226 THE BIOLOGY OF THE SEA-SHORE 



(19 1 3), who has studied these structures in the lug-worm 

 Arenicola and other forms, including the bivalve Solen 

 vagina, states that they have nothing to do with the perception 

 of oscillations in the water, but effect an increase in the 

 tonus of the longitudinal muscles on the side that is under- 

 most and cause their contraction. There is also a musculo- 

 sensory regulation which always tends to keep the tail 

 parallel to the head, and these two factors co-operate to enable 

 Solen to bore in the sand in a vertical, positively geotropic, 

 fashion. 



The tough cuticle which is so characteristic a feature of 

 Crustacea would not seem to point to a high degree of 

 sensitivity in these animals. As a matter of fact, however, 

 many of the numerous setae which are scattered all over the 

 body and limbs of a typical Crustacean have a sensory 

 function and render these animals more sensitive to stimuli 

 of touch, at least, than might be expected. These setae 

 are hollow extensions of the integument, movably articu- 

 lated at the base, and contain a prolongation of some of the 

 cells of the skin, among which lie one or more nerve fibres. 

 When feathered or provided with secondary barbs the 

 setae will react to movements or vibrations of the water. 

 Some setae of this type are thought to have an auditory 

 function (Caiman, 1909). 



The so-called " ear " of Crustacea, really a statocyst 

 or equilibrating organ, is a small, usually open sac placed, 

 in typical cases, upon the basal joint of the first pair of 

 antennae and having its inner surface lined with sensory 

 cells of the kind described above, each of which contains a 

 nerve fibril. Within the cavity of the sac are a number of 

 tiny sand-grains, the contact of which with the sensory hairs 

 results in impulses being transmitted along a nerve to the 

 cerebral ganglia, from which messages are then sent out to 

 the muscles. 



The antennules themselves are thought to be responsive 

 to changes of a chemical nature in the surrounding water. 

 In the case of the fresh-water crayfish the setae on the 

 exopodites of the antennules have been definitely shown to 



