THE ENCYSTMENT OF MACBOBIOTUS. 7 



Pond at Nerston, near Glasgow, 1904 ; Loch Tay, 1905 ; 

 pond near Edinburgh ; Shetland ; Franz Josef Land ; Spitz- 

 bergen. 



Encystment. — The eggs of M. dispar appear to be very 

 rare. They have only been seen on three occasions, and only 

 once was the embryo seen in the advanced stage of develop- 

 ment which led to the discovery of its connection with the adult 

 animal. 



While carefully searching for eggs in a very abundant collec- 

 tion of the animals from the pond at Nerston, I noticed many 

 little dark-coloured skins, bearing six stumps of limbs without 

 claws, which, to my surprise, proved to belong to the Macro- 

 biotus. I was thus led to study them carefully, and observed the 

 remarkable series of changes which I now recount, 



Most of the animals appeared to be moulting ; the normal, 

 smooth, transparent skin was loosened, and the animal in its 

 new skin could be seen moving inside. This may be regarded as 

 an ordinary moult, as many emerged from the old skins, and 

 continued the metamorphosis outside. Very often, however, the 

 whole process took place within the original skin. After this 

 moult the skin of the animal differs from the original skin — it 

 is yellow and punctate, while the old skin is hyaline, colourless, 

 and smooth. The new skin appears also to be viscous dorsally, 

 as there is generally much extraneous matter adhering to the 

 back, which becomes of a rich umber colour. Gradually it con- 

 tracts, till it is little more than half its original length, but 

 remains relatively broad ; it assumes a definite form (figs. 3 

 and 4), only slightly reminiscent of the adult form ; the colour 

 darkens till it becomes grey, purple, or well-nigh black ; finally 

 it becomes quite rigid, with brittle shagreened skin, and six little 

 stumps of legs, without claws (fig. 3). This I call the outer cyst. 

 The last legs have been drawn in before the hardening of the 

 cyst, so that they do not show conspicuously like the others, and 

 the claws of the other legs have also been withdrawn. If one of 

 these cysts is squeezed shortly after its formation it breaks up 

 and reveals a normal adult, having all the usual organs — claws, 

 pharynx, teeth, stomach, eyes, &c. If it is examined at a some- 

 what later period it is found that it contains, instead of the adult 



