U3 



the body-wall of the polyp, but are free below, where the 

 septa fuse with one another. Each segment is further sub- 

 divided by the smaller septa, typically three in number. The 

 septa throughout alternate with mesenteries. The latter are 

 in pairs with their muscles — except on the directives — on the 

 sides facing one another. It follows hence that half the septa 

 are entocoelic and half exocoelic (Fig. I.). As already seen 

 the orders of septa vary greatly in individuals, so that it is 

 impossible to characterise any one order as exocoelic. From the 

 alternate arrangement of mesenteries and septa it follows that 

 the highest numerical order in any part is the exocoelic one. 

 As the growth of any corallite proceeds, more and more septa 

 up to six cycles appear. The former exocoelic order of septa 

 becomes entocoelic by the development of new pairs of mesen- 

 teries. The increase of mesenteries takes ^XsLce pan pasiti 

 with the formation of new septa. An examination of i6 

 corallites has failed to reveal a single case of the growth of 

 the new septa preceding that of the new mesenteries or vice 

 versa. The mesenteries in every case are perfectly distinct on 

 the external body-wall between the tentacles and the upper 

 edge of the epitheca. The mesenterial filament is developed 

 very shortly after the mesentery is formed, but the definite 

 formation of the muscular fibres takes place later, and they 

 gradually increase throughout life. 



Tentacles. — The tentacles arise over the entocoelic septa 

 alone, and are accordingly half as numerous as the whole 

 body of septa. An inner cycle of larger tentacles, corre.'-pond- 

 ing to the septa which reach the columella, and an outer 

 cycle may usually be traced. The tentacles are retracted by 

 the longitudinal muscles of the mesenteries in an acrecbolic 

 manner (Fig. II.). The invagination is never complete, a 

 central portion and two pockets on either side of the septum 

 beneath being found. Secondary pockets also occur, the 

 muscles seeming to be attached in clumps. A pair of 

 mesenteries passes across towards the stomodoeum on each 

 side of the base of a tentacle, some of their longitudinal 

 muscles continuing a direct course up the tentacle. Below 

 the outer cycles the mesenteries may to some degree extend 

 into the tentacles, but with increase of size even in the 

 expanded polyp come to pass around the base. 



AH the tentacles are covered with round, knobbed batteries 

 of nematocysts, which gradually decrease in size from their 

 tips. At the base these pass imperceptibly into the ectoderms 

 of the external body wall and peristome, and except in the 

 youngest tentacles do not cover over the attachments of the 

 mesenteries. 



Stomodoeum.— The stomodoeum is a slit, one-third to two- 

 fifths of the long diameter of the calicle in length, with no 



A2764. C 



