Chemorece'ption 449 



flagellates Bodo and Chilomonas, and many small ciliates, for instance, often 

 aggregate at a given distance from the edge of the cover slip. Aggregation 

 also occurs in oxygen gradients produced by other causes, e.g., by the removal 

 of O2 from the environment. 



The effectiveness of various ions on ciliary reversal of Paramecium has been 

 studied by Mast and Nadler^-' and by Oliphant.'*** Monovalent cations are 

 effective (K+>Li+ >Na+ >NH4+), bivalent cations are not effective, and 

 anions have little or no effect. 



Sponges may react to certain irritant substances (strychnine, cocaine, ether, 

 chloroform) by contraction so that the size of the orifice, and consequently 

 the flow of chemical through to the animal, is reduced. Since there are no 

 special sense cells and no nerve cells it seems that the contractile cells must 

 be stimulated directly. 



Coelenterates are quite sensitive to meat juices, which, if added to the 

 aquarium water containing sea anemones, may cause a great expansion and 

 waving about of the tentacles. If meat juice is placed close to the tentacles 

 the mouth may open. Some anemones, at least, can discriminate between 

 food and inert bodies. Filter paper if soaked in meat juice may be accepted 

 by the tentacles and then rejected either before or after ingestion. However, 

 a hungry anemone may swallow even stones. Nevertheless, there is excel- 

 lent evidence of chemoreception in the tentacles of most or all anemones and 

 around the mouth of some, but this sensitivity is not present in the remainder 

 of the body. Jellyfish also react to meat juices. It has also been noted''*^ that 

 isolated oral arms of Aurelia and Cyanea give a normal grasping reaction to 

 meat juice but not to sugars, starches, and glycogen. 



The orientation to food by a pond planarian, P. In^tihris, has been studied 

 in considerable detail by Koehler.^" The food is placed some distance from 

 the animal and the initial reaction is one of increased random motion on the 

 part of the animal, called orthokinesis (see Chapter 11, pages 386-387). The 

 next phase of the reaction consists of crawling along a very convoluted path, 

 the turning of the animal occurring sooner if it moves into a region of lower 

 concentration of the stimulating agent or agents than if it moves into one of 

 higher concentration. This type of activity, bringing the animal closer to 

 the food in a very indirect manner, is called klinokinesis. When the food is 

 a short distance away, i.e., 8 cm., the planarian follows a path which leads 

 straight to the food but it stops frequently and waves its anterior end from 

 side to side, a type of behavior which indicates the comparison of intensities 

 of stimulation at different times, called klinotaxis. When the animal is but 

 several centimeters from the food, the path may be direct without any turn- 

 ing of the anterior end. This type of behavior indicates the simultaneous 

 comparison of intensities and is called tropotaxis. The olfactory receptors 

 are concentrated in regions at the sides of the head, in the auricular organs, 

 and the taste receptors are located in the center. Chemoreceptors are also 

 distributed generally over the external surface of flatworms. 



Most molluscs apparently locate food by means of their chemical senses, 

 and terrestrial snails are reported to ignore food if they are separated from it 

 by glass. The location of specific chemoreceptors has not been well demon- 

 strated in most molluscan groups. The osphradia of most marine molluscs 



