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NATURAL HISTOEY. 



I. Abdomen composed of distinct segments ..... 

 II. Abdomen with no distinct segments or rudimentary: 



A. Abdomen distinctly separated from the cephalothorax, pedunculate 



B. Abdomen not separated from the cephalothorax : 



* Furnished with tracheae ...... 



+ With no distinct organs of respiration : 



a. Body indistinctly ringed ; four pairs of stumpy legs 



b. Body worm-like ; internal parasites . 



c. Cephalothorax of four segments ; legs long ; abdomen rudimentary 



Order 1. ARTHROGASTRA. 

 ,, 2. ARANEIDA. 

 3. ACARINA. 

 4. TARUIGRADA. 



,, 5. LlNGUATULIXA. 



,, 6. PANTOPODA. 



The first of these orders includes the Scorpions, and some other Pulmonate forms, together with 

 the long-legged Harvest-men, the Book Scorpions, and some others which breathe by tracheae ; the 

 second is formed by the true Spiders, in which we find lung-sacs as well as tracheae ; the Acarina 

 include the numerous species of Mites, in which tracheae are the sole organs of respiration ; the 

 Tardigrada are the minute creatures known to microscopists as Bear animalcules ; the Linguatulina, 



when adult, are worm-like creatures, with a couple of 

 hooks as the sole representatives of limbs, but in the 

 young state show arachnidan characters ; and the last 

 order consists of some marine creatures, which may be 

 called Sea Spiders, and which have been bandied about 

 between the Crustacea and the present class. 



ORDER I. ARTHROGASTRA. 



This order includes several distinct types, of which, 

 indeed, separate orders have been made by many 

 zoologists, and it is not without some hesitation that we 

 have accepted it in its present signification. The sole 

 important character by which all its members are held 

 together, is the possession of a distinctly segmented 

 abdomen, which is attached to the hinder part of the 

 cephalothorax by its whole width ; in other respects we 

 find a great variety both in external structure and in 

 internal anatomy. 



In respect of the respiratory organs especially, we 

 find two perfectly distinct types, some forms belonging 

 to the group breathing exclusively by lung-sacs, while 

 the rest are as exclusively tracheal in their respiration. 

 These lung-sacs, which occur in the highest types of this 

 order, and associated with tracheae throughout the next, 

 are regarded by anatomists as modifications of tracheary 

 organs. They are situated in pairs in one or more seg- 

 ments of the abdomen, and each of them communicates 

 with the external air by a more or less slit-like opening; 

 or stigma, pierced in the ventral plate of the segment. 

 This aperture leads into a small, usually flattened, sac, 

 the walls of which are folded so as to form a number 

 of delicate lamellae (from 20 to 100, in different cases) 

 UNDER SURFACE OF SCORPIO occixANus. dividing the cavity into so many narrow compartments, 



an, falces, representing antennae ; mx', first pair; mx", ,, ... , , , . 1 



second pair of palpi; p l , p 2 , p 3 , true legs; x, comb; st, all opening into a common chamber which communi- 

 cates with the outer air through the stigma. As 



Professor Huxley remarks, " the organ, in fact, somewhat resembles a porte-monnaie with many 

 pockets." The blood circulates through these delicate membranous folds, and is thus exposed 

 to the influence of the air, which has free access to the cavity of the sac ; it is then con- 

 veyed by sinuses to the pericardial cavity, to pass thence into the heart. According to Pro- 

 fessor Huxley the expiration of the air is effected by the agency of peculiar muscles which act on 



