BEHAVIOUR OF SHORE ANIMALS 225 



than those of worms. The diffuse and superficial system 

 of the forms hitherto studied has been replaced by a definitely 

 centralised and more deeply situated band of tissue or 

 nerve-cord. In certain worms, e.g. Sigalwn, the nerve-cord 

 still retains its connection with the ectoderm, and though 

 it has become completely separated off in the earthworm, 

 in Nereis the brain still retains its connection with the skin. 



Contrasting the so-called " Httle brain " type of the 

 higher invertebrates, which reaches its culminatmg point in 

 insects, and the " big brain " type of vertebrates reaching 

 its highest development in birds and mammals, Sir Ray 

 Lankester notes that the former is distinguished by its 

 wealth of ingrained capacities of instinctive behaviour and 

 its comparative ineducability. The big-brain type, on the 

 other hand, is relatively poor in ingrained capacities of 

 instinctive behaviour, but has a remarkable capacity for 

 learning. 



Sense organs in marine worms (Polychaets) are frequently 

 well developed. Thus Nereis has eyes, tentacles, palps 

 and cirri all situated on the front of the head. The tentacles 

 and cirri are tactile in function, the axis of each organ being 

 traversed by a nerve whose fibres terminate in sensory cells. 

 The palps are probably organs of chemical sense and 

 combine the functions of taste and smell (Benham, 1896). 

 Eyes are not necessarily confined to the first segment of 

 the body but may occur elsewhere, the situation depending 

 on the particular conditions of life. In some of the tube- 

 worms eyes occur on the gill-filaments, their dark colour 

 being in sharp contrast to the bright pink of the blood circu- 

 lating in the filaments. 



Another type of sense organ found in some Polychaets, 

 e.g. the lug-worm Arenicola, is an otocyst, perhaps better 

 called a statocyst. Though not common in worms, a similar 

 organ is of considerable importance in Crustacea. These 

 statocysts are equilibrating organs rather than organs of 

 hearing and probably keep the animal informed as to its 

 position in space. In Arenicola, a pair of statocysts is 

 situated at the base of the prostomium. Von Buddenbrock 



Q 



