MEMOIRS OF THE NATIONAL ACADE:\IY OF SCIENCES. 



433 



In simple poh'ps, and whei'e asexual repi'oduction takes place by columnar gemmation, the 

 disk is circular or slightly oval, and bears only one central mouth; a complete tentacular system 

 belongs to each individual, and forms a closed circle. But where increase takes place by incom- 

 plete fissiparitj' the disk becomes large and irregular in outline, and as a rule bears more than 

 one oral aperture, the whole surrounded b}' a complex tentacular system. In genera like 

 Ifanicinti and Ilxandfina the disk is represented by irregular, meandering, flattened areas. 



Dui'ing the retracted state of the polyps the disk is depressed, its peripheral border resting 

 upon the edges of the septa. On very strong retraction the interseptal discal areas ma}^ be 

 drawn much below the level of the septal edges, and invade the polypal cavity as mesenterial 

 funnels — " Septaltrichter " (see below). As a rule, the retracted disk is almost entirely hidden 





Fig. 2. 



One of the six tentacular systems of an Aetinian polyp, illustrating the order of development of the tentacles, from the stage mth twelve 

 to the stage with forty-eight tentacles, in their relation with the mesenterial chambers, a, Sextant with two entocielic proto- 

 tentacles (I, I) and one exoca^lie prototentaclc (A), b. Two rudimentary tentacles have appeared, one ^11) from the entocoele and 

 the other (B) from the exocadi- of a ncw^ pair of metacnenies. c. The new entoccelic tentacle (II) has now become larger than 

 either of the exocadic tentacles, bnt is a little smaller than the entocielic prototentaeles (I), and constitutes the second I'ycle of tentacles, 

 while the exoccelic tentacles (A, B) constitute the third cycle. <?, Rudimentary tentacles (c, III; III, c) have appeared in association with 

 the entocoeles and exoccelcs of two new pairs of second-cycle mctacnemes. f, The'entoeielic tentacles (III, III), incipient in d, have 

 now become larger than all the e.xocielic tentacles, but are less than the member of the second cycle of tentacles, and constitute the 

 third adidt cycle o£ twelve tentacles. All the exoccelic tentacles (A, B, C, C), though appearing at different times, are now equal in 

 size, and constitute the last or fourth cycle of the adult. 



by the overfolding upper region of the column wall, but usually a small circular opening 

 remains, simulating an oral aperture, and through it the middle of the disk can be seen below. 

 On partial expansion also the disk may be sunk below the upper edge of the column, but 

 on full expansion it usually l)ccomes strongly convex, the middle area raised above the level 

 of the column, and even of the tentacular zone. In Favia fragnui, the disk may project in this 

 ■way as much as 5 mm., and the perioral region becomes extended in a dome-like manner, bear- 

 ing the slit-like mouth ;it the apex. This is noticeable also in Oculina dijf\mt (PI. XXII. tig. 14!>). 

 In transverse sections through the disk thus produced only the complete mesenteries are usuallv 

 included, as the incomplete members do not radiate far aci'bss (PI. XI, tig. 83). 



