428 



THE BIOLOGY OF MARINE ANIMALS 



Echinoderms. The nervous system of echinoderms shows interesting 

 features intermediate between the nervous network as exemplified by 

 actinians, and the central nervous organization of annelids and higher 

 groups. Smith (103) makes a functional resolution of the asteroid nervous 

 system into three components: a sensory-association plexus, which is a 

 true nerve network ; a motor system ; and a central system comprising the 

 circumoral nerve ring and radial nerve cords. Much mechanical activity 

 consists of localized responses of spines, pedicellariae, tube feet, etc., and 

 is an expression of the functioning of a peripheral nerve-net; superimposed 

 on this local activity, or acting conjointly with it, are co-ordinated responses 

 of locomotory reaction-systems, controlled by the central nervous system 

 (c.n.s.) (Asterias, Astropecten, Marthasterias). 



In the lateral and dorsal walls of the arms there is a diffuse peripheral 

 or dorsal sheath plexus. A median radial nerve cord runs along the ventral 



Fig. 10.7. Responses of Body Wall of Ascidiella aspersa to 

 Electrical Stimulation 

 Movements of branchial siphon recorded. Stimuli: single shock, pair of shocks and 

 series of ten shocks. Time marking, above, in seconds. (From Hoyle (58).) 



surface of each arm and connects at its base with the circumoral nerve ring. 



The sensory-association plexus of the dorsal sheath displays typical 

 features of the nerve-net, autonomy, diffuse and decremental conduction, 

 and facilitation. For example, pedicellariae react equally well to stimula- 

 tion whether in the intact animal or borne on isolated pieces of dorsal 

 sheath (Asterias). All pedicellariae immediately about the area of mechani- 

 cal disturbance respond to the stimulus, due to diffuse conduction, but the 

 responses fall off with distance. To repetitive weak mechanical stimuli the 

 responses become progressively greater. Pertinent observations on sea- 

 urchins (Strongylocentrotus, Arbacid) show that responses of the spines are 

 independent of strength of electrical stimulation, but become more exten- 

 sive with increase in frequency and number of stimuli. As in actinians, 

 interneural and neuromuscular processes of facilitation would appear to 

 offer an explanation for these observations (102, 103). 



Superimposed on the diffuse plexus of the dorsal sheath there is a 

 system of through-conduction tracts. These are transversely oriented in the 

 arms, and connect with lateral motor centres. Evidence for their function- 

 ing is provided by experiments such as the following. When a shadow is 

 cast over part of the dorsal sheath, the papulae in the stimulated region 



