PHYLUM PORIFERA 



95 



Structure. -- A longitudinal section of Grantia (Fig. 56) 

 shows that the body possesses a single cavity as in Leucosolenia, 

 but the body wall is much thicker. This condition has been 

 brought about by the folding of the wall of a larval stage which 

 resembles Leucosolenia, resulting in the production of a series 

 of parallel canals. Part of these are incurrent canals and open 

 to the outside (Fig. 59, B, inc) ; the rest open into the gastral 

 cavity (G.C.), are lined with choanocytes (Fig. 54), and are 

 called flagellated chambers or radial canals 

 (Fig. 59, B,/.c). The area covered by 

 collar cells is enormously increased in 

 this way (compare the black layers in 

 Fig. 59, A and B). Water enters the 

 body of Grantia as shown by arrows in 

 Figure 59, B, by way of the incurrent 

 canals (inc.) ; from these it passes 

 through pores, called prosopyles (pr.p), 

 into the radial canals (fl.c), then through 

 the apopyles (ap.p) into the gastral 

 cavity (G.C.), and finally out of the 

 osculum (osc.~). 



As in Leucosolenia, Grantia possesses 

 an outer dermal layer of cells, an inner 

 gastral epithelium made up of collar cells 



which line the radial canals, and a middle jelly-like substance 

 in which are a number of wandering ameboid cells. The last- 

 named cells are considered by some authorities equivalent to 

 the mesoderm of higher animals, but this is probably not the 

 case. 



The skeleton of Grantia consists of calcareous spicules, of 

 which there are four varieties: (i) long, straight mona.wn rods 

 guarding the osculum, (2) short, straight monaxon rods surround- 

 ing the incurrent pores, (3) triradiate spicules always found em- 

 bedded in the body-wall, and (4) T-shaped spicules lining the 

 gastral cavity ; four- and five- rayed spicules may also be found. 



FIG. 55. A simple 

 sponge. (After Minchin.) 



