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THE AMERICAN NATURALIST [Vol. LIV 



has frequently been objected to the trilobites as ancestors 

 of the Crustacea, that they had wide pleural extensions 

 and a large pygidium. To the first it may now be replied 

 that some trilobites did get rid of the pleural extensions, 

 but that, on the other hand, most Crustacea retain some 

 remnants of them. I have already shown above that the 

 large pygidium in trilobites was more primitive than the 

 trunk with numerous free segments, and it may further 

 be pointed out that some orders of Isopoda do have a 

 pygidium. 



Akachnida 



My task in this class is rendered somewhat easier by 

 the fact that the followers of Lankester appear to have 

 accepted his explanation of the descent of the class from 

 the trilobites. While I agree with the general thesis, I 

 must point out that a certain amount of caution must be 

 used, for the connecting links are not nearly so satisfac- 

 tory as one would like them to be, and the trilobites are 

 not nearly so closely related even to the Merostomata, as 

 they are to the higher Crustacea. 



In the first place, while the Trilobita were probably 

 the ancestors of the Arachnida, they do not themselves 

 belong to that class. Lankester advanced six reasons for 

 placing them in the Arachnida, but the first is the only 

 one having any considerable weight, and is the only one 

 which will be discussed here. This point was that they 

 had only one pair, apart from the eyes, of pre-oral ap- 

 pendages, while the Crustacea have two pairs. Re- 

 searches since Lankester 's article was written seem to 

 show that this apparent difference between the Arach- 

 nida and Crustacea is not fundamental, for the chelicerae 

 of the former are probably to be homologized with the 

 antennae, not the antennules of the latter, so that the 

 mouth is in the same position in relation to the append- 

 ages in both groups. Further, the mouth does not occupy 

 a constant position in the trilobites, but with the elonga- 

 tion of the hypostoma, is pushed backward, so that from 

 one to four pairs of appendages may be attached in front 

 of it. 



