THE EVIDENCE FROM DEVELOPMENT. 213 



possesses no addenda or belongings save bristle-like processes attached 

 to its broad and divided extremity. 



In 1778 there was figured by a Dutch naturalist a new form of 

 Crustacean which was met with in 1822 in large numbers in the Cove 

 of Cork by Mr. Vaughan Thompson. These beings were referred to a 

 genus Zoea, which had been constructed for their reception. Later re- 

 search, however, showed that theZoeas were merely the young or larval 

 crabs, just described, and the further development of the Zoeas was 

 in due course satisfactorily traced. For, after repeated moults, the 

 Zoea becomes the Megalopa (Fig. 130, b). Its body has now assumed a 

 shape distantly resembling that of the mature crab, and its five pairs 

 of walking legs are well developed. It possesses, however, an 

 appendage, unknown in the adult crab, in the shape of a jointed tail 

 provided with appendages ; and as the Megalopa, the crab bears a 

 very decided resemblance to one of its tailed neighbours, such as the 

 hermit-crab, lobster or shrimp. Ultimately the body widens, after 

 further moultings ; the tail decreases in size, loses its appendages, and 

 becomes tucked up under the body, to form the characteristic little 

 " purse " of the adult crab ; and, finally, with the proportional growth 

 and development of other regions and parts, the features of adult 

 crab-life (Fig. 130, c) are duly produced. Thus a crab's body really 

 consists of a greatly broadened head and chest, and the jointed tail 

 we see in the lobster or shrimp is represented in mature crab- 

 existence by the little appendage or " purse," which, on examination, 

 will be found to bear rudiments of the tail-appendages so typically 

 developed in the long-tailed neighbours of 

 the crab. It likewise becomes clear from 

 the foregoing life-history, that the crabs, 

 in respect of the modification and dis- 

 appearance of their tail, are a later and 

 higher race than the lobsters, shrimps, 

 and prawns. And geology confirms this 

 surmise, inasmuch as the lobster-races 

 were developed ages before the crabs. . FIG. 131. MYSIS. 

 Fossil kith and kin of the lobsters occur 



very early in the stratified rocks, the crabs being late productions ; 

 so that the idea of the crabs having originated from a tailed Zoea-like 

 or lobster-like race is fully supported by the best of evidence. 



The concluding life-histories which may be glanced at, by way of 

 summarising the ways of the crustacean evolution, are those of the 

 Mysis or opossum-shrimps (Fig. 131), and a peculiar genus of prawns 

 known as Penceus (Fig. 132). The first-mentioned animals are common 

 in the lakes of modern Europe and of North America, and also flourish 

 in the Arctic Seas. It is a warrantable inference that the Mysis 

 relicta of the lakes, is simply a variety of the Mysis oculata of the 



