TRILOBITA. 341 



most anterior pair are very large, and form the so-called operculum 

 (cf., Limulus). The surface of the body was covered with scales. 

 Some of the Eurypterids reached a length of six feet. 



This order is sometimes placed near the Crustacea, but the general 

 opinion seems to be that which links them through Limulus to 

 Arachnoids. 



Order 3. Trilobita. Trilobites, e.g. , Calyniene, Phacops, Asaphus. 



Extinct forms chiefly found in Cambrian and Ordovician strata, but 

 extending up to the Carboniferous. The body as found is divisible into 

 three parts, — the unsegmented head shield, often prolonged backwards 

 at the angles ; the flexible thorax of a varying number of segments ; the 

 unsegmented abdomen, or pygidium. A median longitudinal ridge, or 

 rachis, divides the body into three longitudinal portions. 



Traces of limbs are only rarely preserved. In the head region there 

 are four pairs, apparently simple. Antennte have been recently found 

 in this region. The thorax and abdomen were fiu-nished with biramose 



Fig. 109. — Vertical cross section of a Trilobite, Calyniene. 

 (After Waixott. ) 



/., Intestizie ; s., shield ; X., endopodiLe ; e.y exopodite ; 

 ^., epipodial parts. 



appendages with long jointed endopodite, short exopodite, and a gill (or 

 epipodite ? ) of varying shape. In the abdominal region, the gills were ^ 

 perhaps rudimentary. 



Trilobites are often found rolled up in a way that reminds one of some 

 wood lice. So abundant are they in some rocks that even their develop- 

 ment has been studied with some success. 



The limbs seem to be more like those of Crustaceans than those 

 of Arachnoids, and the recent discovery of antennae accentuates 

 the resemblance ; but the marked affinities with Limulus, accord- 

 ing to the views of most authorities, justify the continued asso- 

 ciation with Arachnoids. It is, perhaps, most logical to regard 

 Trilobites as an offshoot from a stock ancestral to both Arachnoids 

 and Crustaceans. 



