SECT. II THE OUTER BODY FORM 15 



In the larva of Euphausia they develop early as two 

 limb-like projections posterior to the mandibles, 

 and strongly resemble those of Apus. 



In Apus, only the mandibles work between the 

 labrum on the one hand and the under lip on the 

 other. In Limulus, however, where the under lip also 

 consists of two projections, the mouth is so stretched 

 in the longitudinal direction that the masticatory 

 ridges of five pairs of limbs work as jaws between 

 them and the labrum. That these labial projections 

 in Apus and Limulus are really homologous with one 

 another, and with the under lip of our original 

 Annelid, will be seen to follow as a necessary con- 

 sequence of our explanation of their morphology. 1 



The shield is of great size, and stretches back (as a 

 fold of the fifth segment) over the greater part of the 

 body. Laterally it covers and protects the limbs. 

 Posteriorly, it is armed with thorns, and has a keel 

 along the dorsal middle line due no doubt to the 

 central thorn which it supports. The carrying of 

 these thorns, which are now so slightly developed, 

 may have originally been the chief function of the 

 shield in its early stages ; Fig. 48, page 215, in con- 

 nection with which the origin of the shield will be 

 discussed more in detail, illustrates what we imagine 

 to have been the first step in the formation of the 

 dorsal shield. This function has, however, long given 

 place to that of protective covering of the whole dorsal 

 surface and (laterally) of the gills. The lateral edges 

 of the shield are prolonged into the ridge which, run- 



1 Cf- PP. 39> 4> and 194, also Fig. 43, p. iSS. 



