THE STRUCTURE OF PORITES. 501 



the peripheral fringe looks very like the mesenterial furrows 

 which run. down the sides' of most Porites. The escape of a 

 planula is shown. 



The other two polyps figured by Lesueur, P. recta and " P. 

 clavaria " (non. Lamarck), have much smaller discs and short 

 tentacles inclined to be pointed. Dana figures the polyps of 

 P. levis as contracted down flush with the surface, the external 

 mesenterial furrows radiating like spokes round the ring of knobs 

 representing the contracted tentacles. The disc is fairly large 

 and only furrowed by six mesenteries, whereas in most of the 

 figures referred to, at least in which any furrowing of the disc is 

 at all marked, the disc is furrowed by the full number of mesen- 

 teries. This is the case also in the figures of the long polyps of 

 " P. furcata" given by Agassiz, in which the tentacles are thin, 

 fusiform, and pointed. Saville Kent, in his ' G-reat Barrier Eeef ,' 

 figures the polyps of three Australian species, with thin, cylin- 

 drical tentacles about as long as the diameter of the disc, each 

 tentacle ending in a distinct spherical knob. The only specimen 

 examined by myself which I would provisionally classity with 

 P. recta, Lesueur, has rather a narrow column which suddenly 

 enlarges to carry the twelve tentacles, which stand erect and 

 short, stout and round-topped ; the disc was rather small. 



Agassiz (Z. c.) first figured the nematocysts, or rather the coiled 

 threads of the nematocysts. On my sections these occur chiefly 

 in small groups raised into hemispherical batteries. One large 

 battery occurs at the tip of each tentacle, and a row of smaller 

 batteries runs down its inner face. The stinging-threads were all 

 I could see, and they were mostly coiled in more or less conical 

 spirals, the cones pointing inwards, and each beneath what appears 

 to be a small round aperture on the covering membrane of the 

 battery. Between these batteries the ectoderm was largely com- 

 posed of slime-cells. 



In addition to these small ectodermal stinging-threads, the 

 cavity of the polyp contains great numbers of long, membranous 

 sacs some 40 ju in length, each with a long coiled thread ; the coil is 

 never a regular spiral, and the membranous sac is often collapsed 

 upon the thread. No nucleus or communication with the exterior 

 could be found, the bodies being loosely attached to the endoderm 

 in great numbers in the tentacles, but also, though in smaller 

 numbers, on the mesenteries. Examined with a very high power, 

 the thread, which was as thick as the whole coil of the ordinary 



LINN. JOUEN. — ZOOLOGY, TOL. XXVII. 38 



