158 



CLASS AEACHNIDA. 

 CHAPTER I. 



SCORPIONS AND SPIDERS. 



ARACHNIDA General Characters Internal Structure Habits Distribution Fossil Forms Classification Order 

 ARTHROGASTRA Lung-sacs Classification Family SCORPIONID.E Scorpions Family PHRYNID^E Family CHE- 

 LIFEKID.E False Scorpions Family PHALANGIID.E Harrest-men Family SOLPUGID.E Galeodes Order ARA- 

 NEIDA True Spiders General Characters Internal Structure Spinning Apparatus Habits Classification 

 TiTRAPNEUMONES Family MYGALID.E Bird Spiders Mygale Trap-door Spiders DIPXEUMONES Family SALTI 

 CID.E Saltigradae Family LYCOSID.E - Citigradae Tarantula Family THOMISID.E Laterigradae Crab Spiders 

 Family TEGENARIIDJE Tubitela? House Spider Water Spider Family THEUIDIID.E Imequitelse Malmignatte 

 Family EPEIRIIXE Orbitelae Garden Spider. 



IN both the preceding classes of air-breathing Arthropods, the head is furnished with a pair of 

 jointed organs recognisable as antennae ; in the Arachiiida we find no antennae of the same kind, but 

 the corresponding parts, when present, are converted into a pair of more or less jaw-like organs. Except 

 in two groups the head is always intimately united with the thorax to form a single mass, called the 

 cephalothorax, which bears in front all the organs pertaining to the head, and on its lower surface the 

 thoracic limbs ; in many cases even the distinction between thorax and abdomen is effaced. There 

 are 110 wings. The organs of respiration, when present, consist either of tracheae, or of peculiar 

 sac-like modifications of tracheae, to which the name of lungs or pulmonary sacs has been given. The 

 abdomen is always destitute of limbs. The range of organisation is so great in the Arachnida that it 

 is somewhat difficult to give any general description of the class, and we shall therefore only indicate 

 briefly what is necessary to make the following descriptions of the orders and families intelligible. 



The cephalothorax is usually covered above by a single plate, upon the anterior part of which 

 the eyes, when present, can be seen. These are always ocelli or simple eyes, and they vary in number 

 between two and twelve. The organs representing the antennae are articulated to the front of the 

 cephalothorax above the opening of the mouth, and receive their nerves from the supraoesophageal 

 ganglion ; they usually take the form of jaws, often of formidable dimensions, and in function replace 

 the true mandibles, which are absent. They are commonly called falces. Of the two pairs 

 of organs representing the maxillae and labium of insects, the former (maxillae) retain their 

 position as organs of the mouth, their basal parts closing the mouth behind either as separate pieces, 

 or united into a single plate, while their jointed palpi project, and frequently acquire the form and 

 size of an additional pair of limbs. Of regular limbs the Arachiiida have four pairs, of which the 

 first may be considered to represent the labial palpi, and the others the three pairs of legs of the 

 Insecta. The bases (coxce) of all these limbs surround the sternum, but possess apparently little power 

 of motion. The limbs springing from these coxae consist, in the higher Arachiiida, usually of seven 

 joints, namely, a troc/tanter, which is sometimes elongated, but generally very short, a stout femur, a 

 tibia composed of two unequal joints, and a tarsus, also consisting generally of two joints, which are 

 sometimes annulated, and the last of which bears the claws and often other subordinate organs. In 

 the lower types of the class of course the limbs are frequently simpler in construction, and occasionally 

 they are represented only by rudimentary parts. 



The abdomen is attached to the cephalothorax sometimes by a slender peduncle, sometimes by its 

 whole width ; and in certain of the lower forms of the class the whole animal shows 110 traces of 

 divisions. In some instances also the abdomen itself is clearly divided into a larger or smaller number 

 of segments, whilst in others no trace of segmentation is apparent. The skin covering the body of the 

 Arachnida is generally soft and leathery, with the exception of the limbs, the joints of which are 

 more or less horny tubes. In other cases the whole surface is horny. The skin is changed repeatedly 

 and throughout the life of the animal, there being no fixed period for the final moult, as in 

 the insects ; thus the Arachnida, after having reached the reproductive stage, may continue to 

 live and increase in size, and produce successive broods of young. 



The mouth in the Arachnida leads into a narrow oesophagus, which in some forms (Scorpions) 

 passes directly into the stomach, while in the majority it is clearly separated from the latter. The 

 stomach itself presents the remarkable peculiarity that in most cases it gives origin on both sides 

 to several caaca, often of the same number as the limbs, which in many cases actually penetrate 



