400 



CHORDATE ANATOMY 



react to food placed on the tentacles, but not to food placed near the 



mouth. The chemical sense receptors are 

 therefore probably neurosensory cells located 

 in the tentacles. 



Some flatworms search for bait placed at 

 a distance and are therefore credited with an 

 olfactory sense. 



Annelids are believed to have two sorts 



of chemical sense organs, paired ciliated 



"olfactory organs" near the anterior end of 



the body, and sense buds formed of clusters 



<AefoVV„?:lh"wf„a°^:„S Of cells, scattered in various parts of the 



groove (o) leading from naris body, each ccll terminating in a sensory 



S-s'^cZpl'-atiirATaJr^TS bristle. Tlrese latter are of special interest, 

 Vertebrates," after Jordan- since they resemble roughly the neuromasts 

 vermann.) ^^ vertebrates. 



That the olfactory organs of vertebrates have evolved from any of 

 the many sense organs of invertebrates 

 has not been demonstrated. It is, 

 however, possibly significant that the 

 olfactory receptors of chordates are 

 neurosensory cells which, like those 

 of invertebrates, spin their own 

 neurites, but persist as constituent 

 elements of the olfactory epithelium. 



The olfactory epithelium of most 

 vertebrates is of the simple columnar 

 type, in which neurosensory cells are 

 uniformly distributed among support- 

 ing non-nervous epithelial cells. Each 

 receptor terminates on the surface in 

 a brush of fine hairs, and is prolonged 

 from its basal end into a neurite, which 

 breaks up in numerous telodendria 

 within the olfactory bulb. 



Olfactory organs seem to be lack- 

 ing in the protochordates, yet these 

 respond to chemical stimulation, so 

 that they must be assumed to have 

 chemo-receptors. 



Amphioxus also lacks a specialized 

 olfactory organ and does not hunt for its food, but hes buried in the sand. 

 A chemical sense, however, Amphioxus indisputably has, since it responds 



Fig. 353. — Head of human embryo 

 with pharyngeal floor removed. Cut 

 surfaces lined. Compare with Fig. 

 352; b, lung; cs, cervical sinus; e, eye; 

 h, hyoid arch; hd, hypophysial duct 

 (Rathke's pocket) ; Ig, lacrimal groove, 

 md, mandible; n. naris; on, oronasal 

 groove; tr, trachea. (From Kingsley's 

 "Comparative Anatomy of Verte- 

 brates," after Hertwig.) 



