450 INVERTEBRATE MORPHOLOGY. 



thread. The thickness of the thread may be modified by 

 uniting together the secretions of a greater or less number of 

 the glands, which, moreover, differ among themselves, some 

 producing, for instance, a sticky secretion with which certain 

 of the threads may be covered. In some forms there is situ- 

 ated upon the abdomen just in front of the -m-immcrctc a $ I 

 chitinous plate, the cribeUum, which is perforated, like the 

 spinnerets, by the ducts of numerous spinning-glands. Its 

 presence is associated with that of a calamistrum, a peculiar 

 modification of the metatarsus of the last pair of legs, it being 

 furnished with a double row of bristles which are rapidly 

 waved over the cribellum and draw from its glands their 

 secretion. The threads are used for several purposes, as, for 

 example, to fasten the ova to the body of the parent or to 

 form a cocoon for them, or else to form a snare by which 

 insects may be caught to serve as food. These snares in 

 some cases are composed of an irregular network of threads 

 arranged without any definite pattern, as in Theridium, but 

 some other forms show a certain amount of architectural 

 skill, weaving a platform of felted threads which terminates 

 in a tubelike place of concealment for the spider (e.g., Age- 

 lena, Tegenaria) or webs composed of threads radiating from 

 a central point and united by other threads arranged in a 

 spiral or in concentric circles (e.g., Epeira, the common garden- 

 spider), or else using the threads to form a hinged trap-door 

 covering in a burrow in the earth which serves as a domicile 

 as in the Trap-door Spider. 



The digestive tract expands in the thoracic region into a 

 saclike structure (s) from each side of which three or more 

 usually five csecal diverticula (sd) arise, the anterior pair 

 sometimes anastomosing so as to form a ring, while in some 

 cases (Epeira) secondary diverticula extend from the more 

 posterior ones into the coxal joints of the legs. In the abdo- 

 men the intestine is more cylindrical, giving rise to much- 

 branched lateral diverticula which together form the so-called 

 liver (dg), and having connected with it, just as it joins the 

 end-gut, two elongated Malpighian tubules (mt). The end- 

 gut itself dilates into a large rectal bladder (rb) which a short 



