84 MANUAL OF ZOOLOGY SECT. 



with or without anastomosis or coalescence between neigh- 

 bouring branches. But the general form of the less simple 

 Sponges (Fig. 35 bis] differs widely from that of such a 

 branching cylinder as is presented by Sycon (Fig. 33). 



From the point to which the embryonic Sponge becomes 

 attached, it may spread out horizontally, following the 

 irregularities 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, 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 

 which frequently coalesce when they come in contact. 

 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. 



Sycon belongs to a type of Sponges intermediate between 

 the very simplest forms on the one hand, and the more 

 complex on the other. The simplest and most primitive 

 of known Sponges is. one named Ascetta primordialis 

 (Fig 36). It is vase-shaped, contracted at the base to 

 form a sort of stalk, by the expanded extremity of which 

 it is attached ; at the opposite or free end is the circular 



