GENERAL DESCRIPTION 403 



are short and end, on each side, either in a sharp, piercing claw, as in 

 spiders, or in a pincer-like claw, as in scorpions. The pedipalps are usually 

 sensory in function, but are prehensile organs in scorpions and many other 

 arachnids, and in the male spider have a copulatory function. The four 

 remaining pairs of appendages are locomotory and are usually long and 

 slender. The abdomen in the embryo has often rudimentary appendages, 

 the hinder three pairs of which in the spider become the spinnerets. 



The cuticula of arachnids is often covered with cuticular hairs or 

 scales, which have often an important tactile function. The special 

 sense organs are not well developed. Eyes are generally present, but 

 they are ocelli and not the composite eyes so characteristic of other 

 arthropods. 



Internal Structure (Fig. 654). The digestive tract is often of com- 

 plex structure. Long diverticula may extend from the stomach towards 

 or into the legs, and a network of diverticula in the form of the so-called 

 liver usually occupies a large part of the abdomen; one or more pairs of 

 Malpighian or kidney tubules enter the rectum. 



The respiratory organs are wanting in some mites and other minute 

 arachnids, but are usually present in the form of lungs and tracheae. The 

 lung is a ventral sac, usually near the anterior end of the abdomen and 

 opening to the outside through a pore called a spiracle, which contains 

 numerous leaf-like plates like the leaves of a book, in which the blood 

 circulates. The tracheae are air tubes reinforced on their inner surface 

 by a cuticular lining usually in the form of a spiral thread to keep them 

 from collapsing, which extend from spiracles throughout the body. 

 Scorpions and some of the larger spiders have only lungs; most spiders 

 have both lungs and tracheae; and mites and many other arachnids have 

 only tracheae. 



Circulatory organs are wanting in many arachnids which lack a 

 special respiratory apparatus, but in most of them a tubular heart with 

 lateral valvular openings is present in the abdomen, from the ends of 

 which arteries extend into the surrounding organs. The Tardigrada are 

 hermaphroditic, but with this exception all arachnids are unisexual. The 

 paired gonads lie in the ventral portion of the abdomen and open to the 

 outside by paired ducts or by a single duct in the first or second abdomi- 

 nal somite. The sexes may often be distinguished by their external 

 characters, the male being smaller than the female and often provided 

 with special copulatory organs. 



Most arachnids are oviparous, but the scorpions and a few others 

 bear their young alive. The young usually resemble the parents in 

 appearance, but in a few cases, as in the Linguatulida and the mites, 

 they go through a metamorphosis. The great majority of arachnids are 



