PHYLUM PORIFERA 



101 



HISTOLOGY. -- The sponges are among the simplest of the 

 ETAZOA with regard to the differentiation of their cells, but 

 they seem quite complex when compared with the PROTOZOA. 



a 



FIG. 60. Types of sponge spicules. 

 a, b, monaxon; c, d, tetraxon; e, triaxon; 

 f, polyaxon. (From the Cambridge 

 Natural History.) 



FIG. 61. Piece of net- 

 work of horny fibers from 

 the bath sponge, Eitspongia. 

 (From Sedgwick.) 



The cells of sponges may be separated into three groups: 

 (i) those of the dermal layer, (2) those of the gastral layer, and 

 (3) the ameboid cells in the jelly between the dermal and gastral 

 layers. The classes of cells and the layers to which they belong 

 are shown in Table III. 



TABLE III 



CLASSES OF CELLS FOUND IN SPONGES 



A. Dermal 

 Layer 



B. Gastral 

 Layer 



C. Middle Re- 

 gion 



I. Epithelial stratum 



II. Porocytes 



III. Skeletogenous stratum 



IV. Gastral epithelium 



V. Wandering cells 



VI. Reproductive cells 



1. Epithelial cells 



2. Contractile cells 



3. Gland cells 



4. Spongoblasts 



5. Pore cells 



6. Scleroblasts 



7. Fiber cells 



8. Choanocytes 



9. Ingestive cells 



10. Nutritive cells 



1 1 . Storage cells 



12. Gemmule cells 



13. Sexual cells 



