PRIMITIVE CHARACTERISTICS OF TRILOBITES. 125 



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PRIMITIVE CHARACTERISTICS OF TRILOBITES. 

 TRILOBITES THE MOST PRIMITIVE ARTHROPODS. 



The Arthropods., to make the simplest possible definition, are invertebrate animals with 

 segmented body and appendages. The most primitive arthropod would appear to be one 

 composed of exactly similar segments bearing exactly similar appendages, the segments of 

 the appendages themselves all similar to one another. It is highly improbable that this most 

 primitive arthropod imaginable will ever be found, but after a survey of the whole phylum, 

 it appears that the simpler trilobites approximate it most closely. 



That the trilobites are primitive is evidenced by the facts that they have been placed 

 at the bottom of the Crustacea by all authors and claimed as the ancestors of that group by 

 some; that Lankester derived the Arachnida from them; and that Handlirsch has consid- 

 ered them the progenitors of the whole arthropodan phylum. 



Specializations among the Arthropoda, even among the free-living forms, are so numer- 

 ous that it would be difficult to make a complete list of them. In discussing the principal 

 groups, I have tried to show that the essential structures can be explained as inherited from 

 the Trilobita, changed in form by explainable modifications, and that new structures, not' 

 present in the Trilobita, are of such a nature that they might be acquired independently in 

 even unrelated groups. 



The chief objections to the derivation of the remainder of the Crustacea from the trilo- 

 bites have been: first, that the trilobites had broad pleural extensions; second, that they had 

 a large pygidium; and lastly, that they had only one pair of tactile antennae. 



It has now been pointed out that many modern Crustacea have pleural extensions, but 

 that they usually bend down at the sides of the body, and also that in the trilobites and more 

 especially in Marrella, there was a tendency toward the degeneration of the pleural lobes. 

 A glance at the Mesonacidse or Paradoxidae should be convincing proof that in some trilo- 

 bites the pygidium is reduced to a very small plate. 



In regard to the second antennae standard text-books contain statements which are actu- 

 ally surprising. A compilation shows that the antennae are entirely uniramous in but a 

 very few suborders, chiefly among the Malacostraca ; that they are biramous with both 

 exopodite and endopodite well developed in most Copepoda, Ostracoda, and Branchiopoda; 

 and that the exopodite, although reduced in size, still has a function in some suborders of 

 the Malacostraca. The Crustacea could not possibly be derived from an ancestor with two 

 pairs of uniramous antenna;. 



Although I have defended the trilobites, perhaps with some warmth, from the impu- 

 tation that they were Arachnida, my argument does not apply in the opposite direction, and 

 I believe Lankester was right in deriving the Arachnida from them. If the number of 

 appendages in front of the mouth is fundamental, then the trilobites were generalized, primi- 

 tive, and capable of giving rise to both' Crustacea and Arachnida. As shown on a previous 

 page (p. 119), the "connecting links" so far found tend to disprove rather than to prove 

 the thesis, but the present finds should be looked upon as only the harbingers of the greater 

 ones which are sure to come. 



LIMBS OF TRILOBITES PRIMITIVE. 



The general presence, in an adult or larva, of some sort of biramous limbs through- 

 out the whole class Crustacea has led most zoologists to expect such a limb in the most 



