. Animal biology. Zoology; Biology. 132 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. 154. Skeleton.—The classification of the sponges depends upon
. Animal biology. Zoology; Biology. 132 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. 154. 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/i/fT? Oscu/utn /ffae://ipr/ ccrncf/ '^ Oscu/um "^ ^ Gcfsfra/ cav/^y ?Subafer-mar/ A B C Fig. 54.—Diagrams of canal systems of sponges. A, ascon type. 5, 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 scleroUasts. 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 poly axon (Fig, 55). Many modifications of each type occur. 155. 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 att
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