484 TEILOBITA AKACHNIDA. 



anterior arched, semicircular region, which may be regarded as 

 head or perhaps as cephalo-thorax, and a number of sharply dis- 

 tinct segments, which belong partly to the thorax and partly to 

 the abdomen and are terminated by a larger shield -shaped caudal 

 portion, the pygidiwm (fig. 376). 



At the edge of the pygiclium, the armour of the upper surface is 

 folded round on to the ventral side and leaves only the middle part 

 of the latter uncovered. The lateral regions of the head, the 

 median part of which especially projects as the " glabellum," bear 

 usually upon two protuberances large compound facetted eyes, and 

 are often prolonged into two very long backwardly directed spines; 

 they are also folded inwards on to the ventral surface. With 

 the exception of a plate (hypostoma) comparable to the under-lip 

 of Apus, no trace of mouth parts has been observed for certain on 

 the ventral surface of the head. The number of thoracic (trunk) 

 segments varies considerably, but is tolerably definite for the adults 

 of each species. Their lateral portions are likewise folded inwards 

 on to the ventral surface, and present variously shaped wing-like 

 processes and long pointed spines. 



The Trilobites lived in the sea, probably in shoals in shallow water 

 near the coast. Their fossils are amongst the most ancient remains 

 of animal life, and are found principally in. Bohemia, Russia, Sweden 

 etc., in the lowest strata of the Uebergangsgebirge (Cambrian, 

 Silurian, etc.) They have been divided into numerous families 

 according to the structure of the head (especially of the glabellum), 

 the form of the pygidium and the number of segments. The most 

 important genera are Cafymene Blutnenbachii Brogn ; Olenus gibbosus 

 Wahlb., Ellipsocephalus Hoffii Schlotth. 



Class II. ARACHNIDA.* 



Air-breathing Arthropoda with fused head and thorax, with two 

 pairs of jaws, four pairs of ambulatory leys and apodal abdomen. 



The Arachnida include animals of extraordinarily different form. 

 The head and thorax are almost invariably fused to form a short 

 cepholo-thorax ; but the condition of the abdomen presents very 

 great variations. 



* C. A. Walrkenaer et P. Gervais. " Histoire naturelle des Inbectes Apteres," 

 3 Vols.. Paris, 1837-1844. Halm imd Koch, " Die Araclinidcn, getreu nach 

 der Natur abgebildet uiul heschrieben," Numbers:, 1831-1849. E. Blanchard, 

 " Organisation du regne animal. Araclmides." Paris, 18(JO. 



