ARTHROPODA — CRUSTACEA — TRILOBITES 289 



and stronger ones of the head may, Hke the mandibles of Apus, 

 have torn the food into smaller pieces. The mouth, opening 

 between the upper and lower lips, led into a long intestinal tube 

 which extended backward parallel to the dorsal test, opening 

 through the anus at the postero-ventral end of the pygidium 

 (Fig. 126, D). The digestion of food was very Hkely similar to 

 that of Apus (p. 299), except that in the absence of a stomach the 

 digestive fluid was probably poured into the anterior portion of 

 the intestine. 



Somewhat similar to the condition in Apus may likewise have" 

 been its blood circulation, excretion, its nervous system and sense 

 organs. The intestinal tube, as well as the double chain of nerve 

 ganglia, heart, etc., were located beneath the axial lobe in the 

 median line of the body. 



Respiration took place mostly through the limb setae or 

 fringes, aided probably by an exchange of gases through the 

 chitinous covering of the ventral surface, which was apparently 

 almost as thin as that covering the setae. 



The antennules were probably tactile organs. There is a 

 pair of slightly raised, compound eyes situated at the inner 

 edge of the free cheeks, while the neighboring border of each 

 fixed cheek is raised into a small eyebrow-like mound, the eye 

 lobe or palpebral lobe (see Fig. 128, e.l.). 



For reproduction and development, see page 292. 



1. Give the habitat of Triarthrus ; its geologic range. 



2. Sketch specimen ; label cephalon, thorax, pygidium, gla- 

 bella, axis, glabellar furrows, eye lobes, facial sutures, fixed 

 cheeks, free cheeks, pleural lobes. 



3. Is the skeleton external or internal ? 



4. What is its composition now ? When living ? 



5. How did growth take place ? 



6. Does each entire fossil necessarily represent the death of 

 an animal ? Why ? 



7. After molting or after the death of the animal separation 

 at the uncalcified joints occurs rather quickly. Name the parts 

 of the dorsal shield which would remain intact ; your reasons. 



u 



