NERVOUS SYSTEM AND BEHAVIOUR 447 



organs of crustaceans have an excitatory action on the crustacean heart 

 similar to adrenaline. These organs appear to be concerned with circulatory 

 regulation (3, 10a, 6Sa). 



INTEGRATIVE FUNCTIONS OF THE NERVOUS SYSTEM 



Animals are not mere automatons, passive machines pulled this way and 

 that by each external stimulus to which they are sensitive. Their responsive- 

 ness is variable, and changes during development, and over shorter periods 

 according to their more immediate past history. There are recognizable 

 behaviour patterns characteristic of each species; these patterns are modi- 

 fied by experience and, to the observer, appear directed towards goals, 

 which may be attained in various ways. 



Life histories of many animals show how conative behaviour alters dur- 

 ing development. Free-swimming larvae of benthic species, for example, 

 are frequently photopositive at first, later becoming photonegative. They 

 then go through a searching and settling phase in which they select a suit- 

 able substratum and settle there. If a suitable bottom is not encountered 

 the planktonic life can be prolonged, but discrimination becomes pro- 

 gressively less refined. Such a behaviour pattern is observed in larvae of 

 Spirorbis, inter alia, and increases their chances of finding the optimal kind 

 of substratum. After settling and metamorphosing, other behaviour pat- 

 terns characteristic of the adult appear (67). 



In simplest nerve-nets occur many of the essential features of central 

 nervous activity, namely reflex functioning, co-ordination of different 

 action-systems, and modification of spontaneous behaviour under pressure 

 of external stimulation. Within the nerve-net of anemones there is struc- 

 tural and functional differentiation, recognizable in distinct neural pat- 

 terns and in regional differences in transmission. This nervous network 

 contains a fast through-conduction system, serving quick retractor reflexes 

 and slow co-ordinated contractions. Slow responses of anemones consist 

 of a co-ordinated sequence of contractions in several muscles. The re- 

 sponses to low-frequency electrical stimulation resemble in many respects 

 the spontaneous contractions recorded from the whole animal. An initial 

 parietal contraction is usually followed by slow contraction of the marginal 

 sphincter, and by a peristaltic wave. The slow responses are noteworthy for 

 their extreme variability. Stimulation seems to alter the state of intrinsic 

 excitability, releasing a state of spontaneous activity. Contraction of one 

 set of muscles is often accompanied by reciprocal inhibition of the oppos- 

 ing set, and co-ordination of the separate parietal components is achieved 

 by the through-conduction system (10, 82). 



Hydromedusae show several distinct kinds of responses, and have a 

 specialized nervous system consisting of distinct but intercommunicating 

 parts which transmit excitation to various muscle groups. Symmetrical 

 contractions (beating) of the bell are regulated by a non-polarized through- 

 conducting network, excitation of which produces brief contractions of the 

 circular muscles (Geryonia). The feeding response involves bending the 



