CHAPTER XV. 



Porifera and Protozoa. 



Porifera. 



Sponges. Accustomed to the familiar form of the bathroom sponge, people are 

 usually surprised to learn that there are any other varieties, and that in our 

 own home waters many species (three hundred at least) abound, two at least 

 in fresh water. 



Naturally they are entirely unlike the " Turkey sponge " in form ; many 

 of them appear to be little more than masses of slime on the rocks in fact, 

 the red, orange, yellow, or green colourings on the sides of the rocks at low 

 water are some of the Porifera. 



Their life-story is a fascinating one. A sponge is to all intents and pur- 

 poses a sort of city, traversed in every direction by streets and lanes, or, more 

 properly, canals. The surface is covered with small openings or pores, which 

 communicate with the main canals, the outer ends of which are seen in the 

 fewer but much larger holes in the sponge (oscula). 



The whole city is a living colony of animals joined together in a common 

 mass, to which the sponge is really the skeleton or framework, covered with a 

 slimy flesh, and divided internally into myriads of cells. The minute organisms 

 which inhabit these are like glass flasks, each with a long whip or lash protrud- 

 ing from it, by which they thrash the water in unison, thus producing a current 

 which flows through the main canals and out at the oscula, continuously fed 

 by other water entering through the little pores. This water bears with it 

 all sorts of other microscopic creatures to provide the sponge's food. 



Some of the cells when full grown split into portions, which increase the 

 size of the sponge; others develop into eggs, which are swept out along the 

 current into the open sea, and settle down upon a rock to grow, by division 

 and subdivision, into a new colony. 



Another remarkable fact to be noted is that the substance of these sponges 

 consists of a horny fibre, together with innumerable quantities of flinty spicules 

 of the most beautiful shape. These can only be examined under the micro- 

 scope, which shows them to be of most diverse forms rods, grapnels, anchors, 

 arrows, crosses, etc., generally entangled together forming the skeleton of the 

 colony. Over all this mass of spicules and embedding them is the sarcode, 

 or living matter of the sponge, like a thin jelly. 



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