Htjdatinaea.^ THE INFUSORIA. 355 



(maintaining themselves passive). The alimentary canal 

 has no true stomach, the posterior diminishes, and the in- 

 ternal surface is provided with delicate vibratile cilii ; it 

 sometimes appears grape-like, from having semi-lunar 

 valves, which form little lateral pockets or stomachs : the 

 ova often occupy a large portion of the body. In most 

 cases, the creature fixes itself to a spot by its foot, and lays 

 several eggs upon the same place, one after another, by a 

 sudden contraction ; sometimes, when it is going to lay 

 more eggs, it returns to the original spot. In eleven hours 

 after the eggs were laid vibration of the anterior cilii was 

 observed, by Ehrenberg, within them ; and in twenty-four 

 hours the young escaped from the shell. Many of the ova 

 have a double shell, and leave a bright space between each 

 at one of the extremities, and such ova are found in other 

 rotatory animalcules, having different shapes. In these, 

 the young are slowly developed. Ehrenberg names them, 

 " lasting eggs, or winter eggs." Some eggs are covered 

 with Hygrocrocis, and appear quite hairy; these have 

 been regarded as the normal state of other ciliated animal- 

 cules. Two kinds of diseases destroy the Hydatina, and 

 most of the Rotatoria: 1st, the formation of vesicles, or 

 little bladders, which give rise to the appearance of small 

 rings all over the creature : 2nd, the formation of granules, 

 from which all the internal organs appear as if composed 

 of delicate granules and shagreened ; a third disease 

 may be the overgrowth of algae upon their bodies. Foul 

 water likewise kills them. Fig. 394 represents a vibrat- 

 ing animalcule completely unfolded, seen from the ventral 

 surface. The arrows in the alimentary canal indicate a 

 decussating, or circulating movement of its contents, 



2a 2 



