426 THE BIOLOGY OF MARINE ANIMALS 



responses, anemones also show very slow contractions. These are called 

 forth by low-frequency stimulation and display long latency, smooth and 

 often variable character, e.g. slow contraction of the longitudinal parietal 

 muscles of Metridium. Several electrical stimuli are necessary to evoke the 

 slow response, which tends to increase with number of stimuli. Facilita- 

 tion, however, is not made evident by varying the frequency of stimulation. 



Despite the existence of diffuse conduction in the nerve-net, actinians 

 still display much complexity in their responses. Differentiation of activity 

 is possible because of variation in sensory properties (variation in threshold, 

 adaptation), the existence of facilitation in the nerve-net, occurrence of 

 two modes of muscular contraction, etc. Different parts of the nerve-net 

 are stimulated by different frequencies, thereby regulating the spread of 

 excitation and the preferential evocation of distinct response mechanisms. 

 The strength and nature of the response are governed also by the past 

 history and physiological condition of the animal, thus bringing the re- 

 sponse into line with its present needs (10, 80, 82, 83, 94b). 



The circular (swimming) muscles of jellyfish (Scyphomedusae) form 

 another system that is regulated by peripheral facilitation {Amelia, 

 Cyanea). In these animals, repeated excitation of the sub-umbrella nerve- 

 net recruits more muscle fibres by facilitation, and produces stronger con- 

 tractions (Fig. 10.6). The spontaneous swimming contractions, which are 

 normally initiated by the marginal sensory organs, also depend upon peri- 

 pheral facilitation, which determines the strength of the pulsations. Single- 

 fibre action potentials recorded from the bell of Aurelia show that the net 

 is non-polarized and that interneural facilitation is absent under normal 

 conditions. Each spontaneous beating of the bell is accompanied by an 

 action potential which spreads diffusely over the nerve-net (17, 55, 84). 



In colonial anthozoans the responses of different parts of the colony are 

 more or less co-ordinated by a common nerve-net. Visible effector activi- 

 ties of sea pens (Pennatulacea) are movements of the polyps (protrusion and 

 retraction), contraction of the whole animal, peristaltic waves of movement 

 and luminescent waves. Luminescent waves and possibly general con- 

 tractility are under control of a through conducting nerve-net; peristalsis 

 may be an inherent property of the muscle, while the movements of polyps 

 are largely independent of each other. Other colonial corals show various 

 degrees of co-ordination in the responses of polyps, between those in which 

 polyp retraction is always local to those in which there is a widespread 

 response involving all polyps. These responses are mediated by nerve-nets 

 in which there exist increasing degrees of facility for interneural trans- 

 mission across the colony (51a). 



Balanoglossids. These are sluggish burrowing animals possessing 

 simple and diffuse nervous systems. Underlying the epidermis is a net- 

 work of nerve fibres and various diffuse nerve cords. Nerve cells are widely 

 distributed beneath the epithelia; primary sensory cells in the epidermis 

 connect with the sub-epidermal plexus ; and efferent fibres proceed to the 

 effectors (muscles, and probably glandular and ciliated cells) (19, 66, 100). 



