614 



ARTHROPODA 



sri'.-KTNGDOM VII 



rock that the organs, even though present, cannot be exposed by the ordinary 

 methods. Furthermore, the appendages and ventral structures are so thin 

 and delicate that the most favourable conditions are necessary for their pre- 

 servation. For this reason, great uncertainty has prevailed regarding the 

 presence and character of the legs and various appendages. After a careful 

 preparation of their inferior side, by far the larger number of Trilobites show 

 only the vacant hollow space beneath the dorsal shell, and the hypostoma 

 attached to the reflexed margin of the cephalic shield, as represented in Fig. 

 1266. This common condition of the fossils led Burmeister, in 1843, to the 



assumption that all organs on the lower 

 side, as in Phyllopods, were originally soft 

 and fleshy. Previous to this, however, 

 Linnaeus, in 1759, described what 

 appeared to be antennae, and Eichwald, 

 in 1825, announced both antennae and 

 Altogether the early literature down to 

 1870 contains quite a number of claimants for 

 this discovery. Most of the evidence is manifestly 

 erroneous, and the two or three cases which bear 

 some semblance of validity are too obscure to be 

 of any scientific value. 



Billings, in 1870, published the description 

 and figure of an unusually well-preserved Asaphus 

 (hotelus) gigas from the Trenton limestone of 

 Ottawa, Canada. The ventral side of the specimen 

 showed eight pairs of jointed feet on each side of 

 a median furrow (Fig. 1265, A}. Soon after, 

 /' Woodward described an antenna or pediform 



cephalic appendage, lying beside the hypostoma 

 of another individual of the same species (Fig. 

 s , De Kay. 1265, B). Through the investigations of Walcott 

 ordovician; Ottawa"; "Canada, A, (1875-94) on Ceraurus and Caliimmene, by means of 



Ventral side, showing remains of \ J . /. -, 



jointed feet (after Biiiings). K, Hypo- transverse and longitudinal sections 01 enrolled 

 ?eph l aiicTppenda g e'(after Woodward"! specimens, a number of problems have been settled 



as to the characters of the ventral side. Accord- 

 ing to these investigations, Trilobites possessed a thin, external, ventral 

 membrane attached to the reflexed margin of the cephalon, the thoracic 

 segments, and pygidium. It 

 was supported by transverse c 

 processes which became cal- 

 careous with age, and to these 

 the feet were attached. 



The intestinal Canal, dis- Median vertical section of Ceraums pleurexanthemvs, Green. 

 rnvprprl Vw "RpvnVVi and c > Cephalon with hypostoma below; m, Mouth; v, Ventral mem- 



cover Dy ceyn ana ^v. .^ lntestinal ^ nal . py> Pygidium (a fter Walcott). 

 volborth, is located beneath 



the axial lobe, within the visceral cavity. It begins at the mouth, which 

 Walcott found at the posterior margin of the hypostoma, first bends forward 

 toward the dorsal region, and then extends backward from the glabella to 

 the posterior end of the pygidium, and parallel to the dorsal test, terminating 

 in the anal opening (Fig. 1266). 



FIG. 1265. 



