94 COLLEGE ZOOLOGY 



even under the best conditions. It is made up of two layers of 

 cells : an outer layer, the dermal epithelium, and an inner layer, 

 the gastral epithelium. These layers, as will be shown later 

 (p. 104), are not comparable to the ectoderm and entoderm of 

 the Ccelenterata and other Metazoa. Between these two 

 layers is a jelly-like substance similar to the 

 mesoglea of Hydra (p. 109) in which are many 

 amcba-like wandering cells. 



The gastral epithelium is peculiar, since it 

 consists of a single layer of collar cells, the 

 choanocytes (Fig. 54), which resemble the 

 similar cells of the choano flagellate Protozoa 

 (Fig. 29). The flagella of these collar cells 

 Fig. 54. — A beat constantly, creating a current of water. 

 single collar cell jf a little coloring matter is placed in the water, 



of Leucosolema. . . . 



n, nucleus. (From it will be drawn into the animal through minute 

 the Cambridge i ncurren t pores, the ostia (Fig. so, A, *), in the 



Natural History, . 



after Bidder.) body-wall and will pass out through the openings 

 in a sieve-like membrane stretched across the 

 osculum (Fig. 53, 7). The osculum is therefore the exhalaat 

 opening, and not the mouth, as a casual examination might 

 lead one to believe. The course of the current of water in 

 such a sponge is shown by arrows in Figure 59, A. The 

 presence of the incurrent pores suggested the name Porieera 

 for members of this phylum. 



2. Anatomy and Physiology of Grantia 



Grant ia (Fig. 55) is also known as a simple sponge, though 

 it is more complex than Leucosolenia. It lives in the salt water 

 along the sea-coast and is permanently attached to the rocks and 

 piles just below the low-tide mark. It is shaped like a vase 

 that bulges in the middle, and is about three-fourths of an inch 

 long. Frequently buds occur near the base, and a small colony 

 is formed. 



