524 THE ANIMAL KINGDOM. 



abdominal somites in Eurypterina are covered by a flap (? genital 

 operculum), and probably bore branchiae. In Trilobita the somites of 

 the mesosoma and the unsegmented metasoma (pygidium) have slender 

 jointed limbs furnished with a short external jointed epi- or exo-podite. 

 Two straight or spirally coiled branchial filaments are attached externally 

 to the base of the limb, and appear to resemble closely the branchiae of 

 certain species of Cyamus (Crustacea Amphipoda). 



The chitinoid cuticle is thick and dense in Limulus, Scorpio, &c., 

 delicate in Linguatulina. There are cutaneous glands in Linguatulina 

 and some Acarina. The Solifugae are covered with thick-set hairs, and 

 in Acarina cuticular hairs or processes may attain a great development. 

 The Araneidae and Pseudoscorpionidae possess silk glands. In the former 

 group they are very numerous, and open on the apices of 4-6 spinnerets 

 placed in front of the anus : their secretion is used to form the web and 

 attach the ova. In the latter they open near the genital outlet on the 

 second abdominal somite *. 



The supra-oesophageal nervous centre supplies the eyes and integu- 

 ment : it is connected at the sides and behind the oesophagus to 

 a single nerve mass in Linguatulina, Acarina, Araneidae, Pedipalpi, and 

 Solifugae, from which pass off nerves to the limbs, &c. In Araneidae the 

 posterior nerve to the abdomen has often a ganglionic swelling at the 

 junction of that region to the cephalothorax. In Scorpionidae there is 

 a ventral cord with seven ganglia, the first of which corresponds to the 

 fifth abdominal somite, the preceding parts, including the chelicerae, 

 being supplied from the sub-oesophageal nerve-mass. In Limulus there 

 is an oval oesophageal collar with 3-8 transverse commissures, which 

 supplies the appendages of the cephalothorax, the chilaria, and the 

 genital operculum, and an abdominal cord which gives off five groups of 

 nerves to the branchial feet, the last pair supplying the metasoma and 

 post-anal spine as well. A stomatogastric system exists in Araneidae, 

 Scorpionidae, and Phalangida. Eyes are absent in Linguatulina and some 

 Acarina. They are present in other Arachnida, and vary in number from two 

 (Acarina, Tardigrada) to twelve. They are often grouped in a characteristic 

 manner. In Spiders they are arranged in 2-3 rows across the cephalothorax 

 dorsally and anteriorly. The Scorpionidae and Xiphosura have a pair of 

 central dorsal eyes, and one (Xiphosura) or more pairs placed near the 

 lateral margin of the cephalothorax. The lateral eyes in Scorpionidae and 



1 In a few Spiders a chitinoid plate, the cribellum, lies in front of the spinnerets. It is perfo- 

 rated by the ducts of innumerable glands. Its presence is correlated with that of the calamistrum, a 

 single or double row of long wavy hairs on the dorsal aspect of the penultimate tarsal joint of the 

 fourth pair of ambulatory limbs. One of the calamistra is rapidly vibrated over the cribellum, and 

 draws out the secretion from the glands in the form of threads, used to strengthen the web, to assist 

 in forming the cocoon for the eggs, and sometimes perhaps in making a domicile. See Bertkau, 

 A. N. 48, i, 1882. 



