44 Etisheathed Larvae of some Parasitic Nematodes 



contortus, but at ordinary room temperatures they can withstand air 

 desiccation for a few days and revive on the addition of water. 



Doubtless also the habit of coiling up watchspring-wise on the advent 

 of desiccation and further of the tendency to congregate together as 

 drying-up proceeds constitute additional safeguards to enable the larvae 

 to withstand the adverse effects of desiccation. It is noticeable that 

 when N. americanus larvae are dried on a shde they do not coil up after 

 the manner of G. strigosum, T. retortaeformis and H. contortus. 



The comparative rapidity with which N. americanus larvae can be 

 killed, i.e. by a few minutes' exposure to dry conditions, points to the 

 sheath being very permeable to water vapour and to the contained larvae 

 being very easily injured by withdrawal of water from its tissues. I there- 

 fore carried out a series of experiments to test this, comparing it with 

 G. strigosum and T. retortaeformis larvae at the same time. For this 

 purpose I made use of solutions of common salt of different strengths 

 brought into contact with the larvae, from 15-20 in number in each case, 

 in shallow glass capsules, and noted the effect on the organisms through 

 the microscope. The salt solutions — 5 per cent., 10 per cent., 15 per cent., 

 and concentrated — acted by withdrawing water through the sheaths 

 from within outwards, and had the effect of bringing about a gradual 

 cessation of movement and finally plasmolysed the contained larvae, 

 causing vacuolations within them. 



Records of the action were taken every five minutes and at the end 

 of each test the power of revival was tested by transferring the larvae, 

 after first washing them in a fair bulk of distilled water, to a capsule 

 containing more distilled water. 



Five per cent, saline causes N. americanus larvae to slow down their 

 movements in 20 minutes, and at 35 minutes all are quiescent or quiet. 

 G. strigosum and T. retortaeformis, on the other hand, remain normal 

 in movement for 20 minutes, one or two began to coil at 25 minutes, and 

 a few remained active even for 90 minutes. On transferring to distilled 

 water after 2 hours in the saline, only five N. americanus larvae showed 

 signs of revival and this not very complete, whilst the G. strigosum and 

 T. retortaeformis larvae all revived and swam about well. 



Ten per cent, saline. G. strigosmn and T. retortaeformis larvae with- 

 stand the plasmolysing action of this strength longer than the N. ameri- 

 canus larvae. They also revived in distilled water to a much greater 

 extent than N. americanus larvae. 



Fifteen per cent, saline. As before in the lower percentages the 

 G. strigosum and T. retortaeformis withstood the action and remained 



