"i PHYLUM PORIFERA 83 



immediately below the flagellate endoderm cells of the 

 flagellate canals, and in the same situation are to be found 

 developing embryos (em, em'). 



The simplest sponges are vase-shaped or cylindrical in 

 form, either branched or unbranched, and, if branched, 

 with or without anastomosis or coalescence between neigh- 

 bouring branches. But the general form of the less sim- 

 ple sponges differs widely from that of such a branching 

 cylinder as is presented by Sycon (Fig. 34). 



From the point to which the embryonic sponge becomes 

 attached, it may spread out horizontally, following the ir- 

 regularities of the surface on which it grows, and forming 

 a more or less closely adherent encrustation like that of an 

 encrusting lichen. In other cases the sponge grows at first 

 more actively in the vertical than in the horizontal direction, 

 and the result may be a long, narrow structure, cylindrical 

 or compressed, and more or less branched. Sometimes 

 vertical and horizontal growth is almost equal, so that event- 

 ually there is formed a thick, solid mass of a rounded or 

 polyhedral shape, with an even, or lobed, or ridged surface. 

 Very often, after active vertical growth has resulted in the 

 formation of a comparatively narrow basal part or stalk, 

 the sponge expands distally, growing out into lobes or 

 branches of varying forms, and frequently anastomosing. 

 Sometimes after the formation of the stalk with root-like 

 processes for attachment, the sponge grows upwards in such 

 a way as to form a cup or tube with a terminal opening. 

 Sometimes the sponge grows from a narrow base of attach- 

 ment into a thin flat plate or lamella ; this may become 

 divided up into a number of parts or lobes, which may 

 exhibit a divergent arrangement like the ribs of an open 

 fan. 



