. A text-book of invertebrate morphology. Invertebrates. TYPE 0(ELENTEBA. 113 7. Older Hexactiniae. In the HexactinisB the six primary pairs of mesenteries described as occurring in the two preceding orders are again found (Fig. 65, /, D, and D), and in a few forms (Ealcampa) may be the only ones present. As a rule, however, a varying D. Fig. 65.—Diagrammatic Thansvbbsb Section op an Hbxactinian, Aip- tasia, with only the mesenteries of the first cycle perfect. D = directive mesenteries. / = mesenteries of the first cycle. p = reproductive region of mesentery. 11= " " "second&qu
. A text-book of invertebrate morphology. Invertebrates. TYPE 0(ELENTEBA. 113 7. Older Hexactiniae. In the HexactinisB the six primary pairs of mesenteries described as occurring in the two preceding orders are again found (Fig. 65, /, D, and D), and in a few forms (Ealcampa) may be the only ones present. As a rule, however, a varying D. Fig. 65.—Diagrammatic Thansvbbsb Section op an Hbxactinian, Aip- tasia, with only the mesenteries of the first cycle perfect. D = directive mesenteries. / = mesenteries of the first cycle. p = reproductive region of mesentery. 11= " " "second" 7n^ = mesenterial filament. 111= " " "third " number of secondary pairs develop, each of these appearing in the interval between two primary pairs, so that two cycles of mesenteries (/ and //) may be distinguished. Usually, however, the process of mesentery formation does not stop here, tertiary {III), quaternary, etc., cycles being developed, the pairs of each new cycle appearing in the intervals between the pairs of the cycles already present. Consequently, since there are six primary pairs, the second cycle will consist also of six pairs, the third of twelve, the fourth of twenty-four, and so on. In a few forms, owing to the precocious development of one or two of the secondary pairs on each side, the symmetry becomes converted from an hexamerous one to an octamerous {Aiptasia annulata) or a decamerous one {Tealia). Since the tentacles develop in connection with the spaces between the mesenteries, they are arranged in cycles corre-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original McMurrich, J. Playfair (James Playfair), 1859-1939. New York, H. Holt and Company
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