THE MICROSCOPE IN ZOOLOGY. 165 



the cavity of the body is divided by septa into narrow 

 compartments. The walls contain circular and longitu- 

 dinal muscles, which serve for contraction of the body, 

 which is afterwards expanded by an injection or absorp- 

 tion of water by the mouth. 



The most interesting part of the structure of these 

 animals, to the microscopist, is the multitude of lasso-cells, 

 called also nettling -cells, thread capsules, and cnidce, which 

 stud the tentacles and other parts of the body, and by 

 means of which the prey of the polyp is at once pierced 

 and poisoned. A small piece of the tentacle of a sea 

 anemone placed in a compressorium under the microscope, 

 and subjected to gentle pressure, will show the protrusion 

 of many little dart-like processes attached to thread-like 

 filaments. Many observations indicate the injection of a 

 poison through these darts, which is instantly fatal to 

 small animals (Plate XIV, Fig. 123). 



The polyp has no circulating fluid but the results of 

 digestion mixed with salt water, no bloodvessels but the 

 vacuities among the tissues, and no passage for excrements 

 except the mouth and the pores of the body. Reproduc- 

 tion is both by ova and by buds. 



I. Actinoid polyps are related to the Actinea or sea 

 anemone. The number of tentacles and interior septa 

 is a multiple of six. 



II. Cyathophylloid polyps have the number of tentacles 

 and septa a multiple of four. 



III. Alcyonoid polyps have eight fringed tentacles. The 

 Alcyonium tribe are among the most beautiful of coral 

 shrubs. The Gorgonia tribe has reticulated species like 

 the sea fan, and bears minute calcareous spicules, often 

 brilliantly colored. The Pennatula tribe is unattached, 

 and often rod-like, with the polyps variously arranged. 



VI. HYDROIDS. The type of this class is the common 

 Hydra, which is often found attached to leaves or stems 

 of aquatic plants, etc. It is seldom over half an inch long. 



