352 



A COURSE ON ZOOLOGY. 



the months of March and April millions of young eels, 

 hardly larger than threads, ascend the rivers in compact 

 masses ; under suitable conditions these grow rapidly, 

 and in a few years will weigh as many pounds. Towards 

 autumn the young fish have grown large enough to take 

 care of themselves, and they go down to the sea. 



In conclusion, pisciculture consists in the care given 

 to the breeding of fish. "If fish be hatched in the 

 hope of increasing them, the method of the agricultu- 

 rist must be followed ; he sows his grain in the most 

 favorable ground, and especially in a soil carefully pre- 

 pared for the seed ; to place young fish in a lifeless river 

 is like planting wheat in chalk or in sand" (Blanchard). 



FIG. 313. 



CHAPTER XXXV. 

 Obnoxious Invertebrates. 



WHILE studying worms we came across a certain 

 number of parasites. Besides these there are among 



the invertebrates, and 

 especially among insects 

 and arachnides, a large 

 number of species inju- 

 rious because of their 

 parasitic habits. 



We have already men- 

 tioned the cattle-flies and 

 the bot-flies ; there are 

 certain species that at- 

 tack man. One of these 

 is a Mexican fly, of which the female deposits her eggs 



BLOW-FLY. 



