POLYGASTRIC ANIMALCULES; BACILLARLE. 551 



a subsequent time. Not unfrequently a second generation may be 

 observed within the globes still contained in the parent structure; 

 and, with a good light, a third generation within these again. 

 The animals of this family do not always form, by their aggrega- 

 tion, a globular structure ; but are sometimes united in flat- 

 tened clusters of various forms. 



1130. The family of Bacillariae is a very interesting one, 

 since it includes nearly all the Animalcules, whose fossil remains 

 (Fig. 627) have yet been discovered. A large number of the 

 species composing it were formerly regarded as belonging to the 

 vegetable kingdom ; and, even now, considerable doubt may be 

 entertained as to the claim of all to be regarded as Animals. 

 This claim seems to rest more upon their general organisation, 

 than upon the movements they exhibit ; for the latter are very 

 slight, and not distinctly referable to spontaneous action. In 

 the greater number of species of this order, we find the body 

 inclosed in a siliceous sheath of considerable firmness ; some- 

 times, however, the envelopes are simply membranous : but no 

 other mineral matter than silex has ever been detected in them. 

 Their forms are extremely various. Most of them are elongated, 

 like the Navicula, whose siliceous sheath has somewhat of a 

 prismatic form. It has six openings, two at each end, and two 

 in the centre, the latter being the largest. Through the latter 

 it would appear that food enters the body. That there is an 

 internal digestive cavity, is made apparent by the accumulation 

 of colouring particles, which the transparency of the siliceous 

 shield allows to be distinctly seen. Along the sides of the body 

 are seen dark masses, which appear to be the organs for the 

 evolution of ova ; and the germs are said to escape from these, 

 through the apertures at the extremities. There is much uncer- 

 tainty, however, as to the vital actions of these beings. Many of 

 them have a tendency to aggregation ; and thus very curious 

 forms result. 



1131. The Vorticellince are a very interesting family to the 

 Microscopic observer, from the variety of their forms and actions, 

 and from their great abundance in most pools in which vegeta- 

 tion is going on. The ciliary action is here very distinct and 



