130 



METAZOAN PHYLA 



In the rhagon type the animal is much larger and the whole body 

 forms a rather thick mass penetrated by a complexly branched canal 

 system. In the fresh- water sponges, which may be taken to represent 

 this type, the ostia lead into subdermal cavities. From these cavities 

 incurrent canals run to chambers lined with flagellated cells. After 

 the water has passed these flagellated cells it is carried by excurrent 

 canals into a gastral cavity, which opens to the outside by an osculum. 



152. Skeleton. — The classification of the sponges depends upon the 

 character of the skeleton, which may be made up of spongin or of spicules. 

 The spicules may be either calcareous or siliceous and differ in shape in the 

 different forms. Spongin is a substance which chemically is similar 

 to silk and which is formed by cells known as spongoblasts. Spicule- 

 Oscu/a/^ Oscu/um 



Rac//(f/ ccrncf/ 

 chamber 



Incurrent \/y^ 



'^ Oscu/un? 

 ■^ ^ Gcistral cav/'-f-y 



A B C 



Fig. 51. — Diagrams of canal systems of sponges. ^4, ascon type. B, sycon type. 

 C, rhagon type. (From Wieman, ''General Zoology," A and B after Minchin, and C modified 

 from Parker and Haswell, by the courtesy of McGraw-Hill Book Company, Inc.) The gastral 

 epithelium is shown by heavy black, the dermal epithelium by a light line. Arrows show 

 water currents. 



forming cells are sderoblasts. The spicules may be straight rods with 

 one axis, the monaxon type; or they may have three rays in one plane 

 and be triradiate; or four rays lying in four planes, in which case they 

 are known as tetraxon. They may have six rays, the ends of three axes, 

 in which case they are triaxon; or they may have numerous rays and be 

 polyaxon (Fig. 52). Many modifications of each type occur. 



153. Histology. — There are in the bodies of sponges a number of 

 different types of cells. In the outer, or so-called dermal layer, are flat 

 epithelial cells, contractile cells, gland cells which secrete the material 

 that attaches the animal to its support, and the cells which form the 

 skeleton. In the middle layer are reproductive cells and wandering 

 cells, the latter capable of ameboid movement. The cells of the gastral 

 layer are flat epithelial cells or collar cells. These cells, however, do not 

 work together to the same degree as do the cells in higher animals. The 

 whole is really a great colony of semi-independent cells, and individuality 

 is so little evident that zoologists have not agreed upon what constitutes 

 an individual. H. V. Wilson has found it possible, by gently squeezing 

 sponges through the meshes of fine silk cloth, to separate them into 

 individual cells. These cells will then gather together in small groups 



