PORIFERA. 13 
in some cases the two forms are included in one life cycle. 
Budding is very common, and many of the sedentary forms. 
—‘‘corals””—have shells of lime. 
Porifera.—Sponges, or Porifera, are the simplest many- 
celled animals. In the simplest forms, the body is a 
tubular, two-layered sac, with numerous inhalant pores by 
which water passes in, with a central cavity lined by cells 
bearing lashes or flagella, and with an exhalant aperture. 
But budding, folding, and other complications arise, and 
there is almost always a skeleton, calcareous, siliceous, or 
“horny.” Apart from one family (Spongillidze), all sponges. 
are marine. 
Contrast of Metazoa and Protozoa.—aAll the animals hitherto- 
mentioned have Jdodzes built up of many cells, but there are other. 
animals, each of which consists of a single cell. These simplest animals- 
are called Protozoa. 
Every animal hitherto mentioned, from mammal or bird to sponge, 
develops, when reproduction takes its usual course, from « fertilised. 
egg-cell. This egg-cell or ovum divides and redivides, and the 
daughter cells cohere and are differentiated to form a ‘‘ body.” But 
the Protozoa form no ‘‘ body”; they remain (with few exceptions) 
single cells, and when they divide, the daughter cells almost invariably 
go apart as independent organisms. 
Here, then, is the greatest gulf which we have hitherto noticed— 
that between multicellular animals (Metazoa) and unicellular animals. 
(Protozoa). But the gulf was bridged, and traces of the bridge remain. 
For—(a) there are a few Protozoa which form loose colonies of cells, 
and (4) there are a few multicellular animals of great simplicity. 
Protozoa.— The Pro- 
tozoa remain single cells, 
with few exceptions. Thus 
they form no “body”; 
and necessarily, therefore, 
they have no organs in 
the ordinary sense. They 
illustrate the deginnings of 
sexual reproduction, and ‘N 
they are Met subject to Fic, 17.—Fossil Foraminifera 
natural death in the same (Nummulites) in limestone. — 
degree as Metazoa are. After Zittel. 
The series includes— 
tA cell may be defined as a unit corpuscle or unit area of living 
matter, typically controlled by a single nuclcus. 
