236 THE APODID^ PART ii 



This development of great multitudes of armoured 

 browsing carnivorous Annelids in the palaeozoic seas, 

 supplies us with abundant matter for biological specu- 

 lation. It was perhaps in defence against these 

 powerful marauders that so many Ccelenterata per- 

 fected their nematocysts or stinging cells, that 'the 

 Corals built their stony ramparts, and that many of 

 the Mollusca developed their shells. It may indeed 

 have been the perfection of these defences which 

 led to the dying out (with the exception of Limulus) 

 of these early Crustacea, especially of the giant forms. 

 Whatever the cause, all except Limulus, the Ostra- 

 coda, and the Apodidse (looked upon as the 

 racial form of all other existing Crustacea) gradually 

 died out. 



The first and the last of these still fortunately retain 

 the clearest traces of their origin, and, more or less 

 modified, the browsing habit of life. 



