INVERTEBRATE ZOOLOGY. 109 



rior border of the thorax. Neither piece shows segmentation, 

 although the paired appendages indicate that it is a segmented ani- 

 mal. Is this consolidation a primitive characteristic, or is it a mod- 

 ification ? Is the spider a high or low form ? 



(b) Appendages. The cephalo-thorax possesses six pairs of appendages. 



of which one is prae-oral. 



1. Chelicers (mandibles). These depend downwards from the 

 abrupt anterior end. Thej' consist of a powerful basal joint 

 and a movable tooth or poison fang. A poison gland in the 

 interior connects by a fine duct with this fang. In some 

 members of this class the chelicers bear a double claw. 

 3. Pedipalpi (palpi). These resemble legs in the female, but are 

 modified in the male into spoon-shaped organs, used to con- 

 vey the spermatozoa over to the female during pairing. In 

 some forms these also may become huge doiible claws (Ex. 

 Scorpion). 

 3-6. True legs. These are seven jointed and possess minute 

 toothed claws at their extremity. These are used in running 

 about on the web. 

 The abdomen of the embryo possesses rudimentary appendages, 

 some of which disappear, while others form the spiimerets or spin- 

 ning glands, at the end of the abdomen. How many pairs are 

 there ? In cases where a median spinneret is present, how may it 

 be explained V 



(c) Respiratory and genital openings. Spiders breathe by a system of 



plates, or fan-tracheae, which hang in respiratory chambers, the so- 

 called •• lungs." A few tropical forms have two pairs of these, but 

 North American spiders have only a single pair. Look at the ven- 

 tral side of the abdomen, close to the cephalo-thorax. for a pair of 

 smooth, often shiny plates. These are the opercida. which cover the 

 respiratory chambers. This system is reinforced by a system of sim- 

 ple tracheal tubes, which open by a single median stigma just ante- 

 rior to the spinnerets. The female possesses a median opening as 

 outlet for the eggs, and two lateral openings for the reception of the 

 spermatozoa, and leading into spermothecae. The median opening, 

 or vagina, lies between or just posterior to the anterior pair of spin- 

 nerets, and the lateral openings at its sides. The vasa deferentia of 

 the male open by a median opening between the opercula. 



