352 THE BIOLOGY OF MARINE ANIMALS 



In the higher Crustacea, at least, most of the body surface shows 

 chemosensitivity. This applies especially to contact chemoreception, 

 mediated by pore organs which consist of sensory buds lying beneath the 

 integument and communicating with the exterior through capillary ducts. 

 Distance chemoreception is most acute in the mouth parts and small 

 antennae. The external ramus of the decapod antennule is provided with 

 basiconical hair organs, variously known as aethetascs, olfactory clubs, 

 etc., and presumably having an olfactory role. Osmoreceptors are believed 

 to occur at the ends of the small antennae in crabs (Jasus). Stimulation of 

 this region with dilute sea water (four-fifths to three-quarters full strength) 

 initiates flight reactions. Some threshold values for chemoreception in 

 Crangon are: cucumarin, 1 in 500,000; acetic acid and saccharin, 1 in 100. 

 Glycogen is a strong attractant for Crangon, and for Asterias and Nassarius, 

 all of them predators or scavengers (15, 16, 91, 147). 



The ciliated pit of ascidians appears to function as a chemoreceptor, 

 sampling the water entering the mouth. It is believed to control feeding and 

 release of gametes. When fed with eggs and sperm of their own species, 

 ascidians respond by releasing gametes. Under this mode of stimulation, 

 the neural gland, containing the ciliated pit, releases gonadotrophin which 

 passes to the neural ganglion; the latter, being excited, then activates the 

 gonads by nervous pathways to release gametes (25). 



Fishes. In lower vertebrates the chemical senses are important sensory 

 modalities in determining various aspects of behaviour. Well-defined 

 chemoreceptors for smell and taste are present and, in addition, exposed 

 mucous and external surfaces possess a diffuse chemical sense mediated 

 by free nerve endings (Fig. 8.1). 



The olfactory receptor cells of fishes are localized in an olfactory 

 epithelium lying in nasal pits. Cyclostomes have a single olfactory aper- 

 ture; fishes bear a pair of olfactory pits. In elasmobranchs the olfactory 

 pits lie on the ventral surface of the snout, and sometimes open into the 

 mouth. In teleosts the pits lie on the upper or lateral surface of the head 

 and the aperture into each pit is usually divided by a skin fold into an 

 anterior and posterior opening. Water passes through the olfactory pit from 

 the anterior to the posterior opening (Fig. 8.31). 



Peculiarities of organization are encountered in some species: in the 

 hammerhead shark the olfactory pits are widely separated; in many teleosts 

 each olfactory organ is prolonged posteriorly into a saccular structure; 

 and in the Chinese sole (Cynoglossus semilaevis) the two sacs unite over the 

 roof of the mouth. A naso-vomerine organ (Jacobson's organ), located in 

 the nasal septum of some fishes, functions as an accessory olfactory 

 structure. 



The olfactory epithelium is made up of columnar supporting cells, 

 among which occur the olfactory receptors (Fig. 8.32). These are bipolar 

 neurones, bearing an elongated hair-like structure (the dendrite) which 

 extends to the free surface, and giving off an axon proximally to the 

 olfactory nerve. The distal hairs are stimulated by chemical substances, 



