



A, a primitive lobopod, common ancestral form of the Onychophora and 

 Arthropoda. B, a derived form with longer and slenderer legs. C, a primitive 

 arthropod with sclerotized integument, jointed legs, and gill lobes on the coxae. 

 D, a fairly generalized modern crustacean, Anaspides tasnianiae. 



I Ant, first antenna; 2Ant, second antenna; Mxpd, maxilliped; PIpds, pleo- 

 pods; Prpds, pereiopods; Tel, telson; Urpd, uropod; II-XVIII, body segments. 



perhaps represented by such fossils as the Pre-Cambrian Xcnusion 

 and the Cambrian Aysheaia. The modern Onychophora are probably 

 direct descendants from these early lobopods, and have structurally 

 not progressed much beyond them. Others, however, acquired a 

 sclerotization of the integument, which allowed the legs to become 

 longer and slenderer (B), and finally jointed (C) for more efficient 

 action in locomotion. These jointed-legged forms were the first true 

 arthropods. The segmentation of the legs early took on a definite 

 pattern, which has been preserved in both fossil and living arthropods, 

 most of which retained the walking mode of locomotion, though 

 some may also swim or fly. 



