CALCAREOUS SPOXGE. 25 



brown layer, except around the osculum, where it becomes 

 thin, and ends in a narrow edge. 



d. The circlet of long, slender spicules, which forms 

 a collar or crown (Fig. 13, a) around the osculum. 



e. The radiating tuljes. 



1. AVhen slightly magnified, the inner surface of the 

 cloaca will be seen to be tilled with small polygonal open- 

 ings (Fig. 13, e) the inner ends of the radiating tubes. 

 These are not as regular as they are represented in the 

 figure. 



2. Upon the cut surface of the section of the sponge- 

 flesh along the sides of the cloaca, the radiating tubes will 

 be seen to be laid open longitudinally (Fig. 13, d). 

 They are straight tubes, which penetrate the sponge-flesh 

 at right angles to the long axis of the body, and opening 

 on its outer surface, among the bases of the spicules, 

 establish a communication between the outer surface and 

 the cloaca. 



f. Around the base of the sponge, notice the buds which 

 are to give rise, by their detachment, to separate sponges ; 

 and obsen^e, — 



1. The cloaca of the bud (Fig. 13, g), which is in free 

 communication with the cloaca of the large sponge. 



2. The osculum, spicules, and radiating tubes of the 

 bud, similar in every respect to those of the large sponge. 



3. Notice that there is no boundary- line between the 

 sponge-flesh of the large sponge and that of the bud. 



g. Make a sketch showing all these points. 



III. The spicules. Cut a small piece from the speci- 

 men, and boil it- for a short time in a test tube in caustic 

 potash solution, in order to separate and clean the spic- 

 ules. Allow them to settle to the bottom of the tube, and 

 then draw up some of them with a medicine dropper, and 



