. The causes and course of organic evolution; . lear vesicle already referred toin rotifers. Though various views have been expressed re-garding it, it seems probable that both represent dorsal shellglands. Therefore the embryonic dorsal shell of Crustaceawould be homologous to the dorsal shell of loricate rotifers,of brachiopods, and of molluscs. 531 Causes and Course of Organic Evolution But as Fr. Miiller and Grobben have suggested, this nuchalgland may give origin to the dorsal organ that is so markeda feature in many crustaceans. Now Gosse observed twodorsal bodies in some rotifers that c
. The causes and course of organic evolution; . lear vesicle already referred toin rotifers. Though various views have been expressed re-garding it, it seems probable that both represent dorsal shellglands. Therefore the embryonic dorsal shell of Crustaceawould be homologous to the dorsal shell of loricate rotifers,of brachiopods, and of molluscs. 531 Causes and Course of Organic Evolution But as Fr. Miiller and Grobben have suggested, this nuchalgland may give origin to the dorsal organ that is so markeda feature in many crustaceans. Now Gosse observed twodorsal bodies in some rotifers that closely resemble the onealready referred to, while the dorsal organ in some Crustacea(Amphipoda-Isopoda) is a median dorsal ingrowth, in othersa paired disk, or as in Mysis the paired bodies may be wideapart according to Nusbaum {171, I: 153). This history isof special value in connection with shell origin. For, afterthe dorsal shell has formed in some larvae, it is gradually re-placed by a right and left bivalve shell as in lamellibranchs. ?n-. FiG. 25.—a, compounded diagram of loricate Rotifer; h, of larval Entomos-traca; m, mouth; a, anus; k, excretory tubes; , dorsal brain: , ventralbrain or ganglia; e, eyes; s, lorica or shell; , shell gland. The liver in bothis outlined on the dorsal side of the stomach. Such is well seen in the Ostracoda, etc. But the connectingstage between these two phases that completely harmonizeswith rotiferan conditions is seen in the cirripedes, where, inthe early larva, the lateral parts of the dorsal shield now al-ready begin to bend downwards, and, covering the body lat-erally, foreshadow in position the valves of the shell in theCypris stage. These two shell-valves, which can be broughttogether by an adductor muscle, are directly continuous dor-sally however. Korschelt-Heider incline to regard even the lateral valvesas extensions of the dorsal, for they say, speaking of Limnadiaand Estheria, although here, as in Cladocera, the two
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