SPONGES. 



693 



Fig. 3— Sponge Spicules. 



projections, or else resemble thorny sticks ; others end in lobes instead 

 of points. The spicules either lie embedded separately in the Sponge 

 tissue, giving it firmness, or unite in many dif- 

 ferent ways, forming rosette-like or plate-like 

 structures or else delicate supporting frame- 

 works. Several kinds of spicules may be found 

 in the same Sponge. Besides strengthening the 

 body, they also protect it, as is evident from 

 their position. Spicules of the anchor type are 

 frequently found projecting from the outer sur- 

 face, especially round the oscules and pores, and, 

 in some Sponges, spicules stand out like thorns 

 all over the surface of the gastral cavity. 



The exceedingly fine and brittle nature of 

 Sponge spicules causes them to penetrate any soft 

 object that comes in contact with them. If some 

 Sponges are handled carelessly, a burning irrita- 

 tion is set up by the fine points, which pierce and 

 break off in the skin. One Sponge has on this 

 account received the significant name of Agdas 

 noli tangere. This peculiarity no doubt efficiently protects Sponges from 

 many enemies which might prey upon them. 



In size, Sponges vary greatly ; some are minute objects only to be distinctly 

 seen under the microscope, others form enormous masses several feet high. 



By far the greater number of Sponges live in the sea ; a few species of 

 fresh-water Sjonges, however, occur in our own rivers, lakes, and ponds, and 

 many others are found in various parts of the globe. 



A considerable number of marine Sponges are to be found in our own 

 coasts, but these are less attractive in appearance than some of the tropical 

 Sponges, especially those with elegant framework, 

 produced by combinations of six-rayed spicules. 

 The skeletons of the Venus's Flower Basket (Fig. 

 4, A), the Lacework Sponge, and Bird's-Nest 

 Sponge are familiar objects in museums. Below 

 the beautiful framework of the Sponge itself, a 

 tangle of very fine threads resembling spun glass 

 is to be seen. These are modified spicules, and 

 by their means the Sponges anchor themselves in 

 the mud. The Glass-Rope Sponge (Fig. 4, B) 

 derives its name from the rope-like appearance of 

 the anchoring threads, which are slightly twisted. 

 Among the more massive Sponges is the 

 Neptune's Cup, which may stand several feet 

 high. 



The most familiar of all Sponges, the Bath 

 Sponge, has a skeleton of horny fibres. In a 

 living condition, it forma large 

 dark, often almost black masses, 

 the surface of which is raised up 

 into numberless conical promin- 

 ences and broken through by 

 large oscules ; each oscule is surrounded by an 



Fig. 4.— Six-rated SroNaBS 

 {Bexactinel Uclce). 



A, Venus's Flower Basket 



(KuplecteUd). 



B, The Glass-Rope Sponge 



i^Ryalonema), 



The Bath Sponge 



(En>^potigia officu 



