33S The Field Naturalist's Quarterly November 



Sponges are classified in four main groups, according to the nature 

 of the skeleton. The fleshy ones are a small and unimportant group ; 

 the horny ones include the sponge of commerce ; the calcareous ones 

 are of small size, but of great interest and beauty ; the siliceous sponges 

 are by far the largest group as regards number of genera and species. 

 The Freshwater Sponge (Sftongilla fluviatilis) belongs to this last 

 group, and its structure and physiology may be taken as typical of the 

 whole class. 



To find Spongilla. — The field naturalist may seek for it in either 

 brook, river, canal, or pool. I have found it in all situations, except in 

 small stagnant ponds. It presents an extraordinary variety of aspects. 

 In a large reservoir I once saw a huge colony, in the shape of stag's 

 antlers, attached, 3 or 4 feet below the water surface, to the piles sup- 

 porting a platform. Another large sponge formed a layer several square 

 yards in area, coating the bottom of a streamlet below the sluice-gate 

 of an artificial pool. To the naked eye the sponge patch is of a light 

 yellowish-grey colour. A ready means of distinguishing Spongilla is by 

 its odour. When just taken from the water it has a strong and not 

 unpleasant smell, something like fresh sea-weed, but more pungent. 

 Large specimens, however, such as those described above, are of no use 

 for study. They die speedily, unless they have an unlimited supply of 

 fresh water. Small specimens about the size of a pea are much more 

 useful, as they can be placed under the microscope entire, and they 

 will live for months in an aquarium. They can generally be found 

 attached to the rootlets of trees in the water at the edges of pools, 

 canals, and sluggish rivers, so that to procure them one has only to tear 

 off a handful or two of the roots. 



Place one of these little sponges, still attached to the rootlet, in a 

 glass jar filled with water. At first there will be no visible sign of life 

 about it, but if it be left undisturbed for a few hours and then examined 

 with a pocket-lens, it will present an appearance somewhat like Fig. 17. 

 Now watch it awhile. That curious transparent projection is a tube 

 through which a current of water is issuing with a force that may be 

 judged when I say that I have seen a good-sized Volvox glooa/or, 

 which came within " range," hurled instantly to the opposite side of the 

 aquarium — a distance of nearly a foot ! 



Let us place the little sponge in a glass trough and examine it under 

 a low power of the microscope, first adding to the water a small quantity 

 of Prussian blue. The minute particles of colouring matter render the 

 currents easily visible. Watch these ; at certain points on the surface 

 we shall see the particles entering into the sponge, sucked in by some 

 invisible force ; but no particles will be seen leaving the sponge except 

 through the long transparent tube, but they issue thence in a continuous 

 stream, and at much greater speed than they entered it. 



The surface of the sponge bristles with sharp points, mostly arranged 

 in little groups. These are the ends of the flinty needles (spicules), 

 which by their aggregation constitute the skeleton of the sponge ; they 

 are entirely siliceous, but are held together by a horny cement. (In 



