MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



433 



In simple polyps, and where asexual reproduction takes place by columnar gemmation, the 

 disk is circular or slightly oval, and hears 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 fissiparity the disk becomes large and irregular in outline, and as a rule bears more than 

 one oral aperture, the whole surrounded by a complex tentacular system. In genera like 

 Manicina and MseandrLna the disk is represented by irregular, meandering, flattened areas. 



During 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 may he 

 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 



Fir;. 2. 



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

 to ili,.' stage with forty-eight tentacles, in their relation with the mesenterial chambers, a, Sextant with two entocoelic proto- 

 tentacles (1,1) and one exocoelie prototentacle (A). 6, Two rudimentary tentacles have appeared, one ill) from the entoeoele and 

 the other (B) from the exoceele of a new pair of nietacnemes. e, The new entoceelie tentacle illi has now become larger than 

 either of the exocoelie tentacles, but is a lit tie smaller than the entocoelic prototentacles(I), and constitutes the second cycle of tentacles, 

 while the exocoelie tentacles A B) constitute the third cycle, d. Rudimentary tentacles (c, HI; HI, c) have appeared in association with 

 the entoceeles and exocceles of two new pairs of second-cycle mctacnemes. <, The entocoelic tentacles (III, III), incipient in </, have 

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

 third adult cycle of twelve tentacles. All the exocoelie tentacles I 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, hut 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 he sunk below the upper edge of the column, hut 

 on full expansion it usually becomes strongly convex, the middle area raised above the level 

 of the column, and even of the tentacular zone. In Fa/oia fragum the disk may project in this 

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

 ing the slit-like mouth at the apex. This is noticeable also in OeuUna diffusa (PI. XXII. fit;-, l-t'.t). 

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

 included, as the incomplete members do not radiate far across (PI. XI, tig. S3). 



