. The causes and course of organic evolution; . y terminates in an enlargedsac. In Drepanophorus the cerebral canal is quite exceptional,in that it bifurcates, one branch terminating in a sac with sen- 430 Causes and Course of Organic Evolution sory epithelium, the other being glandular; this in D. crassusextends backwards beyond the brain as a free tube. In severalgenera of this order the cerebral organ lies in front of the brain(Tetrastemyna, sp. of Emiemertes, and of Amphiporus); in othersit lies at the side, and in still others behind the brain—in whichcase it attains a great size. In all
. The causes and course of organic evolution; . y terminates in an enlargedsac. In Drepanophorus the cerebral canal is quite exceptional,in that it bifurcates, one branch terminating in a sac with sen- 430 Causes and Course of Organic Evolution sory epithelium, the other being glandular; this in D. crassusextends backwards beyond the brain as a free tube. In severalgenera of this order the cerebral organ lies in front of the brain(Tetrastemyna, sp. of Emiemertes, and of Amphiporus); in othersit lies at the side, and in still others behind the brain—in whichcase it attains a great size. In all cases the organ is separatefrom the dorsal brain mass, from which it receives nerves.(Benhain in Lank. Zool. IV, p. 185). A comparison with the embryology and structure of thevertebrate ear leads us to believe that here one has to dealwith evolving stages leading toward that organ. The accom-panying diagram, copied from Burgers beautiful work, issuggestive, and can be compared with a figure of the embryonicear in a mammal. c/u. coch. —
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