844 MR JAMES MURRAY ON 



Encystment of Tardigrada and Acarina Compared. 



In the foregoing accounts of the encystment of Macrobiotics dispar and Glyco- 

 phagus domesticus there are many points of correspondence, as well as some important 

 differences. 



The essential points of agreement are the formation, within an ordinary skin of the 

 animal, of an inert protoplasmic mass, protected by a peculiar skin, and which has lost 

 by retrogression all trace of external organs, and apparently of most of the internal 

 organs, and the final emergence, by the opening of a posterior trap-door in the case, of 

 an animal having the full complement of organs. 



The chief difference is that the case of the Mite is a real skin, that of the Tardigrade 

 appears to be merely a dense secretion from the skin. This is not, however, demonstrated, 

 and the outer case of the cyst may consist of a real skin, as well as the secretion. 

 The reason for thinking that it is a secretion only is the withdrawal of the claws, 

 which in ordinary changes of skin are thrown off with the rest. 



Even if the case is only a secretion, there is the original skin of the animal to corre- 

 spond to the hypopial case of the Mite, as encystment is always preceded by an ordinary 

 moult. 



The Mite is known to encyst, when it does so at all, at a definite stage in develop- 

 ment, immediately before attaining to maturity. The Tardigrade is judged from its size 

 to be full grown, but this also is nut yet demonstrated, and it may be that here also it 

 occurs at the corresponding stage of development, just before reaching sexual maturity. 



Simplex Forms of Tardigrada. 



We may now inquire whether the phenomena of encystment throw any light on 

 the puzzling questions of simplex forms. As a simplification of the Tardigrade takes 

 place during encystment, is it possible that the simplex forms are connected with this 

 process ? 



Simplex forms are very common. They are known in nearly every species of 

 Macrobiotus, in several of Diphascon, and it is likely that all Tardigrada possess them. 



The name was given to them by Richters (31), in recognition of the fact that 

 Plate's Doyeria simplex was nothing but a peculiar condition of some species of 

 Macrobiotus. 



The peculiarity of the simplex form is the reduction of the manducatory apparatus. 

 The teeth are reduced in size, have no furca or bearers, and are simply little, straight, 

 pointed stylets, which do not even reach the mouth or gullet, and therefore cannot be 

 functional. The rods in the pharynx are usually quite abortive ; the gullet becomes a 

 very slender tube (fig. 7). This is the usual simplex state, but the reduction may go 

 further, and the teeth, rods, gullet, and mouth may totally disappear ; and though in 

 these cases the muscular bulb of the pharynx usually persists, that is occasionally also 



