306 



FRESH-WATER BIOLOGY 



of the birotulate form, i.e., two little wheels or rotules connected by 

 a shaft, and on the numerous variations of these parts depends the 

 position of the species in the key. 



KEY TO NORTH AMERICAN FRESH-WATER SPONGES 



1(12) Gemmules with acerate spicules only. . . Spongilla Lamarck . . )i 



Spongillidae with long spindle-shaped skeleton spicules, macroscleres, having pointed or 

 rounded ends, and often also with minute simple flesh spicules, or microscleres. Gemmules 

 naked, or with external air-cell layer containing rhabdi, or rod-like spicules. 



2 (5) Sponge branching. . . ' 



Abnormal forms of S. lacuslris occur in which there are no branches. 



3 (4) Flesh spicules smooth. 



Spongilla aspinosa Potts 1880. 



Sponge evergreen, encrusting, thin, sending out numerous long, 

 slender waving branches from a relatively thick basal membrane. 

 Gemmules few, in scattered branches. Skeleton spicules smooth, 

 straight or slightly curved, rather abruptly pointed. Dermal spic- 

 ules minute, smooth, straight or curved, slender, gradually pointed. 

 From clear standing water in New Jersey and Virginia. 



FIG. 560. Spicules of Spongilla aspinosa. Four types of spicules figured 

 here: ordinary skeleton spicules abruptly pointed at both ends; skeleton 

 spicule, acute or rounded at one end; malformations of skeleton spicules, 

 with processes at or near one end; small smooth dermal spicules; globular 

 or discoidal masses of silica frequently observed in this species. X 100. 

 (After Potts.) 



4 (3) Flesh spicules spined Spongilla lacustris (Linnaeus) 1745. 



Branches cylindrical or tapering, and rigid. Prefers rapidly running water. Very abun- 

 dant. Gemmules either apparently wanting or abundant throughout the sponge, with or 

 without a granular crust. Skeleton spicules smooth. Dermal spicules pointed spined acer- 

 ates. Gemmule spicules whether few or many generally cylin- 

 drical, more or less curved, rather sparsely spined. From Boston, 

 Mass., to McDonald Lake, Alaska, in an infinite number of situa- 

 tions and variety of forms. The variety paupercula Bowerbank, 

 made an independent species Spongilla paupercula by Carter, "is 

 perhaps that one of this group of synonyms about whose identity 

 with S. lacuslris there may be most hesitation. Its character is some- 

 what anomalous, as its locality and associations are peculiar. Grow- 

 ing originally in the ponds and reservoirs tributary to the Boston 

 Water supply, Bailey wrote in 1856 that it grew abundantly in the 

 waterpipes by which the city was supplied with water from a small 



!?. - c , , lake." The minute acerates were said to have been smooth which 



mufe spicu!2 D ?f n /Si would separate it clearly from S. lacustris, but Potts was unable to 

 gilla lacuslris, var. man- secure material from the original locality which bore out the con- 

 tana. Xioo. (After Potts.) tention. 



5 (2) Sponge without branches 6 



6 (9) Gemmules in layers or groups 7 



