OBSERVATIONS ON THE LIFE HISTORY OF THE HORSE 

 OXYURID (OXYURIS EQUI) 



By Benjamin Schwartz 

 Of the University of the Philippines 



ONE PLATE AND TWO TEXT FIGURES 



The object of this paper is to record observations that have 

 a bearing on the life history of Oxyuris equi, a nematode of 

 common occurrence in the colon, caecum, and rectum of horses 

 in the Philippine Islands. In view of the fact that several refer- 

 ences to the morphology and biology of Oxyuris equi are not 

 obtainable in the Philippine Islands the work of former investi- 

 gators that may have a bearing on the life history of this para- 

 site cannot be reviewed fully in this paper. 



THE EGG 



The egg of Oxyuris equi is elliptical in shape, slightly asym- 

 metrical, and is enveloped in a firm cuticle having a double 

 contour and also an operculum at one end, the latter resembling 

 that of trematode eggs. Numerous observations on the hatching 

 of the eggs in vitro have shown that the larva invariably 

 escapes from the eggshell through the opening that is covered 

 by the operculum. Empty eggshells that were recovered from 

 the intestines of guinea pigs that were fed eggs of Oxyuris equi 

 almost invariably lacked the operculum. In eggs that hatched 

 in vitro the operculum frequently adhered to the eggshell and 

 in several cases still retained its normal position. Such shells 

 were intact, however, showing conclusively that the larvae must 

 have escaped through the opercular end and that the operculum, 

 after being lifted as a result of the movements of the embryo 

 against its inner surface, regained its normal position following 

 emergence of the larva from the shell. 



Measurements of numerous eggs have shown considerable 

 variation in size, the range of variation being from about 74 to 

 100 ix in length by from 38 to 47 ^ in width. In Table 1 are 

 given the records of size of twenty-three eggs selected at random. 



