BRANCH PORIFERA: THE SPONGES 85 



spherical or disk-like bodies, the gemmnles. These are 

 reproductive bodies. Each gemmule is a sort of internal 

 bud. It is composed of an interior group of protoplasmic 

 cells, enclosed by a crust thickly covered with spicules. 

 In winter the sponge dies down and the gemmnles are set 

 free in the water. In spring the protoplasmic contents 

 issue through an aperture in the crust, called the micro- 

 pyte or foraminal opening, and develop and grow into a 

 new sponge. 



For a good account of the fresh- water sponge, see 

 Pott's " Fresh-water Sponges. " 



A CALCAREOUS OCEAN-SPONGE (Grantia sp.) (fig, 7, D, E, F.) 



TECHNICAL NOTE. For inland schools, specimens preserved in 

 alcohol or formalin must be used. They may be obtained from 

 dealers in naturalists' supplies (see p. 453). Specimens of some 

 species of this genus can be obtained at almost any point on the 

 Atlantic or Pacific coasts of this country. 



Examine the external structure of a specimen. Note 

 the elongate, sub-cylindrical form, the attached base, the 

 free end. Note the large exhalant opening, osteole or 

 osculum, at the free end; the numerous small inhalant 

 openings elsewhere on the surface (best seen in dried 

 specimens). Note the spicules covering the surface of the 

 body, and the longer ones surrounding the osculum. Cut 

 the sponge in two longitudinally and note the simple cylin- 

 drical body-cavity, the gastric cavity or cloaca. Note the 

 thickness of the body- wall ; note the tubes running through 

 the body-wall from cloaca to external surface. Through 

 these tubes water laden with food enters the gastric cavity, 

 where the food is digested, the water and undigested 

 particles passing out through the osculum. Crush a bit of 

 dried sponge, or boil a bit of soft sponge in caustic potash 

 and mount on a glass slide. Examine under a micro- 

 scope and note the abundance of spicules and the variety 

 in their form. Two kinds may always be found, and 



