AND HISTOLOGY OF SPONGES. 237 



For further examination, specimens should be plunged for some 

 minutes, whilst living, into a one or two per cent, solution of osmic 

 acid, and then transferred to weak spirit, about 25 per cent. Thin 

 sections should be made from the surface inwards by a section 

 cutter, a freezing microtome being preferred, and then examined 

 with high powers. 



Those who would only study Sponges for the beauty of the spicules 

 should cut a tolerably thick piece of the Sponge, taking care to in- 

 clude a portion of the dermal membrane, when present, from the 

 surface to the centre, placing it in a test tube and boiling for a few 

 minutes in strong nitric acid in tlie case of siliceous Sponges, but 

 liquid potash when they are calcareous, washing carefully several 

 times the spicules which fall to the bottom of the tube with dis- 

 tilled water, and finally mounting in Canada balsam. Thin sec- 

 tions taken in the same way, as also portions of the dermis, and 

 mounted in cells, brass ones being avoided, with Canada balsam, 

 show the relative position of the spicules. In the case of the 

 Sponges of the Hexradiate group, it is better to mount portions of 

 the dermis and interior part of the Sponge without attempting to 

 cut them, as the process would mutilate the structure, which is in 

 most cases so delicate. 



This group contains, I think, a greater variety of beautiful forms 

 than any other. We have a few of the spicules drawn here (Plate 

 XVIII.) to give some idea of the forms they take ; and you 

 will be struck by them, as in many things, that man's inventions 

 and ideas of form and symmetry have been forestalled by Nature 

 long before, perhaps, he himself appeared upon the surface of the 

 earth. 



One question forces itself upon us when we look at these minute 

 forms — Why is so much beauty hidden from our view, and for what 

 purpose are they so placed, that they can only be seen in many 

 cases by the higher powers of the microscope ? We can only wonder, 

 and find it is as much beyond our grasp, as the definition of an 

 atom, or the realization of boundless space. 



DESCRIPTION OF PLATES XVII., XVIII, 



PLATE XVIL 



Fig. 1. — Hypothetical diagram of Sjjongilla, showing currents (Carter). 

 Fig. 2. — Section of ovary of Spongilla jluviatills, showing position of bi- 

 rotulate spicules and separate spicules of same. 



