the Freshwater Sponges. 405 



of Paramecium Aurelia, Paramecium Colpoda, Chilodon cucullulus ; 

 several species of Trachelius, especially Trachelius ovum; less 

 frequently the various forms of Amphileptus, especially Amphi- 

 leptus Anser g^^^ ^^ ^^ ^^^^^ ^^ diameter, with a bacillar coat to 

 the oesophagus, resembling Prorodon, which was also present ; 

 moreover, Loxodes bursaria, several species of Bursaria and 

 Ophryoglena, Of the Oxytrichina were found principally Sty- 

 lonichia, Urostyla, and Euplotes. 



The Gemmules. — The living sponges are often situated not 

 directly upon wood, stones, or other objects, but separated 

 from them by a peculiar dark brown earthy mass, often several 

 inches thick. This consists principally of the remains of the 

 dead sponge, empty shells of gemmules with their amphidisks, 

 various siliceous needles and decayed gelatinous substance ; some- 

 times it contains brown gemmules, the contents of which are sus- 

 ceptible of development. In many the developing power of the 

 contents is extinct, and they consist merely of extremely slender 

 acicular crystals and a detritus-like mass ; the crystals are too 

 minute to allow of the determination of their shape, yet the 

 angles are perfectly distinct in some of them. In a few instances 

 the dead and broad sponge retained exactly the form and colour 

 of the living, but the microscope showed that the cells were 

 absent : between these acicular skeletons gemmules were also 

 present. The dead branched sponges, which exist mostly at 

 the bottom of the water, are frequently so densely covered with 

 gemmules as to appear grey or greenish; the points of the 

 needles then project beyond the gemmules ; these again are 

 often entirely covered by new sponge-formations, and are not 

 visible until the sponge is broken. In the lowest layers of the 

 living broad sponge, which bound the dead layers, large numbers 

 of shining white gemmules are sometimes found ; they resemble 

 in general the ordinary brown gemmules; their shell is very 

 firm, and when pressed offers considerable resistance; but the 

 amphidisks are remarkably distinct. Their contents consist 

 of the well-known globular masses, composed of smaller or 

 larger fat-like granules and albuminous matter, — are of about 

 the size of the largest sponge-cells, and when pressed are easily 

 broken up. Other gemmules found here are distinguished by 

 a very soft transparent shell, which is immediately burst, even 

 when the glass-cover is carefully laid upon the object ; their 

 amphidisks are also very distinct, but the globular masses con- 

 tained within them do not break up very easily. When a piece 

 of one of these sponges containing the above- described forma- 

 tions is dissected under water with fine needles, isolated whitish 

 ill-defined globular pieces, of about the size of the gemmules, 

 are usually detected, with the following properties. Even under 



