OP TIIE NEMATOIDS, PAEASITIC A]ST) FEEE. 617 



tinuation of their kind by the ordinary processes of reproduction. Whilst the sum total 

 of these periods of active life, peculiar and necessary to the species, is in all probability 

 pretty definite, the period of time over wliich its fragmentary existence may be extended 

 is altogether variable and indefinite, owing to the uncertain length and number of the 

 interpolated periods of desiccation and apparent death. Whilst we meet with this admi- 

 rable adaptation to external conditions in the animals of these genera, there are many 

 species of the genera Mononchus and Borylahmis found in some of the same situations, 

 which are nevertheless frail and incapable of resisting desiccation. Many species of the 

 genus Plcctus also are to be met with which do lead a constantly submerged existence, 

 and to whom therefore this remarkable tenacity of life would not be a matter of so 

 much importance. These facts show that M. Davaine's conclusions on this subject were 

 drawn from insufficient data. 



An examination of tufts of moss from the roofs of houses* or from old walls, as well 

 as of specimens of the yellow lichen, Parmelia x>arietina, from the same situations, has 

 invariably revealed to me three principal kinds of animal occupants — specimens of Ro- 

 tifers, of peculiarly slow-moving arachnidal Tardigradaf, and two or three different 

 kinds of Anguillulidce. Precisely the same varieties of animal life are spoken of as 

 existing in the tufts of moss examined by Spall anzajs^i, and also in those which were 

 experimented upon by Doyere and Gavaeret. Moreover I have found specimens of 

 lichen brought from Sweden tenanted by just the same types. In all the specimens of 

 moss and lichen of the kind above mentioned that I have examined, I have invariably 

 found that the free Nematoids present were representatives of my two genera Plectus 

 and Aphelenchus, It seems therefore highly probable that the so-called " Anguillulae" 

 of preceeding observers were also representatives of these genera. The specimens met 

 with were of very different ages, many containing well-developed ova within them, and 

 the property of reviviscence seems common to old and young alike. Possibly there 

 may be a slight difference in the degree of tenacity of life enjoyed by the immature and 

 the adult animals respectively, but, at all events, no notable difference exists, such as we 

 have ascertained to be the case with Tylenchiis tritici. After what has already been said, 

 it would be useless for me to give further details concerning the powers of resisting the 

 effects of prolonged and complete desiccation possessed by these animals whilst still in 

 their natural habitat, whether this be sand, moss, or lichen, but I can add a few inter- 

 esting facts concerning the extreme curtailment of this power when the animal is 

 allowed to undergo the process of desiccation on a slip of glass, isolated from all other 

 materials and thus freely exposed to the air ; and also concerning the results of other 

 experiments, made with free Nematoids not belonging to one of the four genera before 

 named. 



In submitting the animals to these tests, I was careful to select active uninjured 



• Belonging to the genus TorUda : often found in hcmisphcrieal masses. 



t For some particulars concerning the anatomy of these singular animals see Archiv fiir llicrosk. Anat, 

 erstc Band, 18G5. 



MDCCCLXVL 4 Q 



