TYPES OF C(ELENTERA A SEA-ANEMONE. 163 



extend in a vertical ridge along them, are turned away from one another, 

 and run on the inter-septal surfaces, whereas in the other mesenteries 

 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 tseniolae of jelly-fish, and the mesenteric with 

 the gastric filaments. 



From the above description it will be noticed that the funda- 

 mental radial symmetry of the Ccelentera has here become profoundly 

 modified. 



Development. Comparatively little is known in regard to the early 

 stages of development in sea-anemones. From the fertilised ovum a 

 blastosphere may result which by invagination becomes a gastrula. In 



FlG. 8l. Z, Diagrammatic section of Zoantharian ; A^ of 

 Alcyonarian. After Chun. 



The line -6" in Z is through the siphonoglyphes (a), the line 

 T-T passes through two inter-septal spaces. The retractor 

 muscles are represented by dark thickenings on the mesen- 

 teriesall on one (the ventral)! side in the Alcyonarian. The 

 line S-S in A represents the axis of symmetry. 



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 established by 

 a process known as delamination, in which a single layer of cells is 

 divided into an inner endodermic and an outer ectodermic layer. 

 According to Goette, the development is in essentials the same as that 

 of the 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 

 asZoantharia or Hexacoralla from the Alcyonaria or Octocoralla, like 

 Alcyonium and the related forms. This contrast is not very satis- 

 factory, but it rests on such distinctions as the following : 



