SECT, xii RELATION OF APUS TO CRUSTACEA 197 



surface required, in consequence of the suppression of 

 the gills on the anterior limbs, is obtained by a com- 

 plicated increase of the surface of the gills on the 

 other limbs, and of the gills only. 



We have, then, compared the appendages of Limu- 

 lus with those of Apus, and shown how they throw 

 light upon one another, how. they are both deduciblc, 

 some along one line of special differentiation, some 

 along another, from the parapodia of our original 

 Crustacean-Annelid. The type is clearly the same 

 in both, as is also the type of the whole organisation 

 of the two animals. 



The Musculature. The musculature of Limulus is 

 very specialised, in accordance with the specialisation 

 of the outer body and the high development of 

 the exoskeleton. We will not here venture on the 

 task of endeavouring to trace the separate muscles 

 from their Annelidan origin. In all such attempts, 

 the limitation of the movements of the body must 

 be borne in mind. The fact that the body of Limulus 

 is no longer capable of any bend, except in the 

 sagittal plane, would alone bring about very profound 

 changes in the musculature, which originally moved 

 the Annelid freely in any plane. 



It must here suffice to refer to what was said above 

 (p. 184) about the sinewy mass found within the bend 

 of the intestine for the attachment of the muscles. 

 For the origin of this mass from the Annelidan ven- 

 tral muscle bands we refer to the derivation given in 

 Part I. of the similar mass found in the same place in 

 Apus. And further we can point to the entapophyses 



