OF THE NEMATOIDS, PAHASITIC AKD FREE. 619 



more quickly than those of the other four genera when thus exposed ; and when water 

 is added its imbibition, instead of being slow and gradual, is almost instantaneous, giving 

 rise to a few contractions and movements which are at first simulative of returning 

 vital movements. But this temporary activity almost immediately subsides, and then, 

 though watched for many hours after, no movements can ever be discerned. 



The members of the genus Bhahditis seem to occupy a kind of intermediate position. 

 Thus, I have succeeded in reviving some specimens after fifteen minutes' exposure on a 

 slip of glass, but in the few trials I have made, have never succeeded in doing the same 

 after they have been exposed for twice that time. In their anatomy, also, these animals 

 arc somewhat allied to those of the four genera above mentioned, and the nature of their 

 habitats being much the same, this remarkable tenacity of life is useful to all alike. 



It seems to me that the increased tenacity of life exhibited by the members of the four 

 genera Tylenchiis, Aphelenchits, Plectus, and Cephalohus, is partly connected with the 

 power they possess of retaining their tissues in a moist condition for a longer time than 

 the others, owing to the comparative, or even total absence in them of the integumcntal 

 pores which appear to be present in most of the other species of free Nematoids. This 

 view is supported by what I have just stated of the difference in the rapidity of imbibing 

 fluid exhibited by desiccated animals belonging to these two classes, and is still further 

 borne out by other observations tending the same way. Thus I have seen adult specimens 

 of Tylenchus tritici, when immersed in strong glycerine, instead of shrivelling up almost 

 immediately as the great majority of free Nematoids would do, continue to swim about 

 for twenty minutes in this dense medium before any shrivelling of their bodies took 

 place. Moreover, I have immersed them, as well as specimens of the genera Plectus 

 and Aphelenchus, in a magenta colouring solution and taken them out of it after the 

 same period alive and active, and with their bodies perfectly uncoloured save for a very 

 short distance from mouth, anus, or vulva ; whereas other species not belonging to one of 

 these four genera would have had their whole bodies perfectly coloured, and have been 

 dead in two or three minutes. Plectus parietinus, however, only resists the glycerine for 

 a minute or so, though it may remain alive in the magenta solution for half an hour and 

 then only have the anterior portion of its oesophagus coloured, together Avitli the external 

 parts of the rectum and vagina, and the narrow portions (Plate XXVIII. fig. 14, V, b') 

 of the vessels in communication with the lateral cervical spaces*. 



This power can, however, be looked upon only as a mere accessory contributing to the 

 greater hold on life possessed by these animals, as it fails to account for facts, when we 

 take into consideration the prolonged periods of ordinary desiccation — to say nothing 

 of absolute desiccation in vacuo — ^Avhich these animals will undergo and still recover. It 

 is not only that they have this power of resisting the efiects of desiccation, but they are 



* It is this fact whicli makes mo imagine that these spaces must be perforated, although I have been unable 

 to detect apertures even with the highest magnifying powers. May not these vessels be homologous with the 

 more abortive, lateral, excretory tubes of Leptosomaium elongatum (Trans, of Linn. Soc. vol. xxv. pi. 12. fig. 156),- 

 since wo kpow for certain that the similar vessels of Tylenchus tritici terminate posteriorly in ctccal extremities ? 



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