128 METAZOAN PHYLA 



through the body instead of carrying the animal about. These same 

 cells are also digestive. In all other Metazoa digestion is carried on by 

 cells of entodermal origin. In the sponges the layer which at the begin- 

 ning seems to be entoderm comes to lie on the surface of the body and to 

 perform the functions which we generally associate with ectoderm. 

 The middle layer is not differentiated in the way the mesoderm is in all 

 triploblastic animals and so is not recognized as a germ layer. The 

 sponges are, if anything, diploblastic, but this term is not strictly apphca- 

 ble because of the difference in the manner of development of the two 

 body layers. In view of these facts it seems best to include sponges in 

 the Metazoa but to separate them as a distinct group from all the rest 

 and to call them Parazoa — literally, animals set off at one side. The rest 

 of the Metazoa are called Enterozoa, or animals with a digestive cavity. 

 The group Parazoa contains but the one phylum, the Porifera. 



149. Classification. — Porifera (po rif er a; L., porus, pore, and ferre, 

 to bear) is divided into three classes : 



1. Calcarea (kal ka' re a; L., calcarius, limy). — Sponges which possess 

 spicules of carbonate of lime; all marine. 



2. Hexactinellida (hex ak ti nel' li da; G., hex, six, actinos, ray, ella, 

 Latin diminutive, and eidos, form). — Sponges with siliceous spicules 

 having three axes; confined to the deep sea. 



3. Demospongiae (de mo spiin' gi e; G., demos, the people, and spongia, 

 sponge). — Sponges with either spicules of silica, which are not triaxial, 

 or a supporting framework of spongin, or both; mostly marine, but with 

 a few fresh-water species. 



150. Structure. — Various types of sponges differ greatly in their 

 general form, in their size, and in their plan of structure (Fig. 50). Some 

 are quite regular in shape, while others are irregular, being branched, 

 often quite complexly so, fan-shaped, or cup-shaped. Some form 

 raised masses, and others spread out like flat discs on the surface to 

 which they are attached. Some are very small and are just visible to 

 the naked eye, while others may be 5 feet in height. They are often 

 brilliantly colored, and among the different species all colors may be 

 seen. The shape of individuals of the same species is not always the 

 same, though in a general way it conforms to a certain type; it may be 

 much modified by environmental factors. 



On the surface of the sponge are very many small openings called 

 ostia and a much smaller number of larger ones known as oscula 

 (Fig. 51). Water enters through the ostia and leaves through the oscula. 

 All openings are surrounded by spicules, which appear like spines, and 

 these may form a barrier over the ostia, protecting them from objects 

 which might do injury to the sponge. 



Within the body of simple sponges is a gastral cavity which opens 

 by an osculum. In more complex sponges there may be many such 

 cavities, each opening by an osculum. 



