PRIMITIVE MULTICELLULAR ANIMALS 



A significant difference in the organization of the Mesozoa as compared 

 with that of the more advanced Eumetazoa is the nature of the inner cell 

 layer. Only in the Mesozoa is the inner cell mass exclusively reproductive; 

 in higher Metazoa the inner cells are concerned with nutrition. 



The Phylum Porifera 



Porifera are the simplest multicellular animals constituting a well-defined 

 group which includes a considerable- number of species. They may be char- 

 acterized as Metazoa with tissues of a very simple sort but without organs; 

 with a more or less extensive system of internal cavities but without a 

 digestive cavity or enteron; and usually with an internal skeleton. The 

 name Porifera ("pore bearers") is derived from the many small openings upon 

 the exposed surface, through which water enters on its way to the internal 

 cavities, and the fewer large openings through which this water is expelled. 



The phylum Porifera includes three classes (Figs. 9.5 and 9.6): the class 

 Calcarea, in which the skeleton is calcareous and in which are grouped the 

 simplest sponges, such as Leiicosolemo and Scypha: the class Hexactinellida, with 

 skeletons of glass; and the class Demospongiae, in which are included the 

 natural fibrous bath sponges and related forms. In general the skeletons of 

 sponges are composed of minute spicules, of proteinaceous fibers as in the bath 

 sponge, or of both spicules and fibers. The spicules of sponges are often of 

 such characteristic shapes that families and even genera may be identified by 

 spicules alone. I'hese spicules, like the skeletons of Radiolaria and Foramini- 

 fera (p. 236), occur in the debris upon the bottom of the ocean and are often 

 found in .sedimentary rocks. Fragments of sponges, as well as isolated masses 



273 



