. The Danish Ingolf-Expedition. Scientific expeditions; Arctic Ocean. PENNATUUDA. youngest polyps in each transverse series are those nearest to this line; the median line and its more immediate surroundings is often also sterile of polyps; when this is the ease the naked streak, how- ever, is narrower here than on the opposite surface, with the exception of the lower part towards the peduncle (Renilla, however, is a remark- able exception). Thus the two median surfaces of the stem are gener- ally easily distinguished r). Making the median line our starting-point we might say that the surface
. The Danish Ingolf-Expedition. Scientific expeditions; Arctic Ocean. PENNATUUDA. youngest polyps in each transverse series are those nearest to this line; the median line and its more immediate surroundings is often also sterile of polyps; when this is the ease the naked streak, how- ever, is narrower here than on the opposite surface, with the exception of the lower part towards the peduncle (Renilla, however, is a remark- able exception). Thus the two median surfaces of the stem are gener- ally easily distinguished r). Making the median line our starting-point we might say that the surface D shows a centrifugal development of the polyps, the surface Vs. «centripetal one; so far we might well abandon the terms ventral and dorsal side, and instead of them use centrifugal and centripetal side; these latter names, however, are somewhat clumsy in descriptions, as is also the case, I think, with the terms proposed by Bourne2): prorhachis (for D) and metarhachis (for V); dorsal and ventral side are, and will always be, the most handy names, if only a rational choice were agreed upon. In m y choice I have started from the following consideration: the stem of the sea-pens, rhachis -\- peduncle, is the direct product of the individual developed from the egg (the oozooite of bacaze Duthiers); this primary individual is for some time a solitary polyp of the typical octocorallian structure; accordingly, it is bilaterally symmetrical with respect to a plane naturally called a dorso-ventral plane; the mouth and pharynx (stomodaeum) are oval with the longer axis in the plane of symmetry; this plane bisects two median chambers, mutually diffe- rent and differing from all the others: only one of them contains re- tractor muscles, and this one is on the same side as the ciliated groove (sulcus, siphonoglyphe3)) of the pharynx; it has long ago been agreed on calling this csuicar side the ventral side in all polyps of octo- corals. If we will keep this designation here, we must acco
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