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89. — Fbesh-water Sponges. 



By W. Mubton Holmes. 



(Bead October 8th, 1890.) 



Although fresh-water sponges resemble many marine forms 

 in constitution and general appearance, they differ in one or two 

 important points ; for, whereas amongst marine species we find 

 some with calcareous and some with siliceous spicules, while 

 others possess a skeleton of horny fibres, the skeleton of fi-esh- 

 water sponges, upon which the sponge-flesh or sarcode is sup- 

 ported, is invariably composed of siUceous spicules slightly con- 

 nected together by means of firmer sarcode. The main divisions 

 of the skeleton are made up of several spicules lying side by side, 

 and sometimes overlapping at their ends, the smaller divisions 

 consisting either of a single spicule or of smaller bundles. These 

 spicules in different species vary in size and shape, and are 

 sometimes more or less spiny, but these differences are not by 

 themselves sufficiently constant or positive to serve as a means 

 of classification. 



Another particular in which fresh-water sponges differ from 

 marine is the presence, at certain seasons, of peculiar bodies 

 known as statoblasts or gemmules, which have never been found 

 in sponges of marine origin. These gemmules are about -^ of 

 an inch in diameter and nearly spherical, and are found either in 

 continuous layers, as at the base of encrusting species, or singly 

 in the interspaces between the skeleton spicules, or in groups of 

 a dozen or less scattered through the sponge mass, or in smaller 

 groups enveloped in a compact cellular parenchyma. They con- 

 sist of a compact mass of protoplasmic granules enclosed in a 

 chitinous or horny envelope, having a circular orifice known as 

 the hilum, or foraminal aperture, which is sometimes plain, but 

 more frequently has a slightly raised and expanded margin, or 

 prolonged into cylindrical or funnel-shaped tubules. The 

 chitinous coat is surrounded by a crust composed of air-cells, 

 sometimes very minute, and in other species easily visible under 

 a low magnifying power. 



In aU the genera of fresh-water sponges, with the exception 

 of Spongilla (which are surrounded by acerate or cylindrical 

 spicules only), a number of peculiar spicules are found embedded 

 in this crust in a radial direction, and their varying forms con- 

 stitute the basis of the classification now adopted. 



Another form of spicule, but occurring in some species only, 

 is the dermal or flesh spicule, which exists in more or less 

 abundance on the outer dermal film, or lining the canals in the 

 deeper portions of the sponge. They are always much smaller 



