58 Notes on Mr. Cross's Acarns. 



Each of the eight legs of the Acarus sent by Mr. Cross, is 

 formed of seven joints, without including the tarsus. The 

 first is triangular, and may be considered as the hip ; the se- 

 cond and third are longer than the hip ; the fourth is longer 

 than the two preceding ; the fifth is shorter than the fourth ; 

 the sixth and seventh are longer and thinner than the others, 

 and the seventh is terminated by a little transparent tarsus, 

 which appeared to us to be bi-lobate, and provided with a 

 single claw, bent inwards. Upon the upper edge of each of 

 these joints, except the one which forms the hip, are one or 

 two straight and stiff hairs. 



The actual length of the body and head, is half a millemetre. 



The Acarus of Mr. Cross appears to constitute a new spe- 

 cies of that genus. The species to which it most nearly ap- 

 proaches, of such as are already described and figured, are 

 those found in cheese and in flour, or, perhaps more nearly, 

 Hermann's Acarus dimidiatus. It differs from the two first 

 in the absence of a false corselet, in the two longer and more 

 slender articulations which precede the tarsus, in the form of 

 the body, which is shorter, more ovoid, and more tumid, and 

 finally, by the long and numerous hairs with which the back 

 is covered. 



It is distinguished from the Acarus dimidiatus, (which has 

 a spherical body, with a false corselet, more coloured than the 

 rest of the abdomen), by the want of the little short hairs 

 which cover the surface of the eight limbs of the latter ; but 

 resembles it in the numerous diverging hairs which cover the 

 back. 



We propose to give to this species, supposing that it ulti- 

 mately appears not to have been previously described, and 

 retains its novel mode of originating, the name of the Rough 

 Acarus, (Acarus horridusj. 



Thus far we have limited our remarks to the facts connect- 

 ed with zoological details ; we have examined, described, and 

 figured, counted and measured, all the constituent parts of 

 this little animal. We have by these means really proved that 

 the phial presented to the Academy by Mr. Roberton, does 

 certainly contain the animalcule or the Acarus announced to 

 us ; and which, indeed, may be seen by the naked eye, as a 

 whitish speck. 



We now request permission to say a few words on the sin- 

 gular origin, or rather the singular creation, of an animal, 

 which, although microscopic, is so complicated in its struc- 

 ture, and holds so elevated a rank in the scale of organization. 

 Its parts, as we have seen, consist of, First, a body ; second- 

 ly, a head, formed of two lips, two mandibles, two feelers, a 



