SECT, xi DEVELOPMENT 161 



3. The Dorsal SJiield. As a dorsal shield is present 

 in most Nauplii, it has been generally concluded that 

 the original Crustacean possessed such a protective 

 covering. When we come to discuss the relation 

 between Apus and the Trilobites we shall find that 

 this was by no means the case. Only in so far as 

 Apus is the original of all living Crustacea (excepting, 

 perhaps, the Ostracoda 1 ) can it be said with truth 

 that the racial form possessed a dorsal shield, at least 

 as a fold of the fifth segment. The shield of the 

 original Crustacean-Annelid was itself a different 

 structure. From it not only the shell fold of Apus, 

 but also the different forms of bivalve shell have been 

 developed, as will be described in detail on pp. 213-216. 

 Again, a further false conclusion has often been 

 drawn from the great size of the shield in the 

 Nauplius, viz., that it must have been of about the 

 same size, relatively, in the original Crustacean as in 

 the Nauplius. Hence it has been concluded that, for 

 instance, the Estheridae, which have a large dorsal 

 shield, are more primitive than the Apodidae with 

 their relatively smaller shield. The great size of the 

 shield in the Nauplius, however, admits of a much 

 simpler explanation. The shield is, as we have seen, 

 a dorsal fold of the fifth segment. Hence, in larvae 

 in which only the first five or six segments are 

 developed, it must necessarily appear relatively of 

 very great size. 



1 Our reason for excluding at least some of the Ostracoda from 

 the other modern Crustacea which we deduce from Apus will 1>e 

 discussed in a special section of Part II. 



M 



