26 



EVOLUTION IN THE PAST 



POD-SHRIMPS 



OSTRACODS 



passed away ; but many new genera had appeared. The new 

 forms, with few exceptions, were endowed with eyesight, and 

 were more broadly developed in the region of the tail {Asaphus, 

 Trinucleus, Ceraurus, etc.). The power to roll up in wood- 

 lice fashion was also more common (Calymmene) ; but this 

 mode of meeting dangers was not suggestive of a grand 

 future. 



Numerous " pod-shrimps " were disporting in the waters, 

 and of larger size than their Cambrian ancestors {Ceratio- 

 caris). 



Ostracods, some dwelling in rock-pools, others careering 

 in large companies about the deep, were in a vigorous con- 

 dition. In fact, their shells were never so varied at any other 

 time (Beyrichia, Bairdia, etc.). They were mostly minute 

 animals, but some monsters — nearly an inch in length — were 

 among them (Leper ditiidce). In some cases — ^in the style of 

 lampshells and bivalved molluscs — hinges were developed 

 on the tiny sheU ; and in some shells there was a small 

 perforation through which the antennae could be thrust 

 without disturbing the valves (Cypridina). 



Attached to rocks, sheUs, and seaweeds, barnacles ex- 

 hibited their peculiar structure in various stages of develop- 

 ment. These animals had early departed from strict crusta- 

 ceans, and were fast becoming an isolated group, leading 

 lives similar to those of rock-clinging moUuscs. Some were 

 protected by loosely-arranged shelly plates (Lepidocoleus) ; 

 whilst others were more or less completely encased in plates 

 closely set and overlapping (Turrilepas). None of these 

 barnacles possessed stalks for clinging to their resting-places. 

 They may, indeed, have been capable of locomotion, but 

 probably they did not much indulge in this exercise. Others 

 had stalks of a rude, primitive kind (Polliceps) ; and in a few 

 forms this organ was weU developed (Scalpellum). 



There is no sign of small crustaceans with affinities to 

 SCORPIONS trilobites and king-crabs, such as had appeared in the Cam- 

 brian. But the small scorpion-like animals (Eurypterids) 

 were on the increase {Echinognathus, etc.). Sea-scorpions, 

 so-caUed, did not, however, make their presence greatly felt 

 in Ordovician times. Their descendants, as will be seen, 



BARNACLES 



SEA- 



