582 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Genus M/EANDRINA Lamarck." 



Polyps verrucose, incompletely separated, forming' mainly continuous, meandriform discal and 

 columnar systems, and giving rise to massive, convex, g-ibbose, subplane or subspheroidal colonies, fixed 

 by a comparatively narrow or broad base; perithecal continuatiofa of the g-astro-coelomic cavity and 

 mesenteries at the margin of the colony. Contiguous column walls united along a narrow, common, 

 thecal ridge; on retraction capable of folding over the tentacles and covering the lateral margins of the 

 disk; no sphincter. Tentacles in two, alternating, slightly entacmseous rows, entoccelic and exocoelic, 

 introvertible, short, rounded terminally, stem with irregular urticating areas. Disk with numerous, 

 closely arranged, small oral apertures. Stomodseal walls deeply ridged. 



Mesenteries all complete, with occasional incomplete developing pairs, arranged in irregular stomo- 

 dseal systems; all filamentiferous; directives absent; increase irregularly by addition of single unilateral 

 pairs. Mesenteries and filaments protrusible. Septal invaginations entoccelic and exocoelic, dicyclic, 

 interseptal loculi incompletely separated below. 



Asexual reproduction by continuous incomplete fission. Polyps monoecious. 



ExAMi'i-E. — Jl^vaiidriiKi Inlii/rinthicui (Ell. c'i Sol.). 



MyEANDKIXA LABYRIXTIIICA (Elli.'j&Solander). 

 (Pis. XX-XXII, figs. 138-147.) 



External characters. — The colonies :ire iiias.sivc, sub.sphcvoidal, the upper surface uniformly 

 rounded, not thrown into <;il)ho.'<ities. The .species occurs in ahundancc on the reefs around 

 Jamaica, often forminj^' blocks several feet across, and Kxed )>y a narrow, irregular base; even 

 large colonies are free or are readily detached when collecting. 



The external appearance of the living colonies varie.s greatly according to the condition of 

 expansion or retraction of the superficial soft ti.ssues. The general relations during partial 

 retraction are Itest conceived of as a double .svsteni of meandriform depressions, separated by a 

 fringe of short tentacles. One series of grooves is formed by the continuous, narrow, discal areas, 

 the other by the united column walls of two contiguous polypal systems. The discal troughs are 

 much deeper than the colunuiar, and occur within the united calicis; the slight columnar depres- 

 sions run along the common thecal edges or collines. iind the line of union of the two adjacent 

 column walls is clearlv indicated at the bottom. 



On full expansion the superticial tissues become distended to such a degree that they are 

 raised for several millimeters at)ove the skeleton, the discal region increasing from 2 to 8 mm. 

 across. Along the line of union the two contiguous cohunn walls remain atHxed to the skeleton, 

 so that the walls on each side rise almost vertically from the line of attachment, and may actually 

 apply themselves to one another })y their outer surface, or remain separated only l)y very steep 

 valleys. In the former case the two contiguous fringes of tentacles intermingle in such a way 

 that the whole surface of the colony presents to view little more than the enormously enlarged. 

 convex discal areas. During maximum expansion the tentacles and the \erruca' may almost 

 entirely disappear, the walls of the former becoming part of the flat discal tissues. In the 

 fully retracted state the appearances are reversed; the dee])er valleys are now formed by the 

 di.scal areas, and the ridges by the columnar expansions resting on the septa. The column may 

 become partly overfolded, and under some conditions the two overfolding walls connected with 

 each discal .sj'stem may extend horizontally, and almost come into actual contact, so as to 

 completely hide the tentacles and disk. 



The column wall appears thick and opaque when the polyps are retracted, but more delicate 

 and tran.sparent when fully expanded. The surface exhibits verruco.se ridges and smooth 

 furrows, corresponding respectively with the internal septal and mesenterial divisions. The 



« "Colony massive, dcnso, convex, gibbose, subplane or subsplieroidal, largely fixed by its base. The series of 

 corallites unite by their walls, which are compact, and produce long, simple ridged collines. The valleys are 

 sinuous, long, but vary in length, depth, breadth, and meandroid nature. Calices mostly indistinct, some may be 

 circumscribed. Columella formed by masses of spongy tissue well developed. The septa are close, parallel, thei^ 

 inner edge thickened and enlarged transversely; upper margin denticulate, moderately granular laterally. Union of 

 the transverse enlargements (jf neighbouring septa near the columella often occurs, and gives a paliforni appearance. 

 Endotheca and epitheca exist." (Duncan, 188.5, p. 88.) 



