PHYLUM CCELENTERATA 181 



in pairs symmetrically on each side of a vertical plane : their final 

 radial position is secondary. 



The body-wall consists of ectoderm and endoderm separated by 

 a stout mesoglcea containing fibres and cells. The stomodseum 

 consists of the same layers reversed i.e. its lining membrane is 

 ectodermal. The mesenteries are formed of a double layer of 

 endoderm with a supporting plate of mesoglcea. Nematocysts, 

 frequently of a more complex form than those of Hydrozoa and 

 Scyphozoa, are present in the tentacles, body-wall, stomodaeum, 

 and mesenteric filaments. The muscular system is well developed, 

 and contains both ectodermal and endodermal fibres and endo- 

 dermal muscle-processes. The nervous system consists of irregu- 

 larly disposed cells and fibres ; there is no concentration of these 

 elements to form a central nervous system. 



The gonads are developed in the mesenteries, the sex-cells are 

 endodermal, and the ripe sexual products are discharged into the 

 enteron. The impregnated egg develops into a planula, which, after 

 a short, free existence, settles down and undergoes metamorphosis 

 into the adult form. Except in one doubtful instance there is no 

 alternation of generations. 



In some Actinozoa the animal remains simple throughout life, 

 but in most members of the class an extensive process of budding 

 takes place, the result being the formation of colonies of very various 

 form and often of great size. Some kinds, again, resemble Tealia 

 in having no hard parts or skeletal structures of any kind ; but the 

 majority possess a skeleton, formed either of carbonate of lime or 

 of a horn-like or chitinoid material, and developed, in most cases 

 though not in all, from the ectoderm. 



The Actinozoa are classified as follows : 



Sub-Class I, Zoantharia. 



Actinozoa in which the tentacles and mesenteries are usually 

 very numerous and are arranged, as a rule, in multiples of five or 

 six. The tentacles are usually simple, unbranched, hollow cones. 

 There are commonly two siphonoglyphes and two pairs of directive 

 mesenteries: the remaining mesenteries are usually arranged 

 in couples with the longitudinal muscles of each couple facing one 

 another. 



ORDER 1. ACTINIARIA. 



Zoantharia which usually remain simple, but in a few instances 

 form small colonies. The tentacles and mesenteries are numerous, 

 and there is no skeleton. This order includes the Sea-anemones. 



ORDER 2. MADREPORARIA. 



Zoantharia which resemble the Actiniaria in the general 

 structure of the soft parts, but which usually form colonies, and 



