SPONGES. 165 



throw out. The shell-making species have been distinguished as Foraminifers, 

 from the pores above alluded to. Some of the species not secreting shells (as in 

 the genus Amoeba) have been seen to extemporize a mouth and stomach. When 

 a particle of food touches the surface, the part begins to be depressed, and 

 finally the sides of the depression close over the particle, and thus mouth and 

 stomach are made when needed ; after digestion is complete, the refuse portion 

 is allowed to escape. 



The shells of some Rhizopods do not consist of distinct cells : the aggregate 

 living mass secrete carbonate of lime, without retaining the distinction of the 

 zooids. This is the case, as Carpenter has observed, in the Nummulite-like 

 genus Orbitolitcs. 



2. Sponges. — Sponges are regarded as compound animals, con- 

 sisting of an aggregate of zooids related to those of the Rhizopods. 

 The cylindrical water-passages serve to supply water for respiration, 

 and also such food as the water may contain. The material of the 

 Sponge is a little like horn in its nature ; its microscopic characters 

 show that it is not vegetable. Besides the general tissue of the 

 Sponge, there are in most species microscopic spicula through the 

 tissues, which are siliceous (figs. 194, a-h), — some of them acicular, 



Fig-. 194. 



Siliceous spicula of Sponges. 



others with divergent rays. They are in great numbers in some 

 species ; and even the fibres of the Sponge are sometimes siliceous. 

 In some few species the spicula are calcareous. 



There are other siliceous microscopic organisms, called Polycys- 

 tics and Diatoms. The latter are vegetable in nature, and are men- 

 tioned on p. 167. For a notice of the former, see p. 748. 



2. VEGETABLE KINGD.OM. 



The vegetable kingdom is not divisible into sub-kingdoms like 

 the animal ; for the species all belong to one grand type, the Ra- 

 diate, the one which is the lowest of those in the animal kingdom. 

 The higher subdivisions are as follow : — 



I. Cryptogams. — Having no distinct flowers or proper fruit, the 

 so-called seed being only a spore, that is, a simple cellule without 

 the store of nutriment (albumen and starch) around it which makes 

 up a true seed ; as the Ferns, Sea-weed. They include — 



1. Thallogens. — Consisting wholly of cellular tissue ; growing in 

 fronds without stems, and in other spreading forms ; as (1) Algae, or 

 Sea-weeds ; (2) Lichens. 



