\l( \( II M DA. 3 



Infernal organs. — The generative organs open upon the ventral 

 side of the first or second abdominal somite, beneath either the 

 sternum or a movable plate, the genital operculum. The respiratory 

 organs also open upon the ventral side of the abdomen, communi- 

 cating with the exterior by means of apertures (stigmata) situated 

 upon or behind some of the anterior sternal plates. Barely there 

 is a pair of stigmata on the ventral side of the cephalothorax. 

 The organs themselves consist either of horny branching tubes, 

 the tubular trachea', or of pulmonary sacs supplied with tine leaf- 

 like lamella). 



The Arachnida are viviparous or oviparous. The young are born 

 or hatched from the egg in a form substantially resembling their 

 parents, and, except in the case of the Acari (Mites and Ticks), 

 growth is not accompanied by metamorphosis, the characters of 

 the adult being gradually assumed with each successive moult of the 

 integument. 



Classification. — (Setting aside the marine form Limulus, and also 

 the two degenerate groups Tardigrada and Pentastomida, whose 

 claims to be regarded as Arachnida are somewhat slender, the 

 members of this class may be grouped iuto the following orders : — 



1. SfORPIONES; 2. UrOPYGI ; 3. A.UBLYPYGI; 4. ArANE^E ; 5. 



.SoLirroK: <>. Pamugradi ; 7. Pseuboscorpiones ; 8. Opiliones; 

 ■J). Acari *. With the exception of the Palpigradi, which, so far 

 as is known, are confined to Central and South Europe, all these 

 orders are represented by numerous species and genera in British 

 India. They may be briefly diagnosed as follows : — 



a. Abdomen very long, consisting of twelve 



distinct somites, of which the posterior 

 five are narrowed and compressed to 

 form, with the postanal sclerite or 

 vesicle, a distinct tail ; postanal sclerite 

 with two poison-glands ; a pair of comb- 

 like abdominal appendages SCORPIONES. 



b. Abdomen typically short, usually not 



'tailed,' at most the posterior three 

 somites narrowed to form a short mov- 

 able stalk to support the postanal 

 sclerite; the latter never provided with 

 poison-glands ; no comb-like abdominal 

 appendages. 

 a 1 . Postanal sclerite retained as a single 



unsegmented piece or in the form of 



a many-jointed flagellum supported 



by the posterior three segments of 



the abdomen, which are narrowed to 



form a movable stalk UROPYGI. 



. —t . .. 



*»Tbe subclasses and superordina] groups under which these orders have 

 been rightly arranged are, for the sake of simplicity ignored in this volume. 



