7-44 SUMMARY OF CURRENT RESEARCHES RELATING TO 



into two identical halves by a plane wliich passes through the axis of 

 the body, the buccal cleft, the unpaired tentacles and ventral chamber, 

 the groove between the two continuous septa, and the dorsal chamber of 

 multiplication. This symmetry is due to the primitive formation of tho 

 unpaired ventral chamber and the two (buccal and marginal) unpaired 

 tentacles ; it is continued, during life, by the formation of new septa 

 and cavities, with their external or tentacular and internal or mesen- 

 teric appendages, from a single median jwint whence the products pass 

 towards either side. Though the ventral cliambcr undergoes no 

 change, the dorsal one is repeatedly subdivided owing to the forma- 

 tion of new internal septa. Arachnactis and Cerlanthus ai)pear to be the 

 only living Authozoa which preserve this bilateral symmetry intact 

 during the whole of their lives. In others this symmetry is affected 

 by the grcnvth of new septa from other points of the periphery of tho 

 body. 



The Cerianthida) may be defined as free Actiniaj with persistent 

 bilateral symmetry, a terminal pore leading into tho general cavity, a 

 large buccal disc, surrounded by two circlets of tentacles, marginal and 

 buccal, wliich are separated by a wide smooth peristome. The tentacles 

 are arranged by pairs in such a way that a tentacle of each kind opens 

 in each lateral chamber. The septa do not reach to the floor of the 

 gen( ral cavity, with the exception of the two which correspond to the 

 unpaired tentacle, and these form an internal groove which leads to the 

 pore. The genus Arachnacfis (Sars) has a rounded body, a few tentacles, 

 and short similar septa; the animals are pelagic and swim by means of 

 vibratile cilia. Cerianthns (Delle Chiaje) has an elongated body which 

 is surrounded by a sheath formed by mucus and neinatocysts ; the 

 tentacles are numerous; the short septa are either sterile or repro- 

 ductive. The animals live in tubes at the bottom of the water. Bathy- 

 anthus of Moseley is regarded as a doubtful genus. 



The observations of Lacaze-Duthiers have shown that bilateral 

 symmetry is characteristic of larval Anthozoa, and is a point of great 

 importance. Such forms as retain it throughout life may be justly 

 regarded as presenting a primitive arrangement. Haime drew attention 

 to the resemblance between Cerianthns and the rugose corals ; these 

 palajozoic forms reached their highest development in the Silurian 

 period. Another point to be noted is that the development of Arach- 

 nactis is continuous, and that there is no intermediate secondary 

 stage. 



There is a certain primitive conformation common to tho Anthozoa 

 and the Acalephte among the Medusae, whence Arachnactis goes off in one 

 and Pclagia and its allies in another direction. But they always remain 

 fre<!, and produce ova and larvas in that condition. In most Anthozoa 

 and Acalephaj there is a more or less well-marked period of fixation, due 

 to different causes, and characterized by the asexual production of buds. 

 Now, no one will deny that the primitive and ancestral form of the 

 Anthozoa was an animal swimming freely in the sea, provided with an 

 invaginated buccal tube which is retained in that position by vertical 

 septa developed symmetrically on either side of the buccal cleft ; this 

 bilaterally symmetrical form produced eggs and not buds, and the young 

 grew up directly into the likeness of their parent. Nor will any one 

 deny that the fixed state is secondary and is generally characterized by 

 asexual modes of reproduction. 



