EFFECT OF DESSICATION. 



53 



In many cases, however, the circumstances and environment are by 

 no means favourable. It may be stated generally that some degree 

 of moisture is necessary to ensure further growth. In dry localities, 

 the eggs lose their power of development, not merely for the time, 

 but permanently, while in damp localities and in water they retain 

 this power for a considerable period. In this respect the eggs agree 

 with the fuR-grown animals, as do also, even to a greater degree, the 

 embryos, which are frequently hatched in the body of the host and 

 then evacuated. 



We cannot, however, make a hard and fast rule, since there are 

 a number of Helminths whose eggs and embryos can withstand com- 

 plete desiccation with impunity : these are chiefly Nematodes, a group 

 which will be considered later on. The Nematoda, in spite of the 

 simplicity of their organization and development, or perhaps rather 

 because of it, display a variation in the conditions under which they 

 live greater than that of any other group of Helminths. Not only 

 are there parasitic, semi-parasitic, and free-living species, but numerous 

 others, that infest plants, in many of which (wheat, rye, teazle, and 

 clover) they give rise to actual diseases. That these parasites are 

 liable to undergo a process of desiccation at regular intervals is hardly 

 surprising, considering the periodicity of the developmental cycles of 

 the plants which serve as their hosts. As an instance may be cited 

 the wheat-grains which are infested by the young of a Nematode. 

 When the seed is sown, the young parasites are brought into condi- 

 tions favourable for their migration and further development.^ This 

 capability of withstanding desiccation is not, however, confined to 

 Nematodes parasitic upon plants, but is occasionally found in those 

 species that infest animals. 



Via. 32. — A, Eggs from Ascai'is tumbricoides, and £, Trichoceplialus dispar j a, fresh froiil 

 the faeces ; J, after long exposure to the open air. 



To investigate the influence of desiccation upon the capability for 

 development possessed by the eggs of Nematodes, I have made use 



' See the excellent researches of Davaine on Anguillvla trit'u: 

 or (more in detaU) M6m. Soc. Bt^lZ^d^UypMimOS^® 



Vlnstit, p. 330, 1855,- 



