140 TEXTBOOK OF ZOOLOGY 



canal. The eight tentaculocysts are symmetrically located at eight 

 points ou the margin, each between marginal lappets. The tentaculo- 

 cysts are sense organs of equilibrium. The pigment spot over each 

 is likely sensitive to light. Near it is the olfactory pit. 



Reproduction involves both sexual and asexual generations. Germ 

 cells are produced by the pinkish gonads in the gastric pouches, 

 and they pass out through the mouth with the water. Fertilization 

 takes place, and the egg develops into a free-swimming plajiula 

 which after attachment becomes a tubelike polyp that reproduces 

 by budding most of the season. Then the polyps form medusae by 

 strohilization, i.e., constrictions are formed around the body making 

 it resemble a stack of saucers ; the upper one periodically frees itself 

 and swims away. The polyp with all of these constrictions is known 

 as a strohila, and the new medusa is called an epliyra. 



Class Anthozoa. — All animals in this class conform to polyp or- 

 ganization and may be colonial or solitary. They have an ecto- 

 dermal esophagus and longitudinal partitions called septa (mesen- 

 teries) incompletely dividing the gastrovascular cavity. Muscular 

 tissue bands are found in the septa. The mesogloea is quite abun- 

 dant and contains a good many cells that resemble primitive con- 

 nective tissue cells. Many of these animals produce a calcareous 

 external skeleton called coral. Both sexual and asexual reproduc- 

 tion are common. 



Subclass Zoantharia. — This group has numerous paired septa, 

 typically occurring in multiples of six, and plain tubular tentacles. 

 It includes sea anemones and corals. 



Order Actinaria. — These anemones are usually solitary polyps ; they 

 have many complete septa and numerous tentacles but no skeleton. 

 Sagartia, Cerianthus, and Bletridmm are common examples. 



Metridium usually lives attached to rocks or to solid bodies in 

 the water near shore, even in tide pools. They average about three 

 or four inches in height and two or two and a half inches in diam- 

 eter. The free end of the jar-shaped body is covered with tentacles 

 which are provided with nematocysts. The entire body can be 

 expanded and contracted, and it can change its location by "seooch- 

 ing" on its lasal disc (attachM end). The mouth is located in the 

 center of the crown, and food is forced into it and on through the 

 gullet (stomodeum) bj^ action of cilia on the tentacles and part of 

 the lining of the mouth. At each side of the gullet is usually a 



