392 



FISHES 



condition of the Cyclostomes is probably a secondary acquisition. 

 At the earliest embryonic stage at which any trace of an olfactory 

 organ is apparent, there is a median thickening of the epidermis, 

 possibly a vestige of some older sensory organ comparable, it may 

 be, to the so-called olfactory organ of Amphioxus ; on each side 

 of it there is a lateral thickening, the rudiments of the paired 

 organs.i The three thickenings, or " plakodes," then sink inwards 

 to form an olfactory pit. The partial subdivision of the adult 

 organ by a vertical septum, and the presence of two olfactory 



na.oA 



, /fv.10 



ptyf 



Fig. 225. — Side view of the brain of Petrmnyzon, with the olfactory organ and the 

 pituitary caecum in section, chbn, Cerebellum ; crb.h, cerebral hemisphere ; dkn. 

 thalamencephalon ; /, fold in the nasal tube ; gl, nasal glands ; inf, infundibulum ; 

 l.gn.hh^ left ganglion habenulae ; med.nhl, medulla oblongata ; na.ap, naso-pituitary 

 aperture ; ii.ch, notochord ; Xv^-nv^'\ cranial nerves ; Xr^', first ventral spinal 

 nerve ; olf.cp, olfactory capsule ; olf.l, olfactory lobe ; olf.in.iii, olfactory mucous 

 membrane ; opt.ly optic lobe ; ]}ii, pineal body ; pn' , inferior pineal body ; pn.?, 

 parietal eye ; pty.h, pituitary body ; pty.p. pituitary cul-de-sac ; &p. median septum 

 of the olfactory sac ; si^, first dorsal spinal nerve. (From Parker and Haswell, after 

 Ahlborn and Kaenische. ) 



nerves, point to the same conclusion.^ All Fishes possess olfactory 

 organs which are obviously paired. In Elasmobranchs and 

 Dipnoi they retain their primitive ventral position. Many 

 Sharks and Dog-Fishes possess an oro-nasal groove leading from 

 each olfactory organ to the corresponding angle of the mouth. 

 The Dipnoi proceed a stage farther, and, by the conversion of 

 the grooves into short canals, the olfactory pits communicate 

 with the mouth by true internal nostrils, as in the higher 



' Kupffer, Stud, vcrgl. JSntioickl. d. Kopfcs d. Kraniaten, iii. 1895, p. 6. 

 - In Bdellostoiiia the olfactory organ arises as a pair of outgrowths from the 

 pituitary involution (Bashford Dean, Kuptfer's Festschrift, Jena, 1899, p. 269). 



