STRUCTURE AND CLASSIFICATION OF THE ARTHKOTODA. 541 



not the place in which to attempt an outline of the special 

 laws of vertebrate metamerisui. Fusion of adjacent somites 

 has often been erroneously interpreted in the study of 

 Arthropoda. There are^ in fact, very varying degrees of 

 fusion which need to be carefully distinguished. The fol- 

 lowing generalisation may be formulated : — " The homologous 

 meromes of two or more adjacent somites tend to fuse with 

 one another by a blending of their substance. Very gene- 

 rally, but not invariably, the fused meromes are found as 

 distinct separated structures in the embryo of the animal 

 in which they unite at a later stage of growth." The fusion 

 of neighbouring meromes is often preceded by more or less 

 extensive atrophy of the somites concerned, and by a.rrest of 

 development in the individual ontogeny.^ Thus a case of 

 fusion of partially atrophied somites may simulate the ap- 

 pearance of incipient merogeuesis or formation of new 

 somites; and vice versa, incipient merogenesis may be 

 misinterpreted as a case of fusion of once separate and 

 fully formed somites. Moreover the two phenomena, mero- 

 genesis and fusion of meromes, actually occur side by side 

 in some cases, as in the pygidial shields of the Trilobita) and 

 Limulus. 



The most commonly noted cases of fusion of metameres 

 are simply cases of the fusion of the tegumentary meromes — 

 usually the terga only. Such a fusion has really no very 

 serious morphological importance : it is superficial and 

 readily acquired. It amounts to no more than the disposition 

 of chitinous cuticle of equal thickness over the area of the 

 terga of the somites concerned instead of the thinning of the 

 cuticular deposit at the adjacent borders of the somites. 

 The somites consequently lose their hinge; they can no 

 longer be flexed one on the other. Atrophy of the muscles 

 related to such flexure necessarily follows. The mesosomatic 

 portion of the posterior carapace of Limulus is no more than 

 such a superficial fusion : the other meromes of the auky- 

 losed somites (appendages, ueuromeres, blood-vessels, etc.) 

 are unaffected. Such, too, is the case with the pygidial 



