Evolution of Fishes from Invertebrates 65 



and is further innervated by "an antero-dorsal hollow out- 

 growth from the brain." In Cyclostomes the olfactory 

 organ is similarly unpaired in the embryonic state and re- 

 mains so in the hag-fishes; but in the lampreys as adult 

 life is reached a vertical median septum divides the organ 

 into halves, and these are supplied by two olfactory nerves 

 from the brain. In gnathostome fishes two distinct and 

 paired olfactory organs, that are innervated by two ol- 

 factory nerves, are present. In many teleosts an evident 

 homologue to the head glands of metanemerteans is seen 

 in the glandular "nasal sacs," which similarly are back- 

 ward and dorsal prolongations from each nasal organ. 



(b) The eyes or optic organs. In metanemerteans 

 these show great variation, alike as to number, paired or 

 median position, complexity of structure, and function. 

 Even in one genus, like Geonemertes^ they may vary from 

 the most common number four, to as many as thirty or 

 forty (55: no). At times the anterior of the four usually 

 present are paired with each other and are well developed, 

 while the posterior pair may be smaller, more or less ap- 

 proximated, and even placed in submedian position with 

 each other. This anticipates and resembles the parietal 

 and pineal eyes of the cyclostomes and higher fishes. In 

 structure they show all of the fundamental parts that 

 characterize fishes and higher vertebrates. In position 

 they lie above and in front of the brain. 



(c) The auditory organs. In the great majority of 

 metanemerteans paired structures open along the postero- 

 lateral part of the head and have been termed the cerebral 

 organs. In 19 14 the writer drew attention to their struc- 

 tural similarity to the embryonic ear of vertebrates 

 (7:429). They attain their highest development in 

 meta- and hetero-nemerteans, where they appear as 

 swellings behind or beside the front part of the dorsal brain- 

 lobes. They open by small external pores along the sides 

 of the head, and at the bottom of the head-grooves. In 

 cyclostomes and elasmobranchs the orifice of the ear in the 

 embryo is in line with the first inpouching of the branchial 

 system, and so occupies the same position as in nemerteans, 

 since the writer regards the head-grooves and the first 



