THE SILKWORM 73 



press, and just behind it a pair of small, much divided glands 

 open into the common tube. The common duct can be 

 traced forwards to the spinneret, which opens on the labium, 

 beneath the head (see p. 51). There is yet another pair 

 of salivary glands, which open independently into the mouth, 

 and probably form the only part of the apparatus which has 

 any digestive function, but these can only be made out by 

 a practised dissector. The large convoluted glands already 

 described, whatever may have been their original function, 

 are now solely employed in the elaboration of silk, and may 

 conveniently be called the silk-glands. The walls of the 

 glands are lined by a regular layer of cells, which secrete fluid 

 silk, and store it up till it is wanted. When exposed to air, 

 this fluid silk solidifies, and forms a tough substance, about 

 as firm as horn, which, drawn out into fine threads, constitutes 

 the silk fibre. If w& magnify a thread cut from a silkworm's 

 cocoon, we soon discover that it is double, each thread con- 

 sisting of two flattened, ribbon-like filaments, and that the 

 component filaments are glued together. Hot water dissolves 

 the glue, and sets the filaments free. Some notion of the 

 tenacity of silk may be got by remarking that the secretion 

 when drawn out from the body of a fresh-killed silkworm, 

 stretched between pins, and dried, forms the gut so highly 

 esteemed by anglers. The silkworm draws out its silk so 

 fine that the thread used in a single cocoon is about 4000 

 feet long, say f of a mile ; nevertheless, it is amply sufficient 

 to bear the weight of the full-fed larva. The tenacity of a 

 silk thread is only exceeded by steel wire, drawn copper wire, 

 and aluminium bronze ; platinum wire is inferior to it. The 

 two threads are formed independently in the two salivary ducts, 

 and apparently emerge ready-coated with the glue (this has 

 been both affirmed and contradicted). They are then passed 

 through the press and consolidated. Whether they receive at 

 this point the secretion of the glands which open just behind 

 the press, and what the nature of that secretion may be, are 

 questions on which no certain information is to be had. 

 Finally the thread, fully formed and consolidated, is emitted 

 by the spinneret. We do not know how and where the 

 coagulation of the fluid silk takes place, except that the 

 process is complete before the thread leaves the spinneret. 

 For a considerable part of the three days spent in forming the 



