196 CLASS VI. 
subjected to the will of the animal, for otherwise the vibratile cilia 
would be in a constant motion, which ceases only on death. 
The Wheel-animalcules are capable of contraction in a remark- 
able manner, many of them assuming thereby an oval form. This 
faculty of contraction gave occasion to the name Systolides, by 
which DusarpIn wishes to distinguish this class of animals, but 
which probably will not supersede that of Rotatoria. In some the 
integument is hard and rigid, so as to form a shield or a shell 
(Brachionus, Anurea, &c.). In most there is a caudiform appendage 
on the abdominal surface (EHRENBERG names it processus pediformis 
or pseudopodium), which can be drawn in and out annularly like 
a telescope, and ends in a suctorial disc or in a forceps; by it the 
Rotatories fix the posterior extremity of the body, whenever, being 
at rest, they set the wheel-organ in motion. 
The intestinal canal is straight, in by far the greatest number 
of species, and the anus is found at the hinder end, at the base of 
the tail. At the commencement of the intestinal canal, behind the 
oral aperture, is a muscular organ of cylindrical form armed with two 
lateral horny jaws. LEEUWENHOECK, BAKER and FonrTana took 
this structure for a heart, and its motions of grasping and opening, 
as the first of these authors so aptly describes them!, for the con- 
traction and expansion of the heart; whereon FonTANA expresses 
his surprise that such motions should be dependent upon the will 
of the animal. The lateral jaws indicate a similarity of form with 
articulate animals, the insects and crustaceans, and some writers 
have even supposed that the Wheel-animalcules may be regarded 
as very simply organised crustaceans®. On the whole, by inserting 
these animals between the intestinal and the articulate worms, the 
nearest affinities and natural place of the class are not indicated; 
but in an arrangement that gives the classes in succession, there 
must always be much that is arbitrary, for the affinities cannot be 
represented by a single ascending series. 
The lateral jaws present themselves under two forms. In the 
greater number they consist of two pieces; the posterior serves as 
a pedicle, for the attachment of the muscles of mastication; the 
anterior passes transversely inwards at a right or obtuse angle, and 
1 Sevende vervolg der Brieven, Delft, 1702, 144 ste Missive, bl. 405. 
2 Such was the determination of NirzscH in 1824 on the genus Brachionus. 
