The Responses of Heteroxenia (alcyonaria) to Stimulation and to Some Inorganic Ions

The Red Sea Alcyonarian Heteroxeniafuscescens (Ehrb.) consists of a pinkish brown fleshy syndete up to 5 cm. in length, firmly attached to a stone or dead coral, from which grows a mass of autozooids with columns 2-4 cm. long and 2-3 mm. in diameter (Fig. 1). The first physiological observations on a xenid were made by Keller (1883) on a brown species from Suez that he considered to be H. fuscescens. He described the rhythmical movements in which the tentacles are clapped together (Fig 2A). The zooids beat independently of one another, and the two halves of a disk, split longitudinally, continued to beat as before but independently. Leaning on the Hertwigs' (1879) statement that the nervous system of actinians is best developed in the disk, Keller came to the conclusion that there is here a diffusely spread rhythmical centre. He also pointed out the resemblance to the beat of medusae.