Avoidance of Silt 



253 



crustacean, Asellus, and the nymph of the mayfly, 

 Casnis. Both Hve in muddy bottoms where there is 

 much fine silt. Both possess paired plate-Hke gills. 

 In Asellus they are developed underneath the abdomen ; 

 in C^nis upon the back. In Asellus they are double; 

 in Caenis, simple. In 

 Asellus they are blood 

 gills; in Cccnis, tracheal 

 gills. In both they are 

 developed externally in 

 series, a pair correspond- 

 ing to a body segment. 

 In both they are soft and 

 white and very delicate. 

 But in both an anterior 

 pair has been developed 

 to form a pair of enlarged 

 opercula or gill covers. 

 These are concave pos- 

 teriorly and overlie and 

 protect the true gills. 

 The gills have been ap- 

 proximated more closely, 

 so that they are the more 

 readily covered over ; and 

 they have developed in- 

 terlacing fringes of radi- Fig. 155. The nymph of the mayfly 

 ating marginal hairs, Caenis, showing dorsal gill packets. 



which act as strainers, 



when the covers are raised to open the respiratory 



chamber. 



Such are the mechanical means whereby suffocation 

 in the mud is avoided. It must not be overlooked that 

 there is a physiological adaptation to the same end. A 

 number of soft bodied thin-skinned animals have an 

 unusual amount of haemoglobin in the blood plasma — 



