VII. GRANTIA 



In our previous consideration of Volvox it was noted that each 

 of the somatic cells of the organism " is a balanced structural and 

 physiological unit capable of carrying on the essential life processes 

 independently of the other cells of the colony." A structural and 

 physiological differentiation of the cellular units, save for reproduc- 

 tion, was not apparent. A somewhat different and more advanced 

 condition with regard to the cellular units is to be found in Grantia, 

 which may now be indicated. 



Grantia is a marine animal of low development which belongs to 

 the phylum Porifera, or Sponges, as they are more commonly called. 

 This group is generally regarded as constituting the most primitive 

 group of the true Metazoa, that is, multicellular animals in which 

 the numerous constituent cells of the mature organism are not all 

 alike, as in Volvox, but, on the other hand, are structurally and 

 functionally differentiated, so as to perform certain definite activi- 

 ties for the benefit of the organism as a whole. The differentia- 

 tion of the cells in Sponges is very meager as compared with that 

 present among the cells of the higher Metazoa, such, for example, 

 as a Frog, but it is sufficient to mark the separation from the multi- 

 cellular protozoan types, noted in Volvox. 



The Sponges as a group are characterized by the presence of 

 peculiar secreted spicules in the body wall, which constitute the 

 essential skeletogenous, or supporting, elements. In Grantia these 

 skeletal elements are calcareous in their nature, but in other 

 species of Sponges they may be composed of silica or of a character- 

 istic organic material, known as spongin, which gives the bath 

 sponge of commerce its valuable qualities. 



A. Structure of Grantia 



Grantia is a small, vase-shaped animal measuring about five- 

 eighths of an inch in length and one-fourth of an inch in diameter 

 through the largest part of the body, which is near the center. The 

 base, or closed end of the ' vase,' is normally attached to a solid, sub- 

 merged object, such as a piece of wood or stone, lying some distance 



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