PHYLUM ARTHROPODA 



259 



up into numerous delicate laminae lying parallel with one 

 another, like the leaves of a book. Into the numerous nar- 

 row spaces between the laminae the air penetrates, and oxy- 

 genates the blood which enters the interior of the lamina? 

 from the ventral blood-sinus. In the United States the 

 common scorpion from Florida to North Carolina is Buthus 

 carolinianus ; other species occur in Utah, New Mexico, 

 etc., and in Southern California. 



The Spiders (Fig. 154) differ from the scorpions in hav- 

 ing the abdomen short, rounded, and unsegmented, in having 

 the chelicerae subchelate and 

 provided with poison glands, 

 the ducts of which open at 

 their extremities, and the pedi- 

 palpi simple, the terminal joint 

 in the male being expanded, 

 and the whole appendage be- 

 ing used as an intromittent 

 organ for the transference of 

 the sperm to the genital open- 

 ing of the female. At the 



Fig. 154. — Spider (Epeira diadema). 



extremity of the abdomen is 



a peculiar apparatus, the arachnidium or spinning organ. 

 This consists of four or six appendages, the spinnerets, on 

 the surfaces of which open the numerous ducts of the spin- 

 ning glands secreting the material of which the web is com- 

 posed. The fine threads of viscid secretion issuing from 

 the ducts harden on exposure to the air, and are worked up 

 into the web by means of the posterior legs. There are six 

 or eight eyes on the carapace. The organs of respiration 

 are either four pulmonary sacs similar to those of the scor- 

 pion, or two pulmonary sacs, and a system of tracheae 

 resembling those of insects. 



