138 HALF- AX-HOUR AT THE MICROSCOPE. 



The exact position of the Pycnogons in the animal kingdom 

 has been a subject of much controversy. There can, however, 

 I think, be no doubt that they are rightly placed with the 

 Crustacea, and are not Arachinda. The study of the develop- 

 ment completely proves this. 



The sensory hairs on the limbs of Amuiolhea, their special 

 development at the elbow-joints, and their mode of implantation, 

 are points worthy of careful study in connection with the dense 

 integument. 



Hypopus muscorum — We are here presented with a physio- 

 logical puzzle, which from the interest attaching to it, demands 

 our best consideration. What is Hypopus ? 



According to received authority it is an Acarine parasite found 

 on flies, not uncommonly, although generally believed to be rare, 

 and supposed to be a larval condition of Gamasus Coleoptratoriim^ 

 the fellow sent round on Dr. H.'s interesting slide of " Parasite of 

 Watchman Beetle." See Hypopus in Micro-Die. 



In November, 1869, G. J. Mclntire, in a highly interesting 

 paper on Pseiidoscorpions (which appeared in Sci. Gossip) published 

 a short account of the life-history of a species of Hypopus, which 

 had come under his notice as a parasite on Obisium orthodactyhun, 

 one of the Chelifers. He was, I believe, the first to observe 

 them living. A tolerable figure was given of this, and another, a 

 larger species, casually mentioned. The same author read a paper 

 {Monthly Micro. Jour., Jan., 1874, p. 1) headed " Notes on 

 so-called Acarellus," before the Roy. Micro. Society, in which he 

 details observations which seem to all but prove that the species of 

 Hypopus which have come under his notice, are a stage in further 

 development of Acari resenibling Cheese-mites. The species he 

 so nearly traced out, was found in its early stage on a decayed 

 potato. "About this time (1869), I noticed that many cells were 

 infested with another Acarus " (than the Hypopus of the Obisium) 

 "very like a Cheese-mite." "A group of half-a-dozen of them 

 would be seen devouring the decaying malt and other substances ; 

 in fact, wallowing in the filthy mess they made, like so many pigs ; 

 and in some cells all the corners where the pabulum of the 

 Podurae, or their excrement had accumulated, were occupied by 

 similar groups of these disagreeable-looking Acari." 



Two years ago, however, in the month of September, I picked 

 up a decayed potato, which had such a large population of these 

 Acari upon it, that I was induced to give it some close scrutiny, 

 chiefly with the view to satisfy myself whether these mites were 

 the so-called " Cheese-mites " [Acarus domesticus), or another 

 species. I soon saw that there was a remarkable change going on 

 in the case of the greater number of them. They were casting 

 their skins, and when this operation was complete, they had 



