68 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



attached to a narrow rim-like plate which seems to fit in just ahead of the doublure of the 

 pygidium, or perhaps over it. 



In Ceraurus, Calymene, and Isotclus, the endopodites of the pygidium are similar to 

 those of the thorax, but seemingly more slender, with less well developed coxopodites, and 

 with, in the last-named genus, slender cylindrical segments. Exopodites are not known on 

 the pygidia of any of these genera, but since they are present and like those of the thorax 

 in Triarthrus, Cryptolithns, Neolenus, and Ptychoparia, there is little reason to think that 

 they were absent in Ceraurus or Calymene, though there is some question about Isotclus. 



The limbs are largest and longest on the anterior part of the thorax of a trilobite, and 

 diminish regularly in length and strength to the posterior end of the pygidium. This reg- 

 ular gradation shows, as Beecher'was the first to 'point out, that the growing point of the 

 trilobites is, as in other arthropods, in front of the anal segment. New free segments are 

 introduced into the thorax at the anterior end of the pygidium, and this has led to some 

 confusion between the growing point and the place of introduction of free segments. 



If a new segment were introduced at a moult in front of the pygidium, that segment 

 would probably have less fully developed appendages than those adjacent to it, and so make 

 a break in the regular succession. The condition of the appendages corroborates the evi- 

 dence derived from the ontogeny of the pygidium, and proves that the new segments are 

 introduced at the same growing point as in other Arthropoda. 



Caudal Rami. 



Bernard, who believed that the Crustacea had been derived through an Apus-like an- 

 cestor (1892, pp. 20, 85, 274), pointed out that four or less than four anal cirri were to 

 be expected. Two well developed cirri and two rudimentary ones are present in Apus, and 

 they are also to be found in other phyllopods and some isopods. It is, however, character- 

 istic of the Crustacea as a whole to lack appendages on the anal segment. Caudal cirri 

 (cerci) are much more freely developed in the hexapods than in the Crustacea, particularly 

 in the more primitive orders, Palaeodictyoptera, Apterygota, Archiptera, and Neuroptera. 

 They are supposed, in this case, to be modified limbs, and therefore not homologous with 

 the bristles on the anal segment of an annelid. Doctor W. M. Wheeler of the Bussey In- 

 stitution has kindly allowed me to quote the following excerpt from a letter to me, as 

 expressing the opinion of one who has made an extensive study of the embryology of insects : 



I would say that I have no doubt that the cerci of insects are directly inherited from the insect ancestors. 

 They are always highly developed in the lower insects, and only absent or vestigial in a few of the most 

 highly specialized orders such as the Hemiptera, Diptera, and Hymenoptera. I have further no doubt 

 concerning their being originally ambulatory in function. They are certainly not developed independently in 

 insects. Embryologically they arise precisely like the legs, and each cercus contains a diverticulum of the 

 mesoblastic somite precisely as is the case with the ambulatory legs and mouth parts. 



The "pygidial antennae" seem to be as fully developed in Neolenus as in any of the 

 other arthropods, and may suggest a common ancestry of the phyllopods, isopods, and 

 hexapods, in the trilobites. They were doubtless tactile organs, and while the evidence is 

 chiefly negative, it would seem that they proved useless, and were lost early in the phylog- 

 eny of this group. Possibly the use of the pygidium as a swimming organ proved de- 

 structive to them. 



