36 En^heathed Larvae of some Parasitic Nematodes 



of the intestine become densely crowded with reserve food granules 

 which sharply set off this region from the rest of the body. 



Finally, at the end of this stage of growth, the larva becomes en- 

 sheathed by the replacement of its cuticle by a new one underneath. The 

 mouth and anal apertures close up, and at the same time the enclosed 

 larva shrinks a little in size so that it becomes separated from the old 

 cuticle which encases it as a completely closed sheath. It then wanders 

 upwards from its surrounding medium. 



The ensheathed larvae of G. strigosum and T. retortaeforniis are similar 

 to each other in all essential structures. The former have much longer 

 tails than the latter, and by this means can be easily recognised in a 

 mixture of the two kinds (Fig. 1). 



ATTEMPTS AT SKIN INFECTION. 



(1) A large number of ensheathed larvae were collected from the 

 culture dish lids containing moist blotting-paper by pouring distilled 

 water on to the latter and allowing this to stand for a short time. These 

 were concentrated in a small quantity of water by centrifugalising. 

 A drop of this liquid was placed on the skin of a young rat (10 days old), 

 in the inguinal region, and allowed to remain there for 20 minutes, the 

 animal being held in position during this time. The water was not 

 allowed to dry up, a drop being added to replace that lost by evaporation. 

 It was then killed and the portion of skin was dissected out and fixed 

 in 10 per cent, formalin. This was embedded in paraffin and sectionised, 

 but the sections showed no sign of skin infection. 



(2) The day following this experiment a large number of larvae in 

 water were placed in the bottom of a glass tube, and into the liquid the 

 hind-quarters of another young rat were immersed. The larvae could 

 be seen moving actively in the liquid when examined through a hand 

 lens. The animal was kept in the tube for one hour, during 20 minutes 

 of which the tube was in the 37° C. incubator. After this the rat was 

 chloroformed and the skin from the tail and both hind feet was fixed 

 in 10 per cent, formalin. It was noticed that the larvae left in the tube 

 after taking out the rat were very sluggish in movement and many were 

 practically quiescent. I associated this with the fact that they had been 

 put into the 37° C. incubator and inferred that the increase in temperature 

 had checked their motility. Some of the larvae were placed on a slide 

 in a drop of water and covered with a coverslip ; they then became quite 

 active again. This was no doubt due to the return to normal room-tem- 

 perature as many subsequent ol)servatioiis proved. 



