20 THE VOYAGE OF H.M.S. CHALLENGER. 



DESCRIPTION OF SPECIES. 



ACTINIARIA or MALACODERMATA. 



Polyps with simple unpinnated tentacles and with septa, the number of which is 

 usually a multiple of six ; without skeleton. Body moving freely or adhering to sup- 

 porting substances by means of suction of the pedal disk, rarely firmly fixed. Animals 

 visually solitary, rarely forming colonies. 



In the foregoing diagnosis I have placed the nature of the tentacles first as the only 

 thoroughly positive characteristic of the group ; it is by this that the Actinias are distin- 

 guished from the Anthozoa with pinnated tentacles, the Alcyonaria or Octactiniae. I have 

 been obliged to place second, and to limit the value of the hexamerous arrangement of 

 the septa, to which the chief importance was formerly attached, as the number of the 

 forms in which no settled number or even another number than six is the foundation of 

 the distribution of the septa is continually increasing. I have included the want of the 

 skeleton in the diagnosis, and therefore separated the Actiniae from the Corals, for 

 practical reasons ; the division is not a natural one. There can, however, be no 

 doubt, and this has been settled for some forms by observations, e.g., for Caryophyllia 

 cyaihus and Madrepora variabilis by G. v. Koch (Morphol. Jahrb., Bd. v., p. 316, 

 1880), that many Corals have the septal arrangement of the Hexactiniae, and therefore 

 approach this first section of the Actiniaria more closely than the remaining sections, 

 the Paractiniae, Edwardsiae, Zoantheae, and Ceriantheae. 



Tribe I. Hexactixij;. 



Actiniaria with paired septa. The septa of each pair are usually provided with 

 transverse muscular fibres on those faces which are turned from one another and 

 longitudinal muscular fibres on those faces which are turned towards one another, with 

 the exception of two pairs of directive septa, which are placed opposite one another, 

 and have longitudinal muscles on the faces turned from one another, and transverse 

 muscles on the faces turned towards one another. The number of the pairs of septa is at 

 least six, usually more, and then increasing in multiples of six. Mouth fissure-shaped ; 

 oesophagus with two oesophageal grooves and two oesophageal lappets. 



Ehrenberg had the Hexactiniae and the Corals connected with them in view when he 

 separated his Zoocorallia polyactinia with more than twelve radii from the eight-rayed 

 Octactiniae. In the same way only they can lay claim to the name Hexacorallia bestowed 

 on them by Haeckel (Generelle Morphologic, Bd. ii., 1866). As they form the principal 

 part of the Actiniae, they have long been taken as types for the remainder. After 

 Haeckel had detached Cerianthus by reason of the observations on its development made 

 by Jules Haime, my brother and I pointed out the varying position of the Zoantheaa and 



