Chap, viii.] SILK ORGANS. 267 



of more or less well-marked irritating properties ; a 

 familiar example of tliis is the common toad, the 

 handling of which is often succeeded by inflamma- 

 tion of the eyelid. Experiments with subcutaneous 

 injections of the dermal secretion of the Triton show 

 that it appears to have an effect on the heart, and 

 that of the salamander on the central nervous system. 



Silk organs. The result of the secretion of the 

 silk organs of Spiders is the well-known web ; but 

 the secreted product, when it first appears, is a vis- 

 cous transparent liquid, which rapidly hardens on ex- 

 posure to the air, and then forms threads. The silk is 

 produced in various glands, which, however different 

 in form, are always found distributed among the con- 

 tents of the abdomen ; the secretion makes its way 

 to the exterior through the FO-callecl " spinnerets," of 

 which there are ordinarily three pairs ', these have the 

 form of obtusely conical papillae, the tips of which are 

 provided with a number of pores through which the 

 silk escapes to the exterior. This silk is used in very 

 various ways ; some spiders make cells or tubes for 

 themselves, some scatter the threads about, with the 

 obvious object of entangling an approaching prey, 

 while many make nets for the purpose of entrapping 

 victims. The so-called mason, or trap-door spiders, 

 spin a number of successive webs, which unite to form 

 a door for the pit in which they dwell. Clotho makes 

 a net-like tent, in which the young are concealed. In 

 many cases the webs are spun with considerable 

 rapidity, the common English spiders being able to 

 make one in about an hour. 



Among the Iiisecta, silk-producing glands are 

 best seen in the larvae of the Lepidoptera, where they 

 have the form of two long csecal tubes, placed one on 

 either side of the intestine, and opening by narrow 

 ducts at the base of a spinneret, which is developed 

 on the labiuni. As in the spider, the silk is at first 



