CHAPTEE III 



METAZOA 



Chaeacteeistics. — 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- 

 tiQuations of the primary central cavity lined by endoderm, 

 and no cavities exist lined by mesoderm comparable to a 

 coelom. Eadial 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. 



Q, / Homocoela — Ascetta. 



f \ Heterocoela — Orantia, Leucandra. 



PORIFERA-! rHyalospongiae — Byalonema, Euplectella. 



\ NoN-OALCAREA-j Spiculispongiae — Halisarca, Oscarella, Geodia. 

 [_ Cornacuspongiae — Euspongia, Velinea, Spongilla. 



