324 



WHEEL-ANIMALCTJLES ; HYDATINA. 



tendency to the Vermiform character, exhibited in the elongated 

 shape of the body, the position of the 

 eyes and mouth at one extremity of it, 

 the narrow, straight, intestinal canal 

 having its second orifice near the other 

 extremity, and the mode of locomotion 

 when the tail is detached and the 

 operation of the wheels suspended. 

 This tendency, however, is not so 

 marked in some species of the class, 

 which are in many respects more high- 

 ly developed than this ; for example, 

 the Hydatina senta, an animalcule 

 not uncommon in vegetable infusions. 

 Here the body is somewhat funnel- 

 shaped ; and the cilia are arranged 

 around the wide mouth, in two con- 

 centric rows. The outer row consists 

 of a simple continuous circle arranged 

 upon the edge of the body ; but the 

 inner one is made up of eleven groups 

 of cilia, attached to distinct muscular 



lobes. The integument appears to consist, as in the Rotifer, of 

 two membranes, both of which are soft and flexible. To the inner 

 one are attached four pairs of longitudinal muscles, by which 

 the chief movements of the body are performed. The tail has 

 separate muscles for its retraction or protrusion ; and others are 

 seen at the large extremity of the body, which fold up the ciliary 

 apparatus. The mouth is situated in the centre of the large 

 extremity of the body ; it leads, through a short oesophagus, to 

 the first stomach or gizzard, the jaws at whose entrance are pro- 

 vided with five or six pairs of teeth. Into this cavity a pair of 

 secreting ca3ca open ; and between the termination of these is 

 the commencement of the intestinal tube ; the upper part of 

 which, being very distensible, may perhaps be regarded as a 

 second stomach, since in this the digestive process is principally 

 performed. The intestine terminates, as in the Rotifer, near the 



FIG. 638. HYDATINA SENTA ; a 

 rows of cilia ; b, muscles of the 

 jaws ; c, stomach ; d, enlarged ter- 

 mination of the intestine ; e, anus ; 

 /, salivary glands ; g, ovaria ; h, 

 dorsal vessel. 



