256 THE APODID^E PART n 



accounts for the general resemblance of the latter to 

 one another. 



THE OSTRACODA. 



These animals occur in company with the Trilobites 

 in the very oldest fossilifcrous strata. Balfour sug- 

 gested that they may have had an origin independent 

 of that of the other Crustacea. As, however, we find 

 them possessing the bent intestine, clear traces of the 

 entosternite, the paired and the unpaired eye, we 

 must, according to our theory, deduce them from our 

 bent Annelid. 



There are two ways in which the origin of the 

 bivalve shell may be explained : either (i) that 

 shown in Fig. 57, where it arises simply by the 

 folding together of the horns of the crescent-shaped 

 ridge round the front of the head, or (2) when it arises 

 through the folding down of the two halves of a 

 dorsal shell such as that in Apus. These two 

 methods are quite distinct ; the former bends the 

 dorsal integument of the head-shield alone along the 

 middle line, the latter bends only the dorsal shield as 

 far as its junction with the body. There is, however, a 

 method of combining these two modifications if, after a 

 dorsal shield has been developed, both the head-shield 

 and the dorsal shield are bent. 



We were at first inclined to attribute only the first 

 method of origin to the bivalve shells of the Ostracoda, 

 and to deduce them from some such form as Harpes 

 ungula. It would be an obvious advantage to an animal 

 given to the habit of rolling up for defence, to be able to 



