SENSORY ORGANS AND RECEPTION 351 



Chemoreception has been little studied in marine annelids. All parts of 

 the body of Nereis virens are sensitive to dilute solutions of KOH, HC1 

 and NH 4 C1. Sensitivity is greatest in the palps, less in tentacles and 

 tentacular (peristomial) cirri. This animal also responds to meat juices. 

 Various species of commensal polynoids are attracted by chemical sub- 

 stances given off by their hosts (starfishes, echiuroids, polychaetes) (36, 

 57). 



The chemical senses are well developed in molluscs and are concerned 

 with feeding responses in some species. Osphradia — sensory patches — 

 located near the opening of the mantle cavity in most marine species, 

 respond to tactile and chemical stimuli and test water entering the mantle 

 cavity. Extirpation of the osphradium of carnivorous gastropods results 

 in loss of ability to follow a scent or trail. Taste buds also occur in the 

 buccal cavity of some species. Chemoreceptors of cephalopods occur in 

 pitted papillae lying below the eyes. 



The limpet Patella responds positively when splashed with sea water, 

 and negatively to fresh water. This behaviour pattern is of value to lim- 

 pets living high on the shore, where they are exposed by the tide and 

 washed by the rain. Mantle fringes and tentacles are involved in 

 salinity perception {la). Chiton shows chemical sensitivity to selected 

 stimulatory agents, and reacts negatively to n/500 HC1, n/500 KOH, n/160 

 KC1, and m/1500 picric acid (minimal effective concentrations). These are 

 about the same order of values as for the gustatory sense of man. Sensitivity 

 is greatest in the lips, least in the girdle. The general body surface of Chro- 

 modoris is likewise sensitive to similar chemical reagents. Rhinophores and 

 oral tentacles are the most sensitive regions. These sea-slugs give chemo- 

 positive responses to secretions of other individuals, responses which lead 

 to copulation (6, 33). Pecten reacts strongly when exposed to juices of the 

 starfish, its natural enemy. Patella likewise reacts vigorously to the odour 

 of the carnivorous snail Murex. The presence in minute quantities of some 

 factor behaving as a carbohydrate directly affects the pumping rate of 

 oysters (23, 29). The bivalve Scrobicularia perceives and reacts to osmotic 

 changes in the medium by keeping its valves closed (50#). Stimulus tests 

 reveal that the octopus is sensitive to weak acid, quinine and traces of 

 musk. Chemical sensitivity appears to play but a small part in the life of 

 this animal, however. 



Echinoderms (starfish, urchins and cucumbers) are sensitive to food 

 juices and various reagents, applied directly to the body or arising from a 

 distant source. Chemoreception is important in evoking the normal food 

 reactions of the starfish (Asterias). Starfish respond to juices of shellfish 

 placed near them ; indeed a hungry starfish will follow a piece of meat which 

 is moved about the aquarium. The importance of chemical signals as an 

 indication of prey is also shown by the behaviour of a starfish to small 

 crabs placed on its back. So long as the crabs are uninjured they are carried 

 without concern; but as soon as the crab is injured and its fluids escape, 

 the tube feet reach up and pull the crab towards the starfish's mouth. 



