614 ARTHROPODA SUB-KINGDOM 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 
legs. 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 
(Isotelus) 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, 4). Soon after, 
Woodward described an antenna or pediform 

Fia. 1265. 
Asaphus (Isotelus) gigas, De Kay. 
Ordovician; Ottawa, Canada. 4, 
Ventral side, showing remains of 
jointed feet (after Billings). B, Hypo- 
stoma with antenna or pediform 
cephalic appendage (after Woodward). 
cephalic appendage, lying beside the hypostoma 
of another individual of the same species (Fig. 
1265, 6). Through the investigations of Walcott 
(1875-94) on Ceraurus and Calymmene, by means of 
transverse and longitudinal sections of enrolled 
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 
processes which became cal- 
careous with age, and to these 
the feet were attached. 
Lhe intestinal canal; dis-» yseaien vertical section: of iCeratra: pleurexanthemus, Green. 
govered by Beyrich and g.cephalon, wiih bypesiome on 
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. 1266, 
