734 THE ANIMAL KINGDOM. 



There are three distinct subdivisions of living Zoantharia, the Actini- 

 aria, Antipatharia and Madreporaria. 



The Actiniaria or Malacodermata are distinguished in the first place 

 by the absence of an organic or inorganic skeleton. The animal is usually 

 solitary, and either free, or adherent to the surfaces of foreign bodies, and 

 creeps about by means of a pedal disc, or it lives immersed in sand or mud. 

 It is rarely colonial and fixed. There are six tribes. 



(i) The Hexactiniae have all the mesenteries paired. Two pairs placed 

 one at each end of the mouth, termed directive mesenteries, to each of which 

 corresponds a siphonoglyphe, differ from the remaining pairs which are 

 generally numerous, in having the retractor muscles on the interseptal sur- 

 faces instead of the intraseptal. The number of primary pairs of mesente- 

 ries is six, two directive and two lateral on each side of the body. In the 

 Sagartidae and Amphianthidae the six pairs in question are the only com- 

 plete mesenteries, whereas in other Hexactinians, so far as is known, 

 the secondary mesenteries are also complete : the tertiary and quaternary 

 are always incomplete. The tentacles are numerous, and frequently of 

 great length. There is always a marginal set, and sometimes an interme- 

 diate as well as circumoral, the two latter often termed 'accessory 1 .' They 

 differ, like the mesenteries, in age, and therefore sometimes in size, but 

 there is a general tendency to equalisation in this respect. When 

 they are numerous they are arranged in concentric circles, the oldest 

 nearest to the centre of the disc. Structurally speaking, they are evagina- 

 tions of the oral disc, which correspond to both the intra- and inter-septal 

 spaces. They are usually contractile, and furnished with a terminal pore. 

 They become altered in character in some deep sea forms; i.e. the terminal 

 pore enlarges ; the tentacle itself may be reduced to a short wide-mouthed 

 tube or stomidium, OK to an aperture with ring-like margin, which may 

 almost disappear ; and finally in Liponema it becomes a simple opening 

 in the peristome. When the animal is irritated the tentacles shorten. At 

 the same time the outermost margin of the disc is very generally strongly 

 contracted over the tentacles and mouth by the action of an endodermal 

 sphincter or Rotteken's muscle (p. 240), rarely completely absent. Acontia 

 (p. 241) are present in the Phellidae and Sagartidae, and may be protruded 

 through the mouth, by cinclides, or by rupture of the body-wall. The inner 

 stomata are always, the outer sometimes, present. The genital products 

 are either borne upon all the mesenteries, e. g. in Corallimorphidae, or they 

 are confined to those of lower order, and are at least not developed on the 



1 In the family Stichodactylinae the disc is large and covered with tentacles in radial series 

 either of one form and simple, or of two and then simple, mixed with ramose or foliate tentacles, 

 or of various shapes. So too in the family Thalassianthinae the disc is covered with dendritic 

 appendages. See Andres, ' Le Attinie,' Monograph ix. (i), 1884, p. 264 and p. 299. 



