chap, xii.] NER vous SYSTEM OF ECHINODERMS; 407 



We meet with the same phenomenon in Insects, 

 *but these Arthropods are of greater interest from 

 the point of view that the changes undergone by 

 them during their development afford support to the 

 view that the more primitive forms have a larger, and 

 the more complex a smaller, number of separate 

 ganglia. While the worm-like larva has a ganglionic 

 mass in nearly every one of its segments, the adult 

 insect has a varying number fused together. 



As has been already pointed out in speaking of the 

 Ectimodermata, the nervous system of a starfish 

 is so far extremely primitive in character, that the 

 nerve cord which runs down the ainbulacral groove of 

 each arm lies just below the integument ; in the 

 Ophiuroids this superficial position is lost, owing to the 

 development of a calcareous plate, which forms a floor 

 for the groove. The great development of the test in 

 Echinoids leads to tl*e same result ; but here, as we have 

 already learnt, a compensating arrangement is effected 

 by the development of a plexus of nerve cells and 

 fibres which is superficial to the test. In Holo- 

 thurians the nerve cords are placed more or less deeply 

 in various forms. 



In all cases these radially disposed nerve cords are 

 united with one another by a set of circular fibres, 

 which form the circumoral nerve ring, and it is 

 thanks to this that the apparently independent rays 

 of a starfish or of a brittle star are enabled to act in 

 concert : but, although the nervous system of an Echi- 

 noderm is hereby made a connected whole, it is im- 

 portant to observe that a single arm of a starfish, or 

 even a segment of an Echinus (Fig. 176) is capable of 

 exerting independent movements ; for example, single 

 rays of a starfish have been found to crawl as fast, 

 and in as definite a direction as entire forms ; if turned 

 on their back they succeed in righting themselves, and 

 sometimes, though not always, they attempt to move 



