SUPPORT, MOTION, AND SENSATION 343 



experimental work has been performed, largely by K(jfoid and his 

 students at the University of California, upon co-ordination in the 

 latter form. Euplotcs is characterized by a group of anal cirri, while 

 the anterior surface possesses an undulating membrane, near one end 

 of which lies a co-ordinating center, or motorium. From this, fine 

 protoplasmic threads emanate leading to various parts of the ciliate. 

 Five of these strands lead to five anal cirri. Cutting these proto- 

 plasmic threads causes disruption of the rhythm of their movement, 

 thus furnishing experimental evidence of the existence of a neuromotor 

 apparatus in certain ciliates. 



Co-ordination by a Network 



In some of the most primitive metazoan forms, such as the sponges 

 and the lower coelenterates, there is evidence of a very elementary 

 and simple type of co-ordinating mechanism. A form like Hydra, 

 which makes a variety of different movements, reacts to various 

 stimuli since it feeds, contracts, expands, creeps, and occasionally 

 turns cart-wheels. The mechanism which makes such acrobatics 

 possible in Hydra has been described as a nerve net, and as such forms 

 a part of the sensory-neuro-muscular mechanism, or as it is sometimes 

 called, the receptor-effector system. 



Co-ordination by a Nerve Ring 



Only two of all the great phyla of animals, the coelenterates and 

 the echinoderms, are apparently radially symmetrical. The nervous 

 system of the first of these radially symmetrical groups has just been 

 described and it can be seen how unspecialized are its co-ordinating 

 devices. Turning to the echinoderms, as examples of the second 

 radially symmetrical group, we find that in spite of the fact that 

 embryos of these invertebrates are bilaterally symmetrical, the nerv- 

 ous system of the adults has developed along the lines of radial sym- 

 metry. This type of nervous system is composed of several parts, the 

 relative development of which varies in the different classes, the star- 

 fish having numerous nerve cells lying among the ectodermal cells. 

 Some of these nerve cells may connect with nerves from the fairly 

 definite ridges of nerve tissue known as the radial nerve cords nnming 

 the length of each arm and uniting to join a nerve ring that encircles 

 the mouth. In addition there may be an apical nervous systern that 

 H. \y. H. — 23 



