EEPORT ON THE ACTINIARIA. 3 



become familiar witli the nomenclature, which, taken jsartly from earlier authors, and 

 founded to some extent upon my own observations, will be adopted in the following 

 pages. I shall also be able to interweave short remarks upon the most serviceable 

 methods of investigation. 



The body of the Actinia is shaped like a hollow cylinder, which is usually very long 

 in proportion to its breadth, but which can also be shortened to a discoid form under 

 certain cii-cumstances. It is limited by two terminal surfaces, the " oral disk " or 

 " peristome," and the " pedal disk " or " base," whilst the body wall corresponding to the 

 outer surface of the cylinder is termed the " mural layer," or shortly, the " wall "; the wall 

 is usually separated from the pedal disk, always from the oral disk, by a sharp margin, 

 the two surfaces here meeting at a right or even at an acute angle ; the wall occasionally 

 passes gradually inwards into the base, in such a way that we cannot speak of a separate 

 pedal disk. 



Towards its periphery the oral disk bears the tentacles, which are simply hollow 

 evaginations of the disk. Besides these " marginal " tentacles there are also " circumoral" 

 tentacles, which are united in a corona round the oral opening, and "intermediate" 

 tentacles, which occupy a position between the oral opening and 'the margin of the disk. 

 As the first are always present, and the last two only exceptionally, those may be termed 

 the " primary" or " principal " tentacles, these the " secondary " or •'' accessory " tentacles. 



The oral opening, placed in the middle of the oral disk, leads into a tube which 

 hangs down a little way into the hollow space of the body, and in the older descrip- 

 tions was held to be a stomach, a name which we may now suitably abandon and replace 

 by the term " oesophagus." This ends before it reaches the pedal disk in a free margin, 

 and communicates by a wide opening, the " gastric orifice " or " cardia," with the large 

 hollow space which occupies the inside of every Actinia, and is developed from the 

 primitive intestine of the gastrula, whilst morphologically and physiologically it 

 replaces the intestine and body cavity (enterocoele) of the bilaterals. Leuckart's term 

 " ccelenteron," or " coelenteric space," is therefore specially appropriate to the Actiniae. 



The oesophagus hanging down in the ccelenteron is fastened to its place by the 

 numerous septa (sarcosepta, Hasckel) which spring from the oral disk, wall, and pedal 

 disk, and are attached superiorly to the oesophagus, whilst they end in a free margin 

 below. They therefore divide the peripheral part of the ccelenteron into simple radial 

 chambers, which are closed where they surround the oesophagus and where they pass into 

 the hollow spaces of the tentacles, but which open downwards between the free margins of 

 the septa into the " central stomach," i.e., into that part of the ccelenteron which lies 

 under the oesophagus and is no longer divided into chambers by the septa. 



All the above-mentioned walls and septa of the body of the Actinia are lamelljB of 

 no great thickness, and in many species the wall only is a tough sheath. The 

 firmness of the lamellae depends upon their fundamental substance of connective tissue, 



