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 DUJARDIN 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 

 (Bracliionus, Anurcea, &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 FONTANA took 

 this structure for a heart, and its motions of grasping and opening, 

 as the first of these authors so aptly describes them 1 , 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 2 . 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 Scvende vervolg der Brieven, Delft, i 702, 144 ste Missive, bl. 405. 



2 Such was the determination of NITZSCH in 1824 on the genus Bracltiomix. 



