STRUCTURE AND HABITS OF ARACHXIDA. 159- 



more or less into them. The length of the intestine generally corresponds with that of the abdomen, 

 at the extremity of which it opens. The amount of convolution is not great. Before its termination 

 it generally enlarges into a rather lai-ge cloaca, immediately above which the Malpighian vessels open 

 into the intestine. The so-called fatty body of the Insecta does not occur in this class, but the 

 body-cavity is filled with the lobes of a greatly-developed liver, in which the other internal organs are- 

 imbedded ; the numerous gall-ducts proceeding from this liver unite to form eight or ten main ducts, 

 which open into the sides of the intestine at some distance behind the stomach. Salivary glands, 

 discharging into the cavity of the mouth are also generally present. 



The organs of circulation and respiration show very great differences in the class Arachnida. 

 The lowest types have no special organs of the kind. Others possess a dorsal vessel of very simple 

 construction ; while the higher forms, such as the Spiders and Scorpions, have a regular chambered 

 dorsal vessel or heart, into which the blood penetrates through valvular apertures, while part of it 

 issues again through small arteries given off by the chambers, and the rest through an aorta, 

 which divides into numerous branches, distributed through the body. In the lower forms again 

 respiration is performed by the agency of trachea? like those of insects and Myriopods ; in the more 

 highly-organised groups, while simple tracheae are still frequently present, the chief respiratory organs, 

 are peculiar lung-sacs, of which the ventral surface has from one to four pairs, and which show 

 in their interior several delicate membranous folds. The nervous system exhibits almost an equal 

 amount of variation, but in its highest development it shows an cesophageal ring with a lai-ge 

 ganglion above the oesophagus, from which nerves are supplied to the eyes and falces, and another 

 below the oesophagus, often united with the great ganglionic mass of the cephalothorax, and from 

 these combined nerves issue to the organs of the mouth proper and the four pairs of legs. When the 

 abdomen shows distinct segments, it also contains a ventral chain of ganglia united by commissures. 

 Except in one small and lowly group, the Arachnida are all of separate sexes, and with but few 

 exceptions they are oviparous. Many of them undergo more or less change of character in advancing 

 towards maturity, and in some parasitic forms we find examples of retrograde metamorphosis. 



In their habits the Arachnida are nearly all carnivorous, and, indeed, predaceous, living 

 principally upon various insects and other weaker Arthropods, of which they usually content 

 themselves with sucking out the juices, sometimes, however, devouring part of the solid substance. 

 Among the lower forms, some feed upon solid materials of animal origin, and others upon vegetable 

 matters, whilst some are parasitic, not only upon other Arthropods, but even upon vertebrate 

 animals. They are mostly terrestrial creatures, but one whole order consists of inhabitants of the 

 sea, and a few members of other groups are also aquatic in their habits. Their distribution is 

 world-wide, but, as in most other groups, tropical countries possess the greatest number of species, 

 and present us with the largest and most remarkable forms. Geologically, the class is of great 

 antiquity. Unmistakable Arachnida, of forms which stand high in our classifications, occur in 

 the Coal Measures of various parts of the world. Scorpions and true Spiders are recorded from 

 these deposits both in Europe and in Amei'ica, which also contain other forms, the precise location 

 of which is more difficult. Fossil Arachnida also occur, although sparingly, in later deposits 

 containing insect remains, such as the lithographic slates of Solenhofen and the varioiis Tertiary 

 insect-beds, and, as might be expected, Spiders are by no means uncommon inclusions in amber. 



The vast range of characters presented by the multifarious members of this class, the highest 

 and lowest of which, but for the existence of the intermediate forms, would hardly be 

 referred to the same group, renders the classification of the Arachnida rather complex, and has 

 given rise to considerable diversity in the systems adopted by different authors. Formerly the 

 class was divided into two principal groups, Pulmonata and Tracheata, according as the animals 

 breathed by lung-sacs or by tracheae, but this mode of division has been held to be unsatisfactory on 

 account of the analogy between the so-called lungs and the tracheae, and the further fact that 

 lungs and trachea? co-exist in many forms. The actual difference in the arrangement of the groups 

 by the abolition of this mode of division is so insignificant that it is really of little consequence 

 whether we retain it or not, and as the mode of arrangement in accordance with the general 

 characters presented by the various groups is perhaps more easily intelligible, we may adopt it in 

 the present work. The following table will show the orders into which we propose to divide the class. 



