THE COELENTERATES AND CTENOPHORES 99 



with the septa. The germ cells, which have their origin in the 

 ectoderm, come to lie in the mesoglea near the free margin of the 

 septa. The ruffled free margin of each septum is edged with a 

 thickened mass of epithelial cells called the mesenterial filament, 

 thought to be of use in holding and compressing food particles, 

 thereby aiding in digestion. Near the pedal disc, these mesen- 

 terial filaments are frequently modified to form long threadlike 

 organs, the acontia, which are provided with numerous nettling 

 cells. Through the mouth or through minute pores in the body 

 wall the acontia are thrust to the outside of the body where they 

 serve as efficient defensive organs. 



Reproduction. — Most Anthozoa, with the exception of the sea 

 anemones, possess some sort of skeletal structures, either as 

 solid deposits formed by the zooids or as spicules in the colony 

 wall. These structures are of such diverse natures in the different 

 groups that they cannot be described here. 



Sexual development involves the formation of a planula 

 through cleavage of the fertilized egg. The planula, after a 

 brief free-swimming life, settles down and undergoes a transfor- 

 mation to the polyp form. Budding is of common occur- 

 rence and gives rise either to separate individuals or to colony 

 formation. 



Subclass ZOANTHARIA 



Sea anemones, true stony corals, and black corals are examples 

 of the Zoantharia which may be recognized from other Anthozoa 

 by the presence of numerous simple or unbranched hollow 

 tentacles. The corals are colonial, while the anemones occur as 

 solitary polyps. The esophagus is provided with two siphono- 

 glyphes and the gastrovascular cavity bears numerous paired 

 mesenteries, typically occurring in multiples of six. Except for 

 the directives (mentioned above) the septa are arranged in pairs 

 with the muscle bands of each pair facing one another. Edward- 

 sia is an exception with its eight mesenteries and sixteen or more 

 tentacles. Various other modifications of the rule occur in 

 other representatives of this subclass. 



There is great diversity in extent of development of skeletal 

 material in this subclass. Skeletal structures are lacking in the 

 sea anemones, the stony corals develop massive calcareous 

 skeletons, while the black corals have a branched, hollow axial 

 skeleton of horny material lacking lime spicules. 



