124 ANIMALS KILLED IV, 
tail is detached and, by contracting the anterior 
part, a new ftep is made. The operation is re- 
peated, and the animalcule pafles fo actively 
along as foon to traverfe the field of the micro- 
{cope. . 
This method of progreffion, by means of con- 
tracting and extending the body, is common to 
many infects, but efpecially to apodal vermes, as 
is well known. A circumftance peculiar to the 
wheel animal, is fixing itfelf by the point of the 
tail, which is effential to its regular progrefs, that 
without this precaution, it would have no motion 
except contortion or undulation. When the 
animal has found fome points of fupport, and. is 
fixed by the tail, as to acentre, it frequently {tops 
for fome time, and {tretches out the fore-part, as 
if examining around which way fhould be taken ; 
then, fuddenly detaching itfelf, it advances in a 
given direction. In Leeuwenhoeck’s opinion, the 
wheeler fixes itfelf by the three points terminat- 
ing the tail. At firft, I likewife thought them all 
neceflary ; however, with more attention, I per- 
ceived that the middle point only was ufed. Te 
fee it diftinétly, the drop muft be thin and tranf- 
parent, and free of fand; then we eafily perceive, 
that fo far from the lateral points fixing to the 
place of pofition, they do not even touch it, but 
are at a confiderable diftance, and the middle 
point is the only fixing one. When viewed with 
a 
