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Show 1896.] PLANKTON OP THE PAEROE CHANNEL. 995 form, mouth dorsal, cloaca terminal. Surface smooth. Length of sexual form 12 m m . T E S T clear, transparent, thin. M A N T L E . - I n the sexual form the musculature exhibits an asymmetry similar to that already described in 8. dolichosoma-viryula, musculosa-punctata, and magalhanlca 1. Tbe mouth has a pair of sphincters, apparently formed by splitting of two lateral longitudinal muscle-slips. At least one sphincter surrounds the cloacal aperture ; but the arrangement of the musculature of both apertures was extremely difficult to make out, owing to the bad condition of the specimens. The order, or rather the disorder, of the main muscles is more easily appreciated from drawings than from a description (Plate L. figs. 5, 6, a-f). In addition to these there are two dorsal longitudinal muscle-slips, a dorsal sheet overlying the nucleus, and a fan-like sheet on the right of the nucleus. In the solitary form, extracted with the placenta from the parent, the musculature is much more regular ; it consists of eight complete bands, two large and (?) four small circum cloacal spbincters (the arrangement of which could not be exactly ascertained), a right and a left longitudinal slip of unequal length in connection with the two circumoral sphincters. E N D O S T Y L E fairly long and straight. D O R S A L L A M I N A large (diam. in posterior third about 5 m m . in sexual form), with strongly-marked ridges. No languet was detected. D O R S A L T U B E R C L E large, about 5 m m . in length in sexual form; transversely marked with fine bands of cells. VISCERAL M A S S comparatively small, brownish yellow in life. At first it seemed probable that one was dealing merely with a specimen curiously broken, and that the asymmetry was artificial. But specimens of this species were taken on many occasions, and all possibility of the above explanation was destroyed when I obtained several specimens which presented the same asymmetry, but in a " Spiegelbild," namely the reversal which would be produced by a reflection in a mirror. The same reversal or " inverse image" has been discussed at length by Apstein 2 on the basis of the three asymmetrical genera cited above. As the 'Ergebnisse der Plankton Expedition' are not readily accessible to everyone, and as the point is novel and of some interest, I quote Apstein's conclusions:-" Bei den iibrigen Salpen, die eine symmetriscbe Muskulatur haben, ist Spiegelbild und Kongruenz dasselbe, bei einern unsymmetriscben Korper aber fallen Spiegelbild und Kongruenz nicht zusammen. Ich glaube jedocb, dass bei alien Salpen in der Kette die Individuen der eine Reihe gleich, d. h. kongruent sind, aber zu denen der anderer Reihe spiegelbildlich sich verhalten, aber dass dies in der Muskulatur 1 Apstein, ' Ergebnisse der Plankton Expedition: Die Thaliacea.-B. Ver-teilung der Salpen,' p. 17. 2 Apstein, loc. cit. p. 17. |