40 Gerald F. Hill: 



at the anterior end, and increased gradually to the anus, at which 

 point it was 0.05 nun. in diameter. The tip of tail which was 

 bluntly pointed was about 0.05 mm. from the operculum-like pro- 

 jection of the anus. This larva was apparently somewhat less 

 developed than the larval H. muscae shown in Fig. 5, which 

 measured 0.25 mm. long, and which was probably some days 

 younger (c.f. page 22). 



Fig. 26 shows a larva in a soinewhat similar stage of develop- 

 ment, which was found on February 12th in the abdomen of a 

 Musca domestica fly (see Experiment No. 14, page 32). It 

 measured 0.25 mm. long by 0.049 mm. wide. The oesophagus, 

 which was considerably twisted, joined the intestine about 0.082 

 mm. from the rectum. The anterior end was partly enveloped in a 

 portion of the cast-off cuticle, a poi'tion of which adhered also to 

 the posterior end. The rounded tail, the general appearance o-f 

 the parasite, and the occurrence of such an undeveloped stage in a 

 mature fly of a species not apparently normal for this w^orm 

 suggests that this specimen was an abnormality. 



Figs. 27 and 28 show the next step in the development of the 

 larvae. The lower part of the pharynx is now surrounded by a 

 clear space or vacuole, a feature which has not been observed in 

 lai'val H. musea of less than about 0.65 mm. in length, wliereas the 

 worm figured in Figs. 27 and 28 measured only 0.247 mm. long by 

 0.04 mm. wide at the rectum. Tlie form of tlie oesopliagus and intes- 

 tine are now well defined, and the large nuclei have almost disap- 

 peared from the alimentary tract. Small nuclei only are dispersed 

 through the oesophagus, intestine, and body wall, but those in the 

 rectum and posterior end aro con;^^;id'cia' Iv larger. The body is 

 relatively longer and narrower than in earlier stages. 



At the base of the oesophagus, Avhich was 0.1 mm. from the an- 

 terior end, the diameter of the body was 0.036 mm. Tlie anus, 

 still closed as in the earlier stages, w'as 0.033 mm. from the tip bf 

 the tail. That the worm figured was undergoing a moult was 

 Kliown by the presence of the partly discarded cuticular Jining of 

 the oral cavity. 



This parasite was found on January 4th in a Stomoxys pupa 

 w^hich had developed in the same culture as tlie embryo illustrated 

 by Fig. 20 (i.e.. Experiment No. 12, page 31). It was therefore 

 fourteen days old, and had been harboured by the intermediate 

 liost for a period not exceeding eleven days. This pupa contained 

 upwards of fifteen larval Habronema microstoma, including two 



