PROTOZOA AND SPONGES. 441 



on a prescribed plan, so as to give a certain form and 

 outline to tlie aggregate. 



If you have ever shaken up a box of dressing-pins, 

 and have then endeavoured to take one out, you know 

 how by their mere interlacement they adhere together 

 in a mass, so that by taking hold of one you may lift a 

 bristling group of scores. Somewhat on the same prin- 

 ciple are the calcareous and siliceous pins {spioula) of a 

 Sponge held together by mutual interlacement. Yet 

 their cohesion is aided by the tenacity of the living 

 sarcode which invests them ; for I have found that speci- 

 mens of Oraniia (calcareous Sponges with needles of 

 three rays), when long macerated in water, so that the 

 sarcode is dissolved, have very slight power of cohesion 

 among their spicula. 



To understand the structure of a Sponge we will 

 shave a thin sectional slice from this MaHchondria su- 

 herea. This when alive is of an orange colour ; and is 

 always found closely investing turbinate shells which 

 are inhabited by Hermit-crabs. "We will macerate the 

 slice in tepid water for a quarter of an hour, and then 

 examine it in the live-box. 



The surface is a thin layer of greater density than 

 any other part, and is composed of coloured fleshy gran- 

 ules — omitting for the present, the skeleton. Of the 

 same substance is the whole slice composed, but looser 

 and more open as it recedes from the surface. It is 

 separated by blank spaces which are larger towards the 

 centre, smaller and more numerous as they approach 

 the exterior. 



These openings are sections of so many canals, by 

 which the whole substance of a sponge is permeated. 

 The surface is perforated with minute pores, at which 

 19* 



