38 F. M. CAMPBELL — OBSERVATION'S OX SPIDERS. 



effect would be to place in the same genus spiders which, are 

 utterly dissimilar. 



General Anatomical Stetjctitre. 



Spiders chiefly differ from insects in the following respects. 

 The head and chest are amalgamated (cephalothorax), they have 

 eight legs, and no antennae. 



Respiration is only partly tracheal. The stigmata vary in posi- 

 tion and number. In the garden spider {Epeira diadema) there is 

 but one stigma, which is just in front of the spinnerets. The chief 

 organs of respiration are pulmonary sacs, which are never less than 

 two, nor more than four. These are easily discernible, and are 

 situated under two (generally) smooth pieces of skin near the base 

 of the abdomen on its under side. They contain, with their ends 

 free, a number of membranous sacs, arranged side by side in the 

 form of thin plates (laminae), through which the blood passes. The 

 heart is a muscular tube running along the dorsal part of the 

 abdomen. Blood is admitted by means of openings on each of its 

 sides, and then forced through communicating arteries to different 

 parts of the body. The vascular system is not complete. According 

 to Prof. Huxley,* the principal nerve-centres are " a supra-oesopha- 

 geal ganglion and a single post-oesophageal mass." These are in 

 communication, and from the former run branches to the eyes, 

 while the latter are in connexion with the limbs and abdomen. 

 There are also some free ganglia. The eyes are always simple, with 

 a lens and retinal expansion. Under the eyes, attached to the front 

 portion of the cephalothorax, are two "mandibles," which are the 

 homologues of antennse, and for this reason the name " falces " is 

 applied to them. The first or basal joint is generally stout and 

 strong, while the terminal consists of a sharp claw, which when at 

 rest closes on the basal joint as a blade of a pocket-knife on the 

 handle. At the end of the claw is the opening of the duct of the 

 poison-gland, which runs into the cephalothorax. Its largest 

 portion, which appears to act as a reservoir, is surrounded by an 

 irregular but closely arranged spiral muscle. The function of the 

 falces is to seize and kill the prey, and to bring it within reach of 

 the maxillae. The mouth is just under the attachment of the 

 falces to the cephalothorax. There is an upper and under lip 

 (labrum and labium), to each of which is attached a rough plate, 

 generally somewhat lanceolate, in the centre of which runs a groove 

 terminating in a point.f When the mouth is closed, the two 

 grooves form a tube which opens into the gullet or oesophagus, and 

 that expands into the sucking-stomach. 



At the top of the latter are muscles attached to the cephalothorax, 

 and at the bottom are others fastened between the legs. It would 

 seem that by the alternate contraction and expansion of the sucking- 

 stomach the liquid food is removed from the mouth and driven 



* ' The Anatomy of Invertebrate Animals.' 



t Either of these plates is the so-called ' ' spider's tongue," sold by microscopic- 

 object dealers. 



