SECT. XIII THE TRILOBITES 



now know that the Trilobitcs had ambulatory limbs 

 (see Fig. 51). The two sorts of limbs are, as we 

 shall see, but different modifications of the Anne- 

 lidan parapodium. 



The Annelidan character of the outer form of the 

 Trilobites is not so much disguised as at first sight it 

 seems to be. But for its large head-shield it might 

 well have passed for a flattened Annelid. Anteriorly 

 we have the crescent-shaped head, followed by a 

 variable number of movable segments, and poste- 

 riorly a number of more or less rudimentary seg- 

 ments, often fused together to form a tail-plate. 



Taking the three parts separately, and deducing 

 them from our primitive Crustacean-Annelid, we shall 

 find that much light is thrown upon many hitherto 

 obscure points in their organisation. 



(I.) The Head. — The Trilobite head is composed of 

 the five anterior segments of our Annelid, bent round 

 so that the mouth opens ventrally and faces poste- 

 riorly, as described for Apus. The large labrum was 

 originally the prostomium of the Annelid. Fig. 46 

 is a longitudinal section through a Trilobite, which 

 we had not seen till the first part of this book was in 

 j\IS., and which afforded a most unexpected confir- 

 mation of our argument. Anteriorly and dorsally 

 the bending of the soft cylindrical body gives rise to 

 the glabella, as the characteristic swelling in the 

 median line of the Trilobite head is called. This is 

 the convex surface of the bent Annelidan body, and 

 is retained only in the Trilobites. In Apus it is 

 completely disguised by the growing together of the 



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