56 Notes on Mr. Cross's Acarus. 



Viewed with the naked eye, while still in the phial, it ap- 

 peared merely as a whitish speck ; its specific gravity causing 

 it to remain at the bottom of the vessel. 



The magnifying glass rendered visible a small oval body, 

 bristled with long diverging hairs. 



Having taken it out of the alcohol, dried it as much as pos- 

 sible, and then placed it between two plates of glass, with a 

 thin layer of varnish, in order to render all its parts more 

 transparent, and consequently easier to study, we placed it 

 under the microscope, applying a power which magnified the 

 diameter 280 times. Examined under these circumstances, 

 we saw that the body was of an oval form, and that the sto- 

 mach was slightly flattened, and the back very much arched, 

 particularly towards the posterior part of the body. The dor- 

 sal surface was studded with a profusion of small papilla, a 

 certain number of which, larger than the rest, and distributed 

 here and there among them, served as bases or bulbs to some 

 long hairs or bristles, which pointed in every direction, and 

 the greater number of which were at least as long as the bo- 

 dy of the animal. 



These hairs, standing erect on the arched back of the 

 Acarus, gave it very much the appearance of a microscopic 

 porcupine ; its resemblance to that animal being still farther 

 heightened by the lengthened snout. 



Viewing it as a transparent object, we have not been able 

 to discover any traces of a stomach, ovary, or lateral pulmo- 

 nary lobes. 



The situation of the anus is faintly indicated by a slight 

 indentation situated in the direction of the median line, and 

 at the posterior part of the abdomen. 



We however saw very clearly a large oval egg, like those 

 which are perceived, often to the number of two, three, or even 

 four, in the transparent body of the female domestic Acarus, 

 both of cheese and flour, and in those of the same sex in the 

 Acarus of the human body, as represented in the drawing 

 which we have had the honor to submit to the Academy. — 

 This egg, which is similar in shape at both ends, and -f of a 

 millemeter in length, happens, rather singularly, to occur in 

 the solitary individual sent by Mr. Cross, as if chance were 

 willing to furnish us with a material proof of the well-known 

 mode of re-production of the Acari, in the very species which 

 had been supposed to be producible at will, merely by the 

 aid of elementary molecules floating in space. 



From the anterior part of the body projects a sort of head, 

 terminating in a lengthened snout; the separate parts of 

 which it was very difficult to examine, but we could, howev- 



