CALCAREOUS SPONGE. 29 



3. The tri-racliate spicules (Fig. 14, e), which form a 

 framework or skeleton between the tubes. 



The second, or more rare kind of tri-radiate spicules 

 (Fig. 14, e), which are placed around the wall of the 

 cloaca in such a way that the short branch of the spicule 

 projects into the cavity of the cloaca. 



4. Make a drawing showing these points. 

 V. Histological structure. 



Most of the following points may be made out in a speci- 

 men prepared as above, but they are more satisfactorily 

 shown in stained sections of a specimen w r hich has been 

 hardened in picric acid. 



As eosin is a very convenient staining fluid, which 

 brings out the points to be noticed with sufficient clear- 

 ness, the sponge may be placed for half an hour in a very 

 dilute solution of eosin in water. It should then be 

 returned to absolute alcohol for a few minutes, and then 

 imbedded in paraffine, as above described. Cut a number 

 of transverse sections, mount them in balsam, and examin- 

 ing them with a power of three or four hundred diameters, 

 notice : 



a. The syncitium, or granular protoplasm (Fig. 14, d, d), 

 with scattered nuclei, which covers the outer surface of the 

 sponge, and lines the cloaca, and also tills the spaces be- 

 tween the radiating tubes. On the side of the cloaca it 

 extends, as a thin web, to the tips of the spicules, which 

 project into the cavity. 



b. The layer of cellular epithelium, or endoderm (Fig. 

 14, 1i) which lines the radiating tubes. With a high 

 power, in favorable specimens, each cell may be seen to 

 carry a single long cilium. 



c. The remains of the spicules imbedded in the synci- 

 tium. 



