396 The Animal Kingdom 



Metridium. — The members of this genus have short cylindrical 

 bodies with the flat oral end covered with numerous short hollow 

 tentacles which surround the mouth. The mouth is slitlike in shape. 

 The pedal disc at the base attaches to solid objects. The mouth opens 

 into the gullet which connects to the central gastrovascular cavity also 

 known as the enter on. Along one side of the gullet is a furrow, the 

 siphonoglyph. This is smooth and ciliated and permits water to pass 

 into the enteron. Between the body wall and the gullet are six large 

 vertical septa. Secondary and tertiary septa which do not extend to 

 the gullet are between these larger ones. These serve as supports to 

 the body. Water can pass into the radial compartments formed by the 

 septa through special ostia which are openings just beneath the oral 

 disc. Each septum has a twisted septal filament which is extended 



TENTACLES 



ORAL DISC 



.FILAMENT 



Fig. 130. — The sea anemone, Metridium. A, Entire animal with part of body wall 

 removed; B, cross section to show septa. 



basally as a thread acontium. Each acontium contains nematocysts 

 and gland cells. Acontia can pass through body pores or even through 

 the mouth. With their stinging cells they are very useful in the captur- 

 ing of food. 



Gonads are formed along the edges of the septa. The sexes are 

 separate, and the eggs and sperm are discharged into the water through 

 the mouth. Fertilization occurs in the water. The larvae swim about 

 briefly and settle down to develop into the typical anemone. 



