52 PRIMITIVE ANIMALS 



numerous primitive features, some of which it 

 possesses in common with the Entomostraca. It is 

 interesting to observe that Nebalia, or very closely 

 allied genera, are among the very oldest fossils known, 

 occurring in Cambrian times. It is usually the case 

 that such very ancient animals, which have come down 

 to us practically unchanged from the remotest ages, 

 are rare or at any rate local in their distribution, but 

 Nebalia is a very striking instance to the contrary. 



The other Crustacean type with which we have to 

 deal, Anaspides, a peculiar fresh-water shrimp, is not 

 known to be as ancient as Nebalia, though fossils 

 practically identical with it are fairly common in 

 Permian and Carboniferous formations. The surviving 

 Anaspides and its allies (Fig. 10), unlike Nebalia, 

 are extremely localized animals, occurring only in a 

 few circumscribed localities in the fresh-waters of 

 Tasmania and Southern Australia. The fossil forms, 

 some of which are almost identical with the living 

 Anaspides, inhabited the sea and are found in several 

 localities in Carboniferous formations in England, 

 Central Europe and North America. It was evidently 

 a dominant form of shrimp at the time the Coal- 

 measures were being deposited, but it is not met with 

 in more recent formations, and we have no clue as 

 to its history between the time that it peopled the 

 Carboniferous seas and the present day when it 

 survives only in a few tarns and streams on isolated 



