128 



THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



showing the first maxilliped. In the first to fourth zoex the limb is biramous but in the 

 glaucothoe a pair of broad processes grow out from the protopodite, while the exopodite and 

 particularly the endopodite become greatly reduced. In the adult the endopodite is a mere 

 vestige, while the flat outgrowths from the protopodite have become very large and bear 



setse. 



Summary. 



The limbs of most Crustacea are readily explained as modifications of a simple bira- 

 mous type. These modifications usually take the form of reduction by the loss or fusion of 

 segments and quite generally either the entire endopodite or exopodite is lacking. Modifi- 

 cation by addition frequently occurs in the growth of epipodites, "endites," and "exites" 

 from the coxopodite, basipodite, or both. A protopodite is generally formed by the fusion 

 of coxopodite and basipodite, accompanied by a transference of the proximal end of the 

 exopodite to the distal end of the basipodite. A new segment, not known in the trilobites 

 (precoxal), is sometimes added at the inner end. 



Among modern Crustacea, the anterior cephalic appendages and thoracic appendages of 

 the Copepoda and the thoracic appendages of certain Malacostraca, Syncarida especially, 

 are most nearly like those of the trilobite. The exact homology, segment for segment, be- 

 tween the walking legs of the trilobite and those of many of the Malacostraca, even the 

 Decapoda, is a striking instance of retention of primitive characteristics in a specialized 

 group, comparable to the retention of primitive appendages in man. 



NUMBER OF SEGMENTS IN THE TRUNK. 



Various attempts have been made to show that despite the great variability, trilobites 

 do show a tendency toward a definite number of segments in the body. 



Emmrich (1839), noting that those trilobites which had a long thorax usually had 

 a short pygidium, and that the reverse also held true, formulated the law that the number 

 of segments in the trunk was constant (20 + 0- Very numerous exceptions to this law 

 were, however, soon discovered, and while the condition of those with less than twenty-one 

 segments was easily explained, the increasing numl>er of those with more than twenty-one 

 soon brought the idea into total disrepute. 



Quenstedt (1837) had considered the number of segments of at least specific impor- 

 tance, and both he and Burmeister (1843) considered that the number of segments in the 

 thorax must be the same for all members of a genus. As first shown by Barrande (1852. 

 p. 191 et seq.), there are very many genera in which there is considerable variation in the 

 number of thoracic segments, and a few examples can be cited in which there is variation 

 within a species, or at least in very closely related species. 



Carpenter (1903, p. 333) has tabulated the number of trunk segments of such trilo- 

 bites as were listed by Zittel in 1887 and finds a steady increase throughout the Palaeozoic. 

 His table, which follows, is, however, based upon very few genera. 



