4o6 CHORDATE ANATOMY 



To stimulate the taste cells, substances must be dissolved, so that they 

 can penetrate the pore-like opening of the bud. Only four kinds of sub- 

 stances can be tasted by us, sweet, sour, bitter, and salt. Other flavors 

 are smelled, not tasted. 



VISUAL ORGANS 



Sensitivity to light is a wide-spread, if not a universal, property of 

 hving cells. An Amoeba will not enter strong light; and sea anemones, 

 among other creatures, react to light, although they have no specialized 

 photoreceptors. All flagellates which are phototropic have a red pigment 

 spot or stigma like that of Euglena; and in general, in multicellular crea- 

 tures, only those cells respond to light which contain some such pigment 

 as visual purple, which is altered by light. 



The so-called eye-spots of coelenterates are pigment cells which respond 

 to changes in intensity of light. Many medusae have clusters of pig- 

 mented columnar epithelial cells on the margin of the umbrella, which are 

 interpreted as photic organs, since the animal ceases to be phototropic 

 when these are removed. The jelly-fish Nausithoe has a lens associated 

 with each of these pigment spots. 



Many species of flatworms have, closely associated with the brain, 

 paired pigment spots which are interpreted as eyes. In some species, 

 beaker-Uke clusters of pigmented cells surround the terminations of 

 sensory nerves, and may, therefore, be regarded as among the most 

 primitive of true eyes. 



While, however, photoreceptors occur in coelenterates and all phyla 

 above, true vision is limited to those forms in which the photoreceptors are 

 aggregated into eyes capable of forming an image on a retina. Among 

 invertebrates two types of eyes predominate, beaker eyes and vesicular 

 eyes. Beaker eyes get their name from their shape. They usually have 

 a core or plug of cuticula. Vesicular eyes, however, have a liquid-filled 

 cavity around which the photoreceptor cells are arranged. 



The eyes of annelids are varied. Some free-swimming forms have 

 beaker eyes with spherical lenses and a layer of retinal cells connected by 

 nerve fibers with the brain. In some, the beaker-like eyes are connected 

 with the epidermis; in others the eye sinks below the skin. Nereis has 

 vesicular eyes. The differences among the various types of annelid eyes 

 are so great that it is impossible to believe that the eyes are genetically 

 related to one another. 



Urochordates have pigment spots and an unpaired eye, neither of 

 which can be compared with those of vertebrates. Such forms as Salpa 

 have a median vesiculated eye derived from the brain and usually pig- 

 mented. Only in their origin from the brain do such eyes resemble those 

 of vertebrates. 



