J74 THE APODID^ PART u 



from our bent Annelid. And, secondly, we shall see 

 whether Apus forms a probable starting-point for the 

 modern Crustacea. In both cases we shall find that 

 our theory stands the test. We shall find that the 

 transformation of the carnivorous Annelids into Crus- 

 tacea did not result in only one form of primitive 

 Crustacean, but in several. It was, however, the same 

 Annelid, with the same number (five) of anterior 

 trunk segments bent round towards the ventral sur- 

 face, which gave rise to the whole class. 



The most important and apparently the most suc- 

 cessful modification in early times was the Trilobites, 

 that is, if we may judge from the extraordinary num- 

 bers and varied development of these early Crus- 

 tacea in palaeozoic times. They, however, all died 

 out, leaving, perhaps as their sole modern represen- 

 tatives, some families of the Ostracoda. 



Other modifications of the original Crustacean- 

 Annelid were the Eurypteridae and Xiphosuridae, to 

 which latter the still living king-crab belongs. 



All these groups, however well adapted to their 

 palaeozoic surroundings, have, with the exception of 

 the last-named (and the Ostracoda, which we think 

 may have come direct from the Trilobites), entirely 

 disappeared, and it was the Apodidae which became 

 (with the above exceptions) the sole ancestors of 

 the now living Crustacea, surviving mainly, we think, 

 on account of the advantages afforded by the develop- 

 ment of a dorsal shield. 



We have, then, to try to show first, that these Crus- 

 tacean forms arc dcducible, like Apus, from the bent 



