SECT. XIII THE TRILOBITES 219 



posterior end of the body of Apiis ; our explanation 

 of the one also explains the other. 



We find, in fact, almost the same as we find in Apus, 

 that the posterior segments remain in an undeveloped 

 or larval condition ; although the gradual tapering 

 away and diminution in length of the segments is not 

 visible in all species, yet where it is no longer visible 

 it must be assumed to have secondarily disappeared. 

 In some cases these rudimentary segments develop 

 sufficiently to hinge upon one another and to bend 

 in the sagittal plane, or perhaps the bending may 

 have been effected as in Apus by the development 

 of rings which do not correspond with true seg- 

 ments. In very many cases, however, the segments 

 are so rudimentary that they are unable to bend upon 

 one another, and hence together form the stiff plate 

 under discussion — the pygidium (see Fig. 50). We 

 thus deduce the pygidium not strictly from fused 

 segments but from segments too rudimentary to bend 

 upon one another. 



' It has been noticed as a somewhat remarkable fact 

 that the trunk segments appear after the pygidium, 

 the young larva consisting of the head and the pygi- 

 dium, and between these two the thoracic segments 

 are gradually interposed. This is a most interesting 

 case of the shifting back on to the larva of important 

 characteristics. The pygidium, being probably useful 

 in the rolling up of the larva, is thus very early deve- 

 loped, and is then analogous to the anal segment in 

 the Trochophora larva, although morphologically it 

 is composed of a number of rudimentary segments. 



