424 THE BIOLOGY OF MARINE ANIMALS 



tion relates to medusae and actinians ; variations in detail may be expected 

 in other groups, but the broad pattern is probably similar. 



In Scyphomedusae there is a plexus of bipolar and multipolar nerve cells 

 beneath the epithelium. The bipolar neurones form a network of through- 

 conducting tracts in the sub-umbrella surface, the fibres connecting with 

 one another by axonic synapses. The neurones make contact on the one 

 hand with aggregations of ganglionic cells below the marginal sense 

 organs, and on the other with muscle cells. In actinians there is a wide- 

 spread nervous plexus of bipolar and multipolar, neurones between the 

 epithelia and the muscles. This plexus is particularly well developed in the 

 ectodermal muscle of the disc and tentacles, and connects with an endo- 

 dermal plexus in the column. Primary sense cells, especially numerous in 

 tentacles and oral disc but occurring also in columnar endoderm and pedal 

 disc, feed into the nervous plexus, which connects on the efferent side with 

 muscle fibres (54, 82). 



The chief physiological characteristics of the nerve-net are diffuse 

 spread of excitation, local and equipotential autonomy, so-called decre- 

 mental conduction, and facilitation. Ingenious experiments devised by 

 Parker, in which the body wall of an anemone Metridium was slit in various 

 ways, showed that excitation can be conducted in any direction over the 

 body of the anemone from a point of stimulation, i.e. nerve tracts are 

 diffuse and non-polarized. A mechanical stimulus applied to a tag of the 

 body wall, so long as the latter remains continuous with the column at some 

 point, evokes retraction of the oral disc. Electrical stimulation of the intact 

 column of Calliactis produces a retractile response consisting largely 

 of contraction of the marginal sphincter (Figs. 10.4, 10.5). A single 

 stimulus is without effect, and a contraction is evoked only by a succession 

 of electrical stimuli. This response is frequency-dependent, owing to the 

 operation of peripheral facilitation (p. 385). Under mechanical stimula- 

 tion the strength of the response varies with the strength of stimulation. 

 This is due to the fact that a mechanical stimulus evokes a battery of 

 impulses from the sense organs, and these increase in frequency and num- 

 ber with the intensity of the mechanical stimulus, thus facilitating the 

 response. 



These retractile responses are mediated by fast through-conduction path- 

 ways. Histological evidence for one of these is provided by longitudinally 

 directed tracts of large neurones in the mesenteries of Metridium, neurones 

 which supply the longitudinal retractor muscles. 



Disc and tentacles form a reaction system characterized by greater 

 variety and asymmetrical character of responses. A single stimulus pro- 

 duces a small local response; stimulation at progressively increasing fre- 

 quencies produces stronger and more widespread responses, beginning 

 with local contraction of a single tentacle, extending to more distant parts 

 of the oral disc and finally involving complete contraction of the whole 

 anemone {Calliactis). Similarly, the stronger the mechanical stimulus, the 

 more widely the reaction is propagated around the disc. The responses 



