CHARACTERS AND DIVISIONS OF ALCYONARIA. 325 



With the exception of the two genera Mo?ioxenia (Haimeia, E. 

 and H.) and Hartea, the Alcyonaria are all composite, the tubular 

 polypes being united by a ccenosarc, and their body-cavities being 

 placed in communication by means of anastomosing canals, which 

 ramify in the ccenosarc, and permit of a free circulation of nutrient 

 fluids. The form of the colony differs greatly in different cases, but 

 none possess the power of independent locomotion, most being 

 rooted to foreign objects, or sunk in the mud. The polypes, in 

 most of the essential points of their organisation, agree with those 

 of the Zoantharia, the mouth opening into a tubular gullet, which in 

 turn communicates freely with the body-cavity, and the oesophagus 

 being connected with the body-wall by means of a series of vertical 

 membranous laminae or " mesenteries." The mesenteries, however, 

 are only eight in number, and are not paired, one of the tentacles 

 corresponding with and opening into each intermesenteric chamber. 



The gullet in the Alcyonaria is so placed that its long axis (fig. 

 209) corresponds with the dorso-ventral plane of the body of the 

 polype, and it very commonly is furnished on its ventral side with a 

 ciliated groove (the " siphonoglyphe " of Mr Hickson). 



In many instances the colony in the Alcyonaria is dimorphic, 

 consisting of two sets of zooids, which differ from one another in 

 structure and function. The zooids of the one series (generally 

 called " autozooids) have the normal size, and possess the structures 

 proper to the mature polypes ; while those of the other series (gen- 

 erally called " siphonozooids ") are reduced in size, and want certain 

 structures of the normal polypes, such as tentacles, mesenterial fila- 

 ments, or reproductive organs. No siphonozooids have been recog- 

 nised in Tubipora or in most of the Gorgonidce, and von Koch 

 doubts if the structures described as such by Moseley in Heliopora 

 are really of this nature. 



The mode of increase in the Alcyonaria is typically by basal or 

 stolonal gemmation, the parent throwing out basal extensions, which 

 commonly unite to form a creeping network or a crust-like expansion, 

 from which new polypes are thrown up at intervals. In other cases 

 the stolons are produced from the sides of the polypes at different 

 levels above the base ; and in still other cases (e.g., in Alcyonium) the 

 stolons coalesce to form a common fleshy mass or ccenosarc. In 

 some cases, as in the Heliolitidce, the peculiar mode of increase 

 which has been described by von Koch as " ccenenchymal gemma- 

 tion " is observed, but the nature of this will be described hereafter. 



There are no skeletal structures in Monoxenia (Ifaimeia), but with 

 this exception a corallum of some sort or another is produced in all 

 the Alcyonaria. According to the investigations of von Koch, the 

 skeletal structures of the Alcyonaria may be ectodermal or meso- 

 dermal in origin. The ectoderm commonly produces hard struc- 



