38 LIFE BY THE SEASHORE. 



shades of yellow, brown, and drab. Dried specimens found 

 on the beach are always colourless. In the living sponge 

 the most conspicuous feature is usually the number of 

 openings or oscula, which stud the surface and are raised 

 on little prominences, but in specimens which have grown 

 in a spot where space is limited, as on one of the smaller 

 seaweeds, these oscula are less conspicuous. The surface of 

 the sponge is marked by a distinct network of lines, and 

 when its substance is torn with needles it will be found that 

 it is full of minute flinty spicules. 



Another very common sponge is Graniia compressa, the 

 purse-sponge already mentioned. It is dull in tint, being 

 greyish brown in colour, and rarely grows -in such exposed 

 situations as the crumb -of -bread sponge. It is usually to 

 be found under overhanging rocks with the orifice hanging 

 downwards, and the base attached to the rock surface. It 

 is sac-like in shape, and in the dead state much flattened 

 and compressed. In life, however, the central cavity is full 

 of water, and the sponge is much plumper in appearance. 

 It is of much interest, because it was in it that Robert 

 Grant after whom it is called first discovered that in 

 a sponge currents of water enter the central cavity by 

 minute pores, and leave it by the large osculum. These 

 currents can be readily observed in living specimens placed 

 in sea-water containing solid particles in suspension. The 

 sponge differs from Halichondria in having a skeleton made 

 of spicules of lime and not of flint, and in being usually 

 simple, whereas Halichondria, with its many oscula, is an 

 example of a compound sponge. Occasionally budding 

 occurs, so that there may be as many as seven or eight 

 oscula, but the usual form is that of a slightly stalked sac 

 with one terminal opening. The skeleton, made of three- 

 rayed spicules, may be seen by teasing a little of the sponge 

 substance under a lens, and the fact that it is limy may be 

 proved by adding a drop of dilute acid, when effervescence 

 occurs as in the case of limestone under the same circum- 

 stances. The minute pores in the walls of the body are not 

 easily seen except in dried specimens, and even then they 

 are largely concealed by the spicules. The purse-sponge is 

 very common between tide-marks, but is usually only about 

 /in inch long, and is somewhat inconspicuous. 



