82 



NATURAL HISTORY OF ARTHROPODS. 



FlG. 111. Triarthrus becki, 

 trilobite, natural size. 



up of a varying number of nearly similar segments, and lastly an abdomen, or as it is 

 called in technical works, a pygidium, composed of a number of segments united to- 

 gether. Until very recently nothing has been known of the 

 nature of the imder-surface, or the character of the limbs 



of the trilobites. The first specimen, showing the under- 

 surface of a trilobite, was described by Mr. Billings, but, as 

 the appendages were rather indistinct, the evidence Avas not 

 accepted by all. Mr. Walcott then investigated the subject 

 by means of sections of the fossils, arriving at many satis- 

 factory results, but leaving many other problems still un- 

 settled. Lastly, Professor Mickelborough described another 

 specimen, showing the limbs, but without throwing any new 

 light on the subject. 



The head is rounded in front, and frequently its posterior 

 angles are produced into spines, which sometimes exceed the 

 body in length. The central portion of the head is raised 

 above the rest and constitutes the glabellum. On either side 

 of the glabellum are the eyes, w r hen these organs are present. 

 These eyes are either composed of isolated ocelli, or groups 

 of ocelli, or lastly, compound eyes similar to those of other arthropods, and which, 

 if we may accept the sections of Mr. Walcott, as interpreted by Professor Packard, 

 are essentially similar to those of Limulus as far as the hard parts are concerned. 

 On the under-surface of the head is a shield-shaped upper lip, behind which is the 

 mouth. Of the other organs of manducation nothing certain is known. The 

 thorax is divided into three longitudinal portions by two furrows. The medium 

 ridge is known as the axis, the lateral portions as the pleurae. The joints of 

 the thorax, in many forms, were freely movable on each other, and some of these 

 animals possessed the power of rolling themselves into a ball just as does the familiar 

 pill-bug of to-day, and fossils in this condition are frequently found. On the under- 

 surface of the thorax occur the several jointed limbs, one pair to each segment. Attached 

 to the basal joint of each leg is a peculiar organ, curled in a spiral, which Mr. Walcott 

 interprets as a gill. 



The abdomen is divided in the same manner as the thorax, the axis being continued 

 to the end of the body. Beneath the abdomen, Professor Mickelborough thinks that 

 he has found lamellar appendages like the gills of Limulus ; but 

 Mr. Walcott, in his restoration, continues the ambulatory legs 

 into this region. 



The development of the trilobites has been studied by Bar- 

 rande in Europe, and by Walcott in our own country. In the 

 earliest known stages the body is a small, oval disc, without any 

 distinct segmentation, but with growth, seg- 

 ments appear, their number increasing with 

 age. There is no metamorphosis, the devel- 

 opment being direct. The head of the adult can be resolved into 

 six segments ; the thorax contained from two to twenty-six, while 

 the abdomen was composed of, at the most, twenty-eight segments. 

 FIG. us. Larva of The trilobites made their appearance in the lowest Silurian, and 



Sao hirsutus, natural 



size and enlarged. died ou t at the close of the carboniferous. Both in number of 



. FIG. 112. Larva of tri- 

 lobite, Trinwlcns ornatus, 

 natural size and enlarged. 



