162 ARTHROPODA. 



tion and whether they are all used at the same time. They 

 are probably true abdominal appendages. 



4. On the lower surface of the abdomen, near its anterior 

 end, are two slits, the openings into the lung-sacs or lung-books. 

 They are respiratory in function. 



5. Just in front of the spinnerets is a minute median pore, 

 the spiracle, that is often very hard to find. It is the external 

 opening of a series of abdominal tracheae. 



Make a drawing of a ventral view. 



Montgomery: Studies on the Habits of Spiders, Particularly Those of the 



Mating Period. Proc. Acad. Nat. Sci., Philadelphia, 1903. 

 : On the Spinnerets, Cribellum, Colulus, Tracheae and Lung-books of 



Araneads. Proc. Acad. Nat. Sci., Philadelphia, 1909. 

 : The Development of Theridium, an Aranead, up to the Stage of 



Reversion. Jour. Morph., 20, 1909. 

 : The Significance of the Courtship and Secondary Sexual Characters 



of Araneads. Am. Nat., 44, 1910. 

 Peckham: Observations on Sexual Selection in Spiders of the Family At- 



tidse. Occas. Papers Nat. Hist. Soc., Wisconsin, 1 and 2. 



PHOXICHILIDIUM. 



The exact affinities of the pygnogonids to other forms is 

 not known, but they have certain characters that have suggested 

 a possible relationship to the Arachnoidea. They are frequently 

 found in considerable abundance on the material that is attached 

 to piles. Notice their movements and see how they cling to the 

 material on which they are moving. 



1. The body is very slender and is composed of a number of 

 free segments that form the head and thorax and a small, ves- 

 tigial abdomen. How many free segments are there? At the 

 anterior end is a rather prominent proboscis, with the mouth at 

 its end. 



2. The following appendages will be found: 



(a) The chelicerce. What is their structure? Are they 

 armed with pincers? 



(6) Four pairs of long walking legs. How many segments 

 have they? The viscera extends into the bases of these appen- 

 dages. 



