44 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



4. The other tentacles are much larger, and are capable, 

 in the living animal, of great extension and retraction ; 

 each will be found to be made up of : — 



a. A central axis of endoderm cells, arranged in a sin- 

 gle row. 



h. A transparent supporting layer, which surrounds the 

 endoderm cells, and may be seen in optical section, as a 

 well-defined transparent band on each side of the endo- 

 dermal axis. 



c. The layer of longitudinal muscular fibres, which lies 

 just outside the supporting layer. 



d. The thin layer of ectoderm which forms the outer 

 surface of the tentacle, and is filled with nematocysts. 



5. The ocelli at the l)ases of these tentacles are some- 

 what smaller than those on the club-shaped tentacles, and 

 the lenses may be absent. 



6. The sensory bulb. The tentacles do not spring 

 directly from the edge of the umbrella, but are carried 

 upon Ji somewhat triangular enlargement, the sensory bulb 

 (Fig._22,/). 



This is an enlargement of the margin of the umbrella, 

 at the point where a radiating chymiferous tube (Fig. 

 22, h) joins the circular tube g. The cavity of the bulb is 

 filled by an enlargement of these tubes which sends diver- 

 ticula off towards the bases of the tentacles, and is marked 

 by dark pigment. 



7. In the cut ends of the chymiferous tubes notice the 

 large opaque granular endoderm cells which line them. 



III. The mouth tentacles. Cut off" one of the branched 

 mouth tentacles : mount it in the same way and examine 

 it, first Avith a low power, and then with a higher power. 



1. With a low power notice that the main trunk divides 

 into two equal branches, and each of these again into two, 



