PHYLUM COELENTERATA (CNmARIA). SIMPLE TISSUE ANIMALS 



123 



slitlike mouth. The color is variable, but 

 usually brownish or yellowish. The skin is 

 soft but tough. At either side of the gullet 

 ( stomodaeum ) is a ciliated groove called 

 the siphonoglyph. The internal body cavity 

 consists of 6 radial chambers; between these 

 chambers are 6 pairs of thin double parti- 

 tions called primary septa or mesenteries. 

 Water passes from one chamber to another 



through pores (ostia) in these septa, and 

 all are open below the gullet. Smaller septa 

 project out from the body wall into the 

 chambers, but do not reach the gullet; these 

 are secondary septa. Tertiary septa lie be- 

 tween the primaries and secondaries. There 

 is a considerable variation in the number, 

 position, and size of the septa. 



The free edges of the septa below the 



Tentacles 



Siphonoglyph 



Oral disk 



Cross section 

 through gullet 



Figure 61. Structure of the sea anemone, Metridium, a representative of the class Anthozoa. 

 Left, cross section through the gullet shows the arrangement of the septa. Right, a part of the 

 body has been cut away to show the internal structure. 



gullet in the enteron are expanded int^o 

 thickened structures called digestive fila- 

 ments. These bear the gland cells that se- 

 crete digestive enzymes. Near the base these 

 filaments bear long delicate threads called 

 acontia. The acontia are armed with gland 

 cells and nematocysts. Near the edge of the 

 septa are the gonads. The animals are 



dioecious. Asexual reproduction occurs by 

 budding, by fragmentation at the edge 

 of the basal disk, and by longitudinal 

 fission. 



Sea anemones are among the most beauti- 

 ful and conspicuous inhabitants of tide pooh 

 along the seacoast. When fully expanded 

 they form a sea garden filled with flowerlike 



