SEA-ANEMONES 



195 



the retractor muscles run on the intra-septal surface — those of a pair 

 facing one another. The arrangement of these muscles is of great im- 

 portance in classifying Anthozoa. It is possible that the mesenteries 

 are homologous with the tasniolae of jelly-fish, and the mesenteric with 

 the gastric filaments. But some embryologists maintain that the 

 mesenteric filaments are derived from the ectoderm of the gullet. 



From the above description it will be noticed that the fundamental 

 radial symmetry of the Coelentera has here become profoundly modified. 



Development. — From the fertilised ovum a blastula may result 

 which by invagination becomes a gastrula. In some cases the ovum 

 segments into a solid morula ; this becomes a free planula, in which a 

 cylindrical depression at one pole forms the mouth and gullet. Or the 

 two layers may be estabhshed by a process known as delamination, 

 in which a single layer of cells is divided into an inner endodermic and 

 an outer ectodermic layer. The planula settles down by the aboral 

 pole, and develops like a Hydra-tuba. The larva of Cerianthids is for 

 a time pelagic, and used to be recognised as a distinct genus, Arachnactis. 



Related forms. — The sea-anemones are classified in the sub-class 

 Anthozoa or Actinozoa, and along with many corals are distinguished 

 as Zoantharia or Hexacoralla from the Alcyonaria or Octocoralla, like 

 A Icyonium. 



Anthozoa or Actinozoa 



Zoantharia, Hexacoralla, 

 e.g. Sea-Anemone. 



Many are simple, many colonial. 



The polyps of a colony may give rise 

 to others directly by fission or 

 budding. 



Tentacles usually simple, usually some 

 multiple of six, often dissimilar. 



Mesenteries usually some multiple of 

 six, complete and incomplete. 



Retractor muscles never as in Alcyo- 

 naria. 



Two gullet grooves or siphonoglyphs, 



or only one. 

 No dimorphism. 



Calcareous skeleton, if present, is derived 

 from the basal ectoderm. 



Examples. 



Sea - anemones — e.g. Tealia and 

 Actinia. 



Madrepore corals, many of them reef- 

 building. 



Antipatharians. An aberrant Anti- 

 patharian, Dendrobrachia fallax, has 

 eight feathered tentacles. 



Alcyonaria, Octocoralla, 

 e.g. Dead-Men's-Fingers. 



All colonial, except a small family 

 including Monoxenia and Haimea. 



The polyps of a colony give rise to 

 others not directly, but through 

 stolons or solenia. 



Tentacles eight, feathered, uniform. 



Mesenteries eight, complete. 



Retractor muscles always on one (ven- 

 tral) side of each mesentery (see 

 Fig. loi). 



One (ventral) gullet groove (siphono- 

 glyph or sulcus), or none. 



Frequent dimorphism among members 

 of a colony. 



There are usually calcareous spicules (of 

 ectodermic origin) in the mesogloea. 



Examples. 



Alcyonium (Dead-men's-fingers), with 

 diffuse spicules of lime. 



Tiibipora (Organ - pipe coral), with 

 spicules fused into tubes and trans- 

 verse platforms. 



Coraliium rubrum (Red coral), with an 

 axis of fused spicules. 



Pennatula (Sea-pen), a free phosphor- 

 escent colony, with a " horny " axis, 

 possibly endodermic. 



