Bull. U. S. F. C. 1900, Vol. 2. (To face page 324.) 
Plate A. 
Fig. 1. — Diagrammatic representation of a stage in the development of 
a Hexactinian polyp. Tiie six bilateral pairs of mesenteries num- 
bered I to VI, and indicated by thicker lines, are the protocnemes, 
the fifth and sixth pairs being as yet incomplete; pairs III, III and 
IV, IV, respectively, represent the ventral and dorsal directives. In 
each of the six primary exocceles a unilateral pair of metacnemes has 
appeared, the sequence being bilateral, and from the dorsal to the 
ventral aspect of the polyp. At a later stage all the primary mesen- 
teries become complete, and form a first cycle; the six pairs within 
the primary exocoeles attain practically a uniform size, and constitute 
a second alternating cycle. Other unilateral pairs may arise in the 
exocoeles between the members of the first and second cycles, and 
give rise to third, fourth, or fifth alternating cycles. Owing to the 
presence of directives the polyps attain only a biradial symmetry. 
Fig. 3.— Diagrammatic arrangement of the mesenteries in Cerianthus. 
The four bilateral pairs represented by thicker lines have generally 
been considered to correspond with the Edwardsian mesenteries (I 
to IV) of Hexactinian and Zoanthid polyps, but according to van 
Beneden no such comparison can be maintained. New mesenteries 
are added as bilateral pairs at only one axial region of the polyp. 
According to Carlgren, this is the ventral or posterior aspect. The 
mesenteries remain acyclic and the polyps possess bilateral sym- 
metry. 
Fig. 2. — Diagrammatic representation of mesenteries in a brachytypic 
Zoanthid. As in previous figure the thicker mesenteries numbered 
I to VI are the protocnemes. The fourth (dorsal directives) , fifth, 
and sixth pairs are permanently incomplete, except in the Macro- 
cneminse, where the sixth pair becomes complete. The metacnemes 
are all developed within the exoccele on each side of the ventral 
directives, and are arranged in unilateral pairs consisting of a brachy- 
cneme and a macrocneme. The mesenteries remain acyclic and the 
polyps exhibit bilateral symmetry. 
Fig. 4. — Diagrammatic arrangement of the mesenteries in a polyp 
of the coral Porites. Usually only the six protocnemes are devel- 
oped in adult polyps, the Edwardsian pairs alone complete 
Additional mesenteries sometimes arise as bilateral pairs within 
the entocoele of either the dorsal or ventral pairs of directives; 
in the figure three new pairs' have appeared within the dorsal 
directive entocoele. The mesenteries remain acyclic and the 
polyps exhibit bilateral symmetry. 
