138 PHYLUM CCELENTERATA 



end. A minute spine projects beyond the free surface of the cni- 

 doblast into the water. When the surface of the ectoderm is 

 irritated the tube is evaginated and violently shot out, and the 

 poisonous fluid contained in the cavity is injected into any animal 

 that may be struck. Look for nematocysts which have discharged 

 their spiral threads. 



Exercise 4. Draw the distal portion of a tentacle showing its cellular 

 structure; show the nematocysts at the end, including several 

 which have been discharged. 



Study the finer structure of the wall of the hydranth. It is 

 made up of an outer ectoderm and a much thicker inner entoderm, 

 each composed of a single layer of cells ; the inner ends of the 

 entoderm cells are amoeboid and often flagellate, the function of 

 the flagella being to maintain in circulation the fluids in the gastro- 

 vascular space ; between these two layers is the thin noncellular 

 supporting layer. The hydrotheca incloses aU, but it is not in con- 

 tact with the ectoderm. Study the structure of the stem ; it has 

 essentially the same structure as the hydranth ; note the outer 

 cuticular covering, the perisarc. Note the action of the flagella in 

 a live specimen. 



Exercise 5. Make a drawing showing the cellular structure of the wall 

 of the hydranth and of the stem. 



Study a blastostyle. We note that it is a cylindrical object 

 inclosed within its transparent gonotheca. Budding out on the 

 sides are the young disk-like medusae, those toward the free end 

 being the largest and the oldest. The blastostyle has no tentacles 

 and no mouth. It has an internal cavity which is a part of the 

 gastrovascular space of the colony, and within which the nutri- 

 tive fluids circulate. 



Exercise 6. Make a drawing of a blastostyle. 



Special respiratory, excretory, digestive, and circulatory organs 

 are not present in the hydroid. Respiration and excretion are car- 

 ried on through the surface of the body wall. Digestion, circula- 

 tion, and absorption go qu within the gastrovascular space. The 



