68 ZOOLOGY. 



communicate with four or six small teat-like organs, 

 called spinnerets, placed at the extremity of the body, 

 each of which contains a great number of minute tubes. 

 Through each of these a slender thread of this sticky 

 substance is drawn. The animal, by means of the claws 

 of its hind feet, unites all these filaments together. The 

 spider's thread is thus, small as it is, a rope composed of 

 thousands of strands. 



84. Digestion. The mouth, which is situated in the 

 front of the cephalothorax, is furnished with a pair of man- 

 dibles, which are believed to be the representatives of the 

 antennae of the other Arthropoda, a pair of maxillae, and 

 a labium or lower lip. In the scorpions there is also a 

 labrum or upper lip. In the spiders, the mandibles are 

 each provided with a perforated hooked joint, which 

 conveys poison from a gland at its base. By means of 

 this poison, the spiders are able to kill the insects, on the 

 juices of which they subsist. The mandibles of the 

 scorpions are terminated by pincers. The maxillae are 

 each provided with long jointed appendages, termed 

 palpi. In the female spiders, these are terminated by 

 claws; in the males, by sexual organs. The maxillary 

 palpi of the scorpions are very long, and are furnished 

 with grasping claws. As the food of the spiders consists 

 merely of the juices of those animals on which they 

 subsist, their alimentary apparatus is extremely simple. 

 The intestine is short, and without convolutions. There 

 are salivary glands, and tubes which are believed to 

 represent kidneys. 



85. Circulation. There is usually a heart, similar to 

 the " dorsal vessel " of the Insecta and Myriapoda. Some 

 of the lower groups are without any special blood-vessels. 



86. Respiration is performed variously in the different 

 groups. All, however, breathe air directly. Some groups 

 have tracheae, like those found in the Insecta and Myria- 

 poda ; others have a modified form of these, consisting of 

 a series of bags along the sides of the animal, formed by 

 &n in volution of the integument^ and qpnimunicating \yitli 



