8 U.S. NATIONAL MUSEUM BULLETIN 2 72 



Definition. — Megascleres invariably slender to stout amphioxea, 

 as a rule completely smooth. 



Microscleres always present in dermal membrane, usually also in 

 symplasm; they are rather slender amphioxea, conspicuously different 

 from gemmoscleres and, as a rule, entirely spined. 



Gemmoscleres, whenever present, stout and strongly spined am- 

 phioxea to amphistrongyla with a varying curvature. 



Gemmules abundant in maturing sponge, large and spherical, 

 usually scattered throughout sponge; when occasionally grouped at 

 base never form a distinct pavement layer; usually mth a moderately 

 strong granular pneumatic layer with very small nonpolygonal air 

 spaces, and with gemmoscleres embedded in this layer more or less 

 tangentially; occasionally without any trace of a pneumatic layer and 

 consequently lacking an armature of gemmoscleres; gemmules then 

 either totally unprotected, or encased in surrounding groups of normal 

 or slightly modified megascleres; foramen or foramina simple to 

 shghtly cup-shaped, never distinctly tubular. 



Sponges usually massive, or producing long cylindrical branches 

 arising from an irregular base; often of a drab green to bright green 

 color owing to the presence of zoochlorellae; occasionally without such 

 an association and consequently without particular coloration. Con- 

 sistency moderately firm but often brittle. 



Distribution of genus cosmopolitan, although all species possess 

 pronounced zoogeographical boundaries. 



Discussion. — The genus Spongilla Lamarck, as here restricted, is 

 characterized by the presence of true microscleres in both inner 

 symplasm and dermal membrane, by the structure of the gemmular 

 pneumatic layer which consists of small and nonpolygonal au' spaces, 

 and by the tangential or irregular arrangement of the gemmoscleres. 

 It consists of two distinct groups of species, differing from each other 

 in the characteristics of their microscleres as well as in some other 

 morphological features. The first group, often displaying a branching 

 mode of growth and possessing uniformly spined microscleres, is 

 represented by S. lacustris, S. helvetica, S. aspinosa, S. arctica, and S. 

 inarmata, all in need of additional revision; the second by the closely 

 related species *S'. alba, S. cenota, new species, and S. wagneri, in which 

 the sponges are more or less cushion-like, and the spines on their micro- 

 scleres increase in length towards the center of the spicule. The re- 

 mainder of the species formerly assigned to this genus are now included 

 in Eunapius Gray, Stratospongilla Annandale, and Radiospongilla, new 

 genus, respectively. 



