531 Causes and Course of Organic Evolution 



But as Fr. Miiller and Grobben have suggested, this nuchal 

 gland may give origin to the "dorsal organ" that is so marked 

 a feature in many crustaceans. Now Gosse observed two 

 dorsal bodies in some rotifers that closely resemble the one 

 already referred to, while the dorsal organ in some Crustacea 

 (Amphipoda-Isopoda) is a median dorsal ingrowth, in others 

 a paired disk, or as in Mysis the paired bodies may be wide 

 apart according to Nusbaum {171, I: 153). This history is 

 of special value in connection with shell origin. For, after 

 the 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; d.h., dorsal brain: v.h., ventral 

 brain or ganglia; e, eyes; s, lorica or shell; s.g., shell gland. The liver in both 

 is outlined on the dorsal side of the stomach. 



Such is well seen in the Ostracoda, etc. But the connecting 

 stage between these two phases that completely harmonizes 

 with rotiferan conditions is seen in the cirripedes, where, in 

 the 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 the 

 Cypris stage." These "two shell- valves, which can be brought 

 together by an adductor muscle, are directly continuous dor- 

 sally" however. 



Korschelt-Heider incline to regard even the lateral valves 

 as extensions of the dorsal, for they say, speaking of Limnadia 

 and Estheria, "although here, as in Cladocera, the two halves 

 of the shell at first develop separately, we must, nevertheless. 



