54 MANUAL OF ANIMAL BIOLOGY 



under the surface of the water. The top of the vase has a tiny 

 circular opening, or osculum, through which there is a continuous 

 flow of water outward from the central gastral cavity. The 

 latter extends practically the entire length of the animal's body. 

 The general body plan can, perhaps, best be seen in a longitudinal 

 section passing through the entire length of the animal from base to 

 osculum and thus dividing it into halves. Such a section will 

 reveal a comparatively thick body wall enclosing the gastral 

 cavity. (W. f. 35, C.) 



The outer surface of the body wall appears fairly smooth and 

 continuous when examined with the naked eye. However, when 

 microscopically examined under the proper conditions it will be 

 found to be perforated with thousands of tiny openings, or ostia, 

 which lead into the horizontal incurrent canals. The walls 

 of the incurrent canals contain pores (prosopyles) which connect 

 with flagellated radial canals. The latter lie parallel to the 

 incurrent canals and open into the central gastral cavity but do 

 not extend entirely to the outer body surface. During life there is 

 a continuous flow of water through the incurrent canals, prosopyles, 

 and radial canals, then into the gastral cavity, and, finally, to the 

 exterior through the osculum. This flow of water in Grantia is 

 brought about by the action of the flagellated collar cells, or 

 choanocytes, lining the radial canals and is essential to its life 

 processes, as will be noted in the later discussion. (W. fs. 35, B ; 

 36, B.) 



A study of the early development of Grantia discloses the fact 

 that the body wall consists primarily of three layers, the cellular 

 elements of which reveal considerable structural and functional 

 differentiation. There is, first, an outer dermal region which 

 covers the surface of the body. It contains the incurrent canals, 

 and we may regard these as arising through a series of parallel 

 infoldings of the outer dermal layer. Second, there is an inner 

 gastral region which forms the lining of the gastral cavity. It 

 contains the radial canals, and we may regard these as arising 

 through a series of parallel outfoldings of the inner gastral layer. 

 Finally, there is the so-called middle region which is composed 

 of a jelly-like matrix containing amoeboid cells in abundance. It 

 lies between the dermal and gastral layers and thus separates the 

 incurrent and radial canals, and is perforated by the prosopyles 

 which, as noted above, connect these two systems of canals. 



