REVISION OF FRESHWATER SPONGES OF SPONGILLIDAE 5 



ambiguous term, it is retained in this paper for simplicity; the "bubble 

 cells" or "vesicular cells" (Blasenzellen), recorded as a reliable diag- 

 nostic feature by many previous authors, were found to lack the tax- 

 onomic importance originally accredited to them. 



Megascleres. — Under normal conditions, shape and structure of 

 the skeleton spicules remain intraspecifically constant, although 

 ecomorphic alterations can be brought about by adverse environ- 

 mental conditions; a clear differentiation between amphioxea and 

 amphistrongyla is often difficult in specimens ^vith abruptly pointed 

 amphioxous scleres, and transitional series are frequently produced by 

 the same individual; the relative width of these scleres and the width 

 of their axial canal are subject to great variations and do not repre- 

 sent reliable criteria. 



MiCROSCLERES. — Truc free microscleres ("dermals" or "flesh 

 spicules" of earlier authors) represent a very important and intra- 

 generically constant criterion, which often can also be used for spe- 

 cific discrimination. These scleres must not be confused with occasion- 

 ally abundant immature gemmoscleres (e.g., Radiospongilla cerebellata 

 and some of its congeners) , since the former are always of quite distinct 

 shape and structure; in form these scleres range from smooth or uni- 

 formly spined amphioxea to acerate scleres mth large raylike central 

 spines, and finally to stellate spicules which often are inflated to form 

 a "centrum"; minute birotulate microscleres are characteristic of two 

 spongOlid genera, irrespective of the presence or absence of birotulate 

 gemmoscleres. 



Gemmoscleres. — A new term suggested by the writer and never used 

 before by any other specialist. The gemmoscleres represent the most 

 important taxonomic criterion, in particular in those spongillids which 

 lack true microscleres, and without their presence reliable identifi- 

 cation is usually impossible; there is a full transition in shape and 

 structure of these scleres, leading from smooth or spiny amphioxea 

 and amphistrongyla to scleres with unilateral or concentric aggrega- 

 tions of terminal spines, birotulate spicules with various marginal 

 incisions, birotulates possessing rotules with entire margins, and finaUy 

 to tubelliform and parmuliform scleres; ecomorphic malformations 

 are frequent in adverse environments, resulting in the production of 

 freak scleres; similar malformations can often also be observed as the 

 probable result of hybridization. 



Gemmules. — The most useful diagnostic features of these asexual 

 reproductive bodies are structure and width of their pneumatic 

 layer, structure of their micropyle, shape and curvature of then- foram- 

 inal tubule, presence or absence of foraminal cirri, and the specific 

 arrangement of their gemmoscleres. Their actual size and shape and 

 their relative abundance are features of secondary importance, since 



