Part II. — Stichodactylince and Zoanthece. 135 



Tribe.— HEX ACTINIA, Hertwig, 1882. 



Actiniaria with paired mesenteries. The mesenteries of each pair are provided 

 with longitudinal muscular fibres on the faces turned towards each other, and with 

 transverse muscles on the faces turned away from each other, except in the case of 

 two (sometimes more, one or none) pairs — the directives — in which this arrange- 

 ment of the musculature is reversed, so that the longitudinal muscles are on the 

 faces which look away from each other. The number of pairs of perfect mesen- 

 teries is at least six, but may be eight, ten, or irregular, and they usually increase 

 simultaneously in the same multiples. 



The above definition is, in the main, that adopted by all writers since Professor 

 R. Hertwig founded the tribe. As a result, however, of later investigations it has 

 been found that exceptions may occur in almost every part of the original defini- 

 tion. Many forms are now known in which the hexameral symmetry is replaced 

 by an octameral, decameral, or irregular arrangement ; the directives may be 

 absent, reduced to only one pair, or increased to more than two pairs. Even the 

 increase in pairs of the cycles beyond the primary does not always proceed in 

 regular multiples of the latter, or simultaneously. This is shown in Ricordea florida 

 (PI. xi., fig. 6 ; PI. xii., fig. 1), where the pairs of the third cycle are developed very 

 irregularly and never in proper alternation, i.e. double the number of the first or 

 second cycle, as the rule of symmetry demands ; the hexamerous plan is here 

 likewise departed from. 



Gonidial or oesophageal grooves, included by Hertwig in his definition, are 

 now known to be so variable in number, or even to be absent in so many cases, that 

 their inclusion in the tribal definition is of no importance. Dr. 0. Carlgren 

 (1893) adds that the column-wall and stomodseum are devoid of ectodermal longi- 

 tudinal muscular and ganglionic layers, but, in the present paper, these are 

 shown to occur in several species, and are already known for several others. 



Order. — Stichodactylince, Andres, 1883. 



Hexactiniee in which more than one tentacle may communicate with a mesen- 

 terial chamber. Usually a peripheral series of one or more cycles can be distin- 

 guished from an inner accessory series, the members of which are radially arranged 

 or in groups, and are often of different form. Sphincter muscle either endoder- 

 mal or absent. 



The division of the tribe Hexactinise into the two orders, Actinina? (in which 



X2 



