PHYLUM PORIFERA 105 



Class I. Calcarea. Marine species, mostly white or gray, 

 living in shallow water; spicules of carbonate of lime, either 

 monaxon (Fig. 60, a, b) or tetraxon (Fig. 60, c, d.) ; flagellated 

 chambers large. 



Order 1. Homocoela. Gastral layer continuous. Example: 

 Leucosolenia (Fig. 52, Fig. 59, A). 



Order 2. Heterocoela. Gastral layer discontinuous and re- 

 stricted to flagellated chambers. Example: Grantia (Fig. 55, 

 Fig. 59, B). 



Class II. Hexactinellida. Deep-sea sponges ; spicules 

 triaxon (Fig. 60, e), of silicon; canal system with thimble-shaped 

 chambers. Example: Euplectella aspergillum, Venus' flower- 

 basket (Fig. 62). 



Class III. Demospongls:. Skeleton of silicious spicules, 

 not triaxon, or with spongin, or with both spicules and spongin, 

 canal system derived from rhagon type (Fig. 59, C) ; most highly 

 organized of phylum; majority of existing sponges. 



Order 1. Tetraxonida. Typically with tetraxon spicules. 

 Example: Geodia. 



Order 2. Monaxonida. With monaxon (Fig. 60, a, b), but 

 no tetraxon spicules (c, d). Example: Spongilla (Fig. 59, C). 



Order 3. Keratosa. Main skeleton of spongin. Example: 

 Euspongia, the bath sponge (Fig. 63). 



(5) The Position of Sponges in the Animal Kingdom. — As 

 stated at the beginning of this chapter, sponges are considered 

 many-celled animals. They were formerly, and are even now, 

 placed by some authors in a phylum with the coelenterates 

 (Chapter V). They differ from the coelenterates and other 

 Metazoa so widely in certain important characteristics that 

 most zoologists are inclined to separate them from the Metazoa 

 and call them Parazoa (see diagram, p. 25). 



Sponges differ from coelenterates in the presence of choano- 

 cytes, ostia, and oscula, in their unique method of feeding, in the 

 germ-layers, which are apparently reversed in position (p. 104), 

 and in the absence of a mouth and nematocysts (Fig. 66). The 



