320 SOME COMMON INTERNAL PARASITES [CH. IX 



probably acquired in the same way. Horses suffer from 

 the attacks of a worm of similar habits, Sclerostomum 

 (Strongylus) equinum. The males of this species vary in 

 length from 18 to 35 mm., the females from 20 to 55 mm. 

 When immature they inhabit the blood vessels of their 

 host and may cause serious injuries. The disease produced 

 is known as " colic." The adults make their way to the 

 mucous membrane of the caecum and when mature pass 

 into the large intestine. The eggs are expelled with the 

 faeces of the horse and develop in water, giving rise to 

 minute larvas which enter the horse with his drinking- 

 water, and at once penetrate into the blood vessels of the 

 small intestine. 



A number of Nematodes inhabit the air passages of 

 higher animals. Two species, Strongylus filaria and St. 

 rufescens, occur in the trachea, bronchial tubes and air cells 

 of sheep and goats. The former is from 3 to 10 cm., the 

 latter from 1'8 to 3*5 cm. long. In neither case is the 

 intermediate host known with certainty, though it has 

 been determined that such is necessary. 



The disease known as " gapes " in poultry and other 

 birds is due to a small member of this order, Syngamus 

 trachealis. The length of the male is from 2 to 6 mm., 

 that of the female from 5 to 20 mm. They occur in the 

 windpipe of the birds. Sexual union results in permanent 

 adhesion, hence the adults appear to be double-headed. 

 The eggs escape only by rupture of the body of the female, 

 inside or outside of the body of the host. They develop 

 in water and may even hatch. The free embryo is 0'28 

 mm. long and O'Ol mm. thick. It has however been shown 



