PORIFERA 49 



iridescence. The majority are gray in color. They vary in size 

 from microscopic to several feet in diameter. Their shape, while 

 frequently cylindrical, is quite variable. They may branch to 

 form a network, or assume the shape of a fan, or even that of a hat- 

 crown. (Figure i8.) 



Type of Group — Grantia. — Grantia is a cylindrical, vase-shaped 

 marine sponge, somewhat less than an inch in length. It is readily 

 obtained from submerged piles and rocks alongshore, and since it is 

 convenient in size for external gross study, and can readily be 

 sectioned, it is ordinarily studied in college courses. 



External Anatomy. — Grantia has an outer or dermal layer, 

 consisting of epithelial cells, contractile cells, gland cells and poro- 

 cytes. Lime spicules and spongin fibers are formed by the sclero- 

 blasts, which belong to the inner portion of the dermal layer. The 

 body wall consists of a skeleton of calcareous spicules of which there 

 are four varieties, a long and a short scimitar shape, a trident shape, 

 and a T shape. At times other four- and five-rayed spicules are 

 noted. 



The middle layer is a jelly-like mesoglea, with wandering ameboid 

 cells, which ingest, store and transport food materials. From time 

 to time, germinal cells are here developed from some of the wander- 

 ing cells, and form ova or spermatozoa. Gemmules (statoblasts) 

 are formed in certain species. (See p. 50.) 



The inner or gastral layer lining the radial canals is made up of 

 " collared " flagellate cells, or choanocytes, which create currents of 

 water and bring inward particles of food. 



Ingestion and Digestion. (Figure 19.) — Food is ingested by 

 specialized cells and digested as in protozoa. Some cells are for 

 ingestion; the flagellated collar cells are extremely important in 

 absorbing and taking up food. They form rather dense masses 

 near the nuclei. Certain cells in the mesoglea are for storage, and 

 still others are for nutrition. The sponges make use of detritus 

 coming from dead plant and animal tissues found in sea water 

 along the coast. Food vacuoles are formed as in the Protozoa. 



Circulation. — Circulation is by means of the ameboid wandering 

 cells of the middle layer. Respiration is carried on by the cells of 

 the body wall. Excretion is osmotic by cells, and also by the ex- 

 pulsion of solids through the osculum. 



Reproduction, (i) Asexual. — Buds may arise near the base of 

 the sponge and become detached as a separate individual, or in 



