346 A COURSE ON ZOOLOGY. 



two very narrow canals. The silky matter flows from 

 the two glands, assumes the form of thread in the 

 canals, and dries in the air as it is drawn from the 

 minute orifices in the tubercles. The finest fibres that 

 are used in industry are formed by the twisting together 

 of three or four natural fibres. With these fibres, of 

 which some are white and some yellow, the larva rolls 

 up an entirely closed oval cocoon, in which it shuts itself 

 up during the last metamorphosis, and which it leaves as 

 a perfect moth, after having dissolved, by a peculiar 

 liquid secretion, the gummy matter that holds the fibres 

 together, and prevents them from separating from one 

 another. 



The rearing of silk-worms is an exceedingly important 

 industry in certain countries. It is conducted in silk- 

 worm nurseries, where all the operations are conducted 

 on a scientific basis, in order that the products may 

 have the greatest superiority. However, the accu- 

 mulation of enormous quantities of silk-worms in a 

 limited space is accompanied by great danger of loss 

 in case of epidemic diseases among them, and the rear- 

 ing of the worms must not be undertaken on too large 

 a scale. 



To form an idea of the series of operations conducted 

 in an ordinarily large nursery, we must start with the 

 worms in the eggs. An ounce of these contains about 

 forty-four thousand, and an ordinary nursery employs 

 about ten ounces, while the smaller cultivators cannot 

 use over two or three ounces. In France, where silk 

 culture has reached a high stage of development, the 

 hatching is undertaken when the buds of the mulberry 

 begin to put forth little leaves. The eggs are then main- 

 tained at a temperature between 68 and 77, and the 



