APPLICATION TO METAZOA. 123 



of the metazoa, and the reason for it seems to be not 

 far to seek ; for an animal as large as Actinosphsera, 

 composed as it is of protoplasm, which is externally 

 fluid and radiating, could not maintain its spherical 

 shape without the support given by the intersecting 

 membranes inside of it. This seems to be the pri- 

 mary function of cell walls, wherever we find them. 

 No animal could attain to any more than a micro- 

 scopic size, and possess any permanence of shape or 

 rigidity of body, without the aid of cell walls ; and 

 we find in the simplest of metazoa, and in the sim- 

 plest tissues of all metazoa, that the cell walls form 

 an indispensable means of support. Wherever we 

 find them, they are the means of support, and, in 

 comparison of their supporting function, their func- 

 tion in limiting physiological activity sinks to a sec- 

 ondary importance. Among the protozoa, portions 

 of protoplasm, which are not separated from each 

 other by cell walls, perform separate functions, — as, 

 for instance, the digestive and contractile parts of 

 a vorticella, — thus showing that a physiological limit 

 and a cell wall are not identical. Neither must we 

 suppose that the many-celled animals are of a higher 

 grade of complexity than the single-celled animals, 

 for — leaving sponges aside — none can maintain 

 that the sluggish Hydra is a more highly developed 

 organism than the graceful Stentor or Paramcecium, 



