. The causes and course of organic evolution; . Fig. 19.—a, b, section of ciliated head-grooves in Turbellaria rhabdocoela;c, section of ciliated furrow of Nemertean; d, section of branchial sac of Cyclo-storae; e, do. of caecilian Amphibian. into the pouch, as in nemerteans and cyclostomes, push out-ward and may attain great length as in Ichthyophis of theApoda or Cseciliada so carefully studied by Sarrasin. Theinternal gills are lamellae that arise on the inner ventral sur-face of each pouch, later as a rule than do the external, andwhich become greatly more branched and complex than inthe t
. The causes and course of organic evolution; . Fig. 19.—a, b, section of ciliated head-grooves in Turbellaria rhabdocoela;c, section of ciliated furrow of Nemertean; d, section of branchial sac of Cyclo-storae; e, do. of caecilian Amphibian. into the pouch, as in nemerteans and cyclostomes, push out-ward and may attain great length as in Ichthyophis of theApoda or Cseciliada so carefully studied by Sarrasin. Theinternal gills are lamellae that arise on the inner ventral sur-face of each pouch, later as a rule than do the external, andwhich become greatly more branched and complex than inthe two more ])rimitive grou]>s of the cyclostomes and thenemerteans. Evolution of Animals 447 As previously explained the cerebral organ, that we haveviewed as a probable auditory structure, arises from somepart—usually the cervical or precer\dcal portion—of the grooveor furrow. But, from the simplest vertebrate up to man,the auditory organ arises from the first gill cleft, visceral cleft,spiracular cleft, or hyomandibular cleft, as it ha
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