220 DAVID BRYCE ON THE 



being apparently absolutely dry. In some books of reference 

 I have seen statements which implied that all Kotifera could 

 submit to desiccation and still survive. The real fact is that 

 this remarkable property is practically confined to the Bdelloid 

 Rotifera. In all the numerous samples of dry material which 

 I have examined, I have been able to find and to revive only 

 four different species which did not belong to the order of the 

 Bdelloida, and even in this order the property is by no means 

 universal. As is well known, the reproduction of Bdelloid 

 rotifers takes place in two distinct fashions : in some genera the 

 various species produce living young ; in the other genera the 

 various species produce eggs, whose segmentation does not com- 

 mence until after their extrusion by the parent. It is found as 

 a general rule that viviparous species do not live in situations 

 where they are liable to be frequently dried up. Such places 

 are almost exclusively inhabited by species which are oviparous. 

 There is thus presumption of a direct connection between the 

 habitat and the method of reproduction. In dry mosses one 

 expects therefore to find only oviparous species, and very few 

 exceptions to this rule are known. 



The process of preparing themselves for their recurrent 

 passive condition can be shortly described. By some means 

 or another the animals are sensitive to the approach of the 

 danger point of the evaporation of the water in which they 

 are living. When thus warned they become restless, and 

 finally utilise the telescopic capacities of their structure to 

 withdraw the head and neck, and the foot, within the skin 

 of the central body. From the skin now remaining ex- 

 posed is exuded a liberal covering of some secretion, which 

 hardens into an airproof coating as the animal is gradually left 

 dry by the vanishing water. To be effective, the operation 

 of exuding the secretion seems to demand a certain amount of 

 time. If the animal be allowed to dry up quickly, it invariably 

 fails to revive again if kept more than a very few minutes before 

 being remoistened. On a glass slip that is free from sand or 

 fragments of detritus, the drying takes place too rapidly. If, 

 however, there is present a little sand around which the water 

 clings, the rotifers have a better chance of assuming their air- 



