4 U.S. NATIONAL MUSEUM BULLETIN 2 72 



advice; to Miss I. Bennett, University of Sydney, for helpful criticism; 

 to Dr. J. C. Yaldwyn, Australian Museum, Sydney, for the loan of 

 important literature and advice; to Mrs. Christine S. Gee, Greenwood, 

 S.C., for valuable information on her late husband's research material; 

 to Mr. A. Badger, Columbia, S.C., and Miss P. Walker, Sydney, for 

 their skillful preparation of the majority of illustrations; to Mr. R. 

 Oldfield, University of Sydney, for his craftmanship in the reproduc- 

 tion of the plates; to Miss D. Lissner, the writer's research assistant 

 in Sydney, for her help in the preparation of this paper; and finally 

 to Mrs. D. Knight, Columbia, S.C, and Miss A. Brown, Sydney, for 

 their care in preparing the typescript. 



Financial support from the National Science Foundation, Wash- 

 mgton, D.C. (G-16190 and GB-3394), the University of South 

 Carolina, and the University of Sydney, which greatly facilitated this 

 study, is most gratefully acknowledged. 



Morphological Features and Definitions 



The various morphological criteria of freshwater sponges have 

 never been subjected to detailed comparative studies, even though 

 the nomenclatiu'e employed within this superficial group of sponges 

 remained fau'ly uniform over the past 100 years of systematic research. 

 It is most regrettable that a morphological nomenclatm-e common to 

 both freshwater and marine sponges has not yet been devised, since 

 such a development would greatly assist in the assessment of phylo- 

 genetic relationships within all Porifera. However, so long as additional 

 data are unavailable, it is thought advisable to retain in this paper the 

 nomenclatm'e used by the majority of spongillid investigators. 



Features of systematic importance in freshwater sponges are the 

 following: 



Form and consistency.^ — Characteristic modes of growth vary 

 with the age of the sponge, and consequently are of importance only 

 in mature specimens; growths may range from thin to thick crusts or 

 cushions, be of bulbous or otherwise massive nature, or display 

 branching, subbranching, or pseudobranching projections. Character- 

 istic consistencies are chiefly determined by the amount of spongin 

 present in the skeletal meshwork, only occasionally by other factors. 



Color. — The coloration of spongillids is usually determined by the 

 presence or absence of zoochlorellae within the tissues of the sponge, 

 rarely by the presence of a specific pigment (green or purple) or by 

 adventitious sediments. Apart from a few weU-estabHshed cases, the 

 coloration seems therefore of little taxonomic significance. 



Symplasm and dermal membrane. — Symplasm is herewith 

 suggested as the replacement of the term "parenchyma" of earlier 

 authors; although "dermal membrane" is another outdated and even 



