CHAPTER III 
METAZOA 
CHARACTERISTICS.— Multicellular animals, which pass through 
a unicellular stage, the ovum or egg. This multiplies by 
division, and the cells thus formed, instead of remaining 
equivalent to one another, become differentiated and are 
arranged in tissues. Reproduction takes place by means of 
ova and spermatozoa. 
ACOELOMATA. 
Metazoa in which a two-layered condition is the predomi- 
nant one. . The ectoderm and the endoderm may constitute 
the whole animal, but in many cases an intermediate layer (the 
mesoderm or mesogloea) lies between them. This middle layer 
may be homogeneous, but is more usually invaded by cells from 
one of the two layers, or from both. The cavities of the 
Acoelomata, except certain ectodermal pits, are in all cases con- 
tinuations of the primary central cavity lined by endoderm, 
and no cavities exist lned by mesoderm comparable to a 
coelom. Radial symmetry about an axis passing through the 
mouth is a primitive and common feature of this subdivision. 
The animals which constitute it are exclusively aquatic, and 
almost entirely marine. 
Homocoela—Ascetta. 
Heterocoela—Grantia, Leucandra. 
if Hyalospongiae—Hyalonema, Euplectella. 
Non-CALCAREA~ Spiculispongiae—Halisarca, Oscarella, Geodia. 
| Cornacuspongiae—Luspongia, Velinea, Spongilla. 
CALCAREA 
rortrEra 
