MUSCULAR SYSTEM. 133 



axis, and moves the retroverted spines of its exterior surface 

 so as to push the animal slowly along a rough surface. In 

 the vaginiform zoophytes the internal currents of the body are 

 not produced by the contractions of the internal fleshy parietes, 

 though compared by Cavolini to a heart, but by the action 

 of vibratile cilia, and the open base of the polypi often con- 

 tracts, as if by a movement of deglutition. The polypi 

 being the organs by which the food is attracted, seized, and 

 digested, and the surface of the body aerated, they possess 

 the most complicated structure and the highest vitality. The 

 tentacula surrounding the mouth, as in the flustra carbesia 

 (Fig. 63. d } ) have generally minute vibratile cilia disposed 

 along their sides, by the action of which currents of water 

 conveying animalcules and other food, are directed towards 

 these prehensile and digestive cavities. In these compli- 

 cated and highly irritable polypi of the flustra, we perceive 

 numerous muscular bands (Fig. 63. c,) connecting the lower 

 part of the polypus with the aperture of the cell, and others 

 passing from the same part of the polypus downwards, to 

 connect it with the bottom of the cell (e.) The vibratile 

 cilia move the currents outwards along one side of each 

 tentaculum (d,) and inwards along the 

 other side, and they are generally vi- 

 sible only while they move in the di- 

 rection of the currents which they 

 produce, that is in their forward stroke. 

 When in full activity the cilia are in- 

 visible in their backward movement, 

 from the velocity with which they 

 resume their position, to commence a 

 new stroke forward. And as vibratile 

 cilia are thus generally perceptible only 

 in their slow forward impelling move- 

 ment, they have some resemblance to 

 a stream of globules flowing always in one direction. There 

 are about fifty cilia on each side of a tentaculum in the 

 flustra carbesia, and nearly forty millions on a moderate 

 specimen of the entire animal. In many zoophytes they 

 are much more numerous ; so that these fixed and plant-like 

 animals, though possessing a very low degeee of irritability 

 in their general mass, are well provided with active organs 



