PHYLUM ARTHROPODA. SPIDERS AND THEIR ALLIES 



271 



mouth and pharynx, followed by the hori- 

 zontal esophagus that leads into the sucking 

 stomach, which in turn opens into the true 

 stomach which gives off 5 pairs of ceca or 

 blind tubes in the cephalothorax. The in- 

 testine passes almost straight through the 

 abdomen; it is enlarged at a point where 

 ducts bring into it a digestive fluid from 

 the "hver," and again near the posterior 

 end, where it forms a sac, the stercoral 

 pocket, connected with the rectum, which 

 terminates in the anus. Tubes called 

 Malpighian tubes enter the intestine near 

 the posterior end. The digestive tract in the 

 abdomen is surrounded by a large digestive 

 gland or "Hver." This gland secretes a fluid 

 resembling pancreatic juice, which pours 

 into the intestine through ducts. 



The circulatory system consists of a heart, 

 arteries, veins, and a number of spaces or 

 sinuses. The heart is situated in the abdo- 

 men and is dorsal to the digestive tract. It 

 is a muscular contractile tube lying in a 

 sheath, the pericardium. The heart opens 

 into the pericardium, usually, by three pairs 

 of openings (ostia). It gives off posteriorly a 

 caudal artery, anteriorly an anterior aorta 

 which branches and supplies the tissues in 

 the cephalothorax, and three pairs of ab- 

 dominal arteries. The blood, which is color- 

 less and contains mostly amoeboid corpus- 

 cles, passes from the arteries into sinuses 

 among the tissues, and is carried to the 

 book lungs, where it is oxygenated; it then 

 passes to the pericardium by way of the 

 "pulmonary veins"; and finally enters the 

 heart through the ostia. 



Respiration is carried on by tracheae and 

 book lungs; the latter are peculiar to arach- 

 nids. There are usually two book lungs, 

 which, according to Kaston, have often 

 been incorrectly described in textbooks. He 

 states that each book lung consists of a 

 blood-filled chamber with an air vestibule 

 posterior to it. This air vestibule communi- 

 cates to the outside through a slit in the 

 body wall. From the front wall of the vesti- 

 bule, there have been invaginated many 

 parallel, narrow, air pockets, which have 



pushed forward into the blood-filled cham- 

 ber; these may be considered tracheae which 

 have a flattened fan shape instead of the 

 tubular shape usually associated with 

 tracheae. Air entering through the slit in the 

 body wall circulates through the air pockets 

 where oxygen is taken up by the blood. 

 Tubular tracheae are also usually present but 

 do not ramify to all parts of the body as in 

 the insects. 



The excretory organs are paired Mal- 

 pighian tubules, which open into the intes- 

 tine, and one or two pairs of coxal glands 

 in the floor of the cephalothorax. The coxal 

 glands are sometimes degenerate, and their 

 openings are difficult to find; they are 

 homologous with the green glands of the 

 crayfish (Fig. 114). 



The nervous system consists of a bilobed 

 ganglion brain above the esophagus, a large 

 subesophageal ganglionic mass, and the 

 nerves which run to various organs. Sensory 

 hairs, over the body and appendages, rep- 

 resent the principal sense organs. There are 

 usually 8 simple eyes (Fig. 163), which dif- 

 fer in size and arrangement in different 

 species. In only a few families are distinct 

 images formed; in the others the eyes func- 

 tion primarily for perception of degrees of 

 light, and moving objects such as prey. 



Very fine, erect, hairlike processes set in 

 sockets have been considered organs of hear- 

 ing, but the evidence is still insufficient. The 

 sense of smell is well developed, and an 

 organ of taste is located in the pharynx. 



The sexes are separate, and the testes or 

 ovaries form a network of tubes in the abdo- 

 men. The sperms are ejected upon a special 

 "sperm web," then picked up by the pedi- 

 palps and transferred to the seminal recep- 

 tacles of the female in mating. There is 

 courtship activity before mating which varies 

 with the species. Sometimes the female kills 

 and eats the male after mating. The eggs 

 are not fertilized until laid; the sperms move 

 from the seminal receptacle to fertilize the 

 eggs as they pass through the "uterus ex- 

 ternus" on their way out of the body of the 

 female. The eggs are laid in a silk cocoon, 



