SECT. XTII THE TRILOBITES 217 



the latter the posterior development of the neck-lobe 

 as shown in Fig. 48. 



(II.) T/ie Trunk Segments. The greatest difficulty 

 in homologising these segments with Annelidan seg- 

 ments is that we find the crescent-shaped head 

 followed by segments repeating, in their pleura, the 

 form of the head, whereas at first sight we should 

 expect the head to be followed by a row of Annelidan 

 segments as in Apus, i.e. a continuation of the gla- 

 bella alone, as was no doubt originally the case. The 

 gradual acquisition on the part of the trunk segments 

 of their highly developed pleura repeating the charac- 

 teristics of the wings of the head is probably to be 

 explained as follows: As soon as the typical Trilobite 

 head-shield became an important factor in the struggle 

 of each species for existence, it would tend to appear 

 earlier and earlier in the larva ; the Trilobite Nauplius 

 would then be little more than a generalised Trilobite 

 head with an anal segment. Between these two parts 

 the segments were gradually differentiated, so that 

 the characteristics of the head might very well make 

 themselves felt in the development of the segments, 

 and in this way spread gradually backwards to the 

 posterior end of the body. This, indeed, we find to 

 be the rule in many Trilobites ; the most specialised 

 segments are immediately behind the head, while 

 posteriorly they are more and more simple. In this 

 way then the segments of the Annelidan trunk were 

 gradually provided with the pleura characteristic of 

 the Trilobites ; their pleura being segmental repe- 

 titions of the lateral projections of the head-shield. 



