450 Comparative Animal Physiology 



are probably sensitive to both chemicals and touch. Taste buds are described 

 in the buccal cavity of some species, and ciliated papillae known as rhino- 

 phores and regarded as olfactory sense organs are found on the cephalic 

 tentacles of many land snails. The cephalopods have chemoreceptors in pit- 

 ted papillae just below the eye on each side of the head. 



Pecten reacts negatively to many substances, particularly to the juice of its 

 natural enemy, the starfish. Chiton also reacts negatively to N/500 HCl, 

 N/500 KOH, N/160 KCl, and M/1500 picric acid. Octopus also is sensi- 

 tive to weak acid, to quinine, and especially to musk. 



Starfish have well developed chemoreceptors which are sensitive to meat 

 juices and odors. A hungry starfish can be led about the floor of an aquarium 

 by a piece of meat on the end of a pair of forceps. Asterias will carry small 

 uninjured crabs on its back for an hour or more, but if the crab is crushed so 

 that juices escape, the tube feet reach up immediately and pull the crab 

 toward the mouth. Holothurians are sensitive to various chemicals, and the 

 order of decreasing effectiveness for one series is as follows: HCl, atropine, 

 acetic acid, KOH, KCl, maltose, acetamid, and glycerin. 



Annelids have fairly well developed chemical senses, and there also seems 

 to be some differentiation into organs of taste and smell. Darwin demon- 

 strated that earthworms could distinguish between green and red cabbage, 

 onion leaves and those of either cabbage or horseradish, celery leaves and 

 those of cabbage and parsnip, and also between leaves of carrots and those 

 of celery. These experiments, and others, seem to be explicable on a basis of 

 taste, probably localized in the taste sense organs of the buccal wall and 

 pharynx. Darwin believed that selection in certain instances (as between 

 onions and cabbage) might have been aided by odors. 



There are also, in the cuticle of the body wall of Lumhricus, sense organs 

 which are very similar in structure to vertebrate taste buds and which seem 

 to serve the same function. The earthworm is sensitive to quinine, and the 

 distribution of susceptibility in various parts of the body parallels the distri- 

 bution of the sense organs: the anterior end reacts to 0.04 per cent, the pos- 

 terior end to 0.03 per cent, and the region just behind the clitellum to 0.2 

 per cent. 



The oligochaete Eisenia reacts to acids and is reported to be able to dis- 

 criminate between H2SO4 and either HCl or HNO3. In contrast to the 

 situation in human taste, in Eisenia the cation of salts seems to be more im- 

 portant than the anion. Eisenia also reacts negatively when xylene, turpen- 

 tine, or ether is held near the anterior end, but not when the substance is 

 held near the posterior end. Therefore, it seems that Eisenia has two types 

 of chemosensitive receptors. 



Chemoreception in the Arthropods. The Insects. Among the inverte- 

 brate animals chemoreceptors have achieved a great degree of structural and 

 functional differentiation in the insects. In the terrestrial insects and those 

 aquatic species which are occasionally terrestrial, there is good evidence for 

 the existence of two types of chemoreceptors: the olfactory receptors, which 

 react to very low concentrations of compounds that are volatile at ordinary 

 temperatures; and the taste receptors, which react on contact with liquids or 

 substances in solution that may or may not be volatile at moderate tempera- 



