60 HALF AN HOUR WITH SPONGES. 



red. It occurs in thick, plump bands, about half an 

 inch in width, which alternately swell and contract, 

 meeting, uniting, and separating again, and thus 

 creeping over and covering a considerable portion of 

 rock. The apices on the surface have two or three 

 openings, and the whole structure is thickly invested 

 with granular flesh. The spicules in Microciona are 

 of a very simple kind. Lastly, we may mention 

 Pacliymatisma JoJinstoni as another of this British 

 group, and one of the largest, sometimes obtaining 

 the diameter of a foot. Its shape is of great service 

 to the student, enabling him easily to recognise it. 

 It is of a globose form, and a purplish-grey colour, 

 having its smooth surface dotted with a few minute 

 orifices. The whole flesh is of a dense yellow, in 

 which are set simple and six-rayed spicules, the 

 latter forming beautiful objects for the microscope. 



Before proceeding to notice the British sponges of 

 a larger and more decidedly " sponge-like " form 

 and texture, which, however, are only to be obtained 

 by dredging in deeper water, let us refer to one of 

 the commonest objects to be met with at the sea- 

 side. You cannot fail to have noticed oyster and 

 other thick shells bored and perforated in a very 

 peculiar way, as in Figs. 22 and 23. 



The outsides of oyster shells are often drilled by 

 scores of these holes, and, as the section will show, 

 these are continuous into a series of chambers. 

 When examined fresh, and by the aid of a good 

 pocket lens, you will see these holes coated and lined 



