The cnidae of four species of Mussidae are described and illustrated, Both Weill's (1934) nematocyst nomenclature, later modified by Carlgren (1940), and Schmidt's (1969; 1972; 1974) were used, Three species belonging to the endemic Brazilian genus Mussismilia Ort- mann, 1890, M. hispida (Verrill, 1902), M. hartti (Verrill, 1868) and M. braziliensis (Verrill, 1868), and one belonging to the world wide genus Scolymia Haime, 1852, S. wellsi Laborel, 1967, were studied. Specimens from different localities of the Brazilian coast were observed, Qualitative analysis of their cnidae showed that all the species studied present the same types and distributions of nematocysts, Five types were observed (sensu Schmidt): spirocysts, holotrichs I, holotrichs II, b-rhabdoids, and p-rhabdoids D. Some types presented morpho- logical varieties considered as sub-types. Four structural regions were examined: coenosarc/ edge zone, acrospheres, stomodaeum and mesenterial filaments. The nematocysts of a planula of S. wel/si were also observed. Qualitative and quantitative size analysis of cnidae showed differences at generic level only. The qualitative difference observed was that Mussismilia presents two size classes of p-rhabdoids D in the mesenterial filamcnts, while in Scolymia there is only one. Quantitative size analysis was performed using nested analysis ofvariancc (ANOV A) and the procedure ofStudent-Newman-Keuls (Sokal and Rohlf, 1979). These tests suggested that the size of three types of nematoeysts ean differentiate the two genera: b-rhabdoids from stomodaeum and holotrichs I and II from the mesenterial filaments. The importance of morphometries in the study of eoral cnidae is supported.
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