STRUCTURE OF WHEEL- ANIMALCULE. 319 



joints, each of which has a pair of prongs or points. These 

 joints can be drawn up within each other, like the sliding-tubes 

 of a telescope. Within the external integument there are four 

 longitudinal bands running from end to end ; these are probably 

 bundles of muscular fibre, by the contraction of which the body 

 may be shortened. The cilia are disposed in two circles, form- 

 ing what are termed the wheels. By the successive vibration of 

 these, the appearance of a continual rotation is produced ; and 

 their action creates rapid currents in the surrounding fluid, by 

 which the supply of food is obtained. A sort of whirlpool is 

 created by each wheel, which brings towards the mouth the 

 minute animalcules and other particles floating in the neighbour- 

 hood ; and those which are not swallowed, are carried off by a 

 return-current. Between the wheels, the head is occasionally 

 protruded, bearing the two red spots supposed to be eyes ; and 

 on its under surface there is a projecting spike (c), which is 

 observed to be tubular, and which is believed to act as a syphon 

 for the introduction of water into the general cavity, for the 

 purpose of respiration. In the organisation of the alimentary 

 canal, we observe a striking resemblance to that of the Ascidiform 

 Polypes (Chap. XXI Y.) The oesophagus terminates in a sort of 

 gizzard, provided with regular teeth at its entrance (d). These 

 teeth are two in number on each side, and are fixed upon hard 

 jaws, moved by powerful muscles, so as to work between each 

 other. All the food which is swallowed is submitted to their 

 action, before it enters the first stomach ; and when the cilia are 

 in operation, these jaws are always in regular movement. From 

 the first stomach there passes off in the Rotifer a long straight 

 intestine (/,/), which terminates without any dilatation except 

 near its close (#), just at the commencement of the tail. But in 

 many other Rotifera we find the gizzard opening into a larger 

 cavity, which may be regarded as the true digestive stomach. 

 Near the termination of the intestine is the opening of the pas- 

 sages, by which are extruded the eggs (&), that are formed in the 

 large ovaria. These eggs often attain so great a degree of deve- 

 lopment, while yet within the body of the parent, that the ciliary 

 movements of the embryo may be seen ; and the young may be 



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