350 THE BIOLOGY OF MARINE ANIMALS 



avoiding injury and escaping from enemies. In general, chemoreception 

 plays an important role in conjunction with other sensory modalities by 

 enlarging the animal's sensory field and increasing its environmental 

 experience. 



On the basis of human experience the sense of taste is referred to chemical 

 appreciation of substances in actual contact with the receptors, whereas 

 smell refers to detection of substances at a distance. Taste or gustation, 

 therefore, is concerned with contiguous substances proceeding into solu- 

 tion and thus influencing the taste receptors, whereas smell or olfaction 

 refers to the detection of air-borne particles emanating from a distant 

 source. In the case of aquatic organisms this distinction breaks down, since 

 both taste and smell depend on particles in solution, whatever their source, 

 but it is convenient to retain this distinction between contact chemore- 

 ceptors (taste) and distance chemoreceptors (smell). A further distinction 

 between taste and smell lies in threshold of sensitivity. Taste is concerned 

 with substances in relatively high concentration and shows low sensitivity. 

 The olfactory receptors, on the other hand, are much more sensitive and 

 can detect substances in extremely low concentrations. In addition, animals 

 possess a general chemical sense, dependent upon the general irritability of 

 protoplasm. This general sense detects noxious chemical agents and has 

 low sensitivity. 



Chemoreception in Invertebrates. In protozoa and sponges there are no 

 specialized chemoreceptors, and chemosensitivity is a general property of 

 the body surface. Specialized sensory cells for chemoreception have been 

 described in higher metazoans ; these may be widely dispersed or localized 

 in distribution. The chemical sensitivity of many marine invertebrates has 

 been investigated, but little is known about the functioning of chemo- 

 receptors in these animals (123). 



Sponges, which lack a nervous system, react to chemical irritants, such 

 as ether and chloroform, by contraction of the osculum. Consequently 

 the flow of water and contained irritant matter through the sponge is 

 reduced. Specialized receptors are absent and the contractile cells respond 

 directly to the irritant substances. 



Among coelenterates, sea-anemones and medusae show pronounced 

 responses to meat juices, which represent a normal stimulus for these 

 animals. When meat juices are placed in the vicinity of anemones, they 

 react by expanding the body, opening the mouth and actively waving the 

 tentacles. Discrimination is shown between food and inert substances, the 

 latter usually being rejected. The chemical stimuli emanating from food- 

 stuffs set up prolonged excitation, and the food objects are usually ac- 

 cepted. Chemoreception is localized in the tentacles and oral region, the 

 latter being the more sensitive. Proteins, certain kinds of mucus, and 

 soluble protein derivatives are most effective in producing a feeding- 

 response, whereas starch and sugars are without effect. Again, the isolated 

 oral arms of Scyphomedusae (Awe Ha, Cyaned) give normal grasping 

 reactions to meat juices, but not to sugars, starch and glycogen (115). 



