4 



Mr. G. J. Romanes and Prof. J. C. Ewart. [Mar. 24, 



They dimmish in size as they proceed outwards, and at the ends 

 of the arms are scarcely visible. 



Nervous System of Echinus. 



The internal nervous system of Echinus consists of five radial 

 trunks, which may be traced from the ocular plates along* the ambu- 

 lacral areas external to the radial canals to the oral floor, where they 

 bifurcate and unite with each other so as to form the pentagonal nerve- 

 ring. This ring lies between the oesophagus and the tips of the 

 teeth which project from the lantern. Small branches leave the ring 

 and supply the oesophagus, and lateral branches arise from the 

 several trunks to escape with the pedicels through the apertures of the 

 pore plates. Each trunk lies in a sinus situated between the lining 

 membrane of the shell and the ambulacral radial canal. The lateral 

 branches which accompany the first series of pedicels through the 

 oral floor are large and deeply pigmented. The branches within 

 the auricles are small ; those external to the auricles gradually 

 increase in size until the equator is reached, and from the equator 

 to the ocular plates they diminish in size. At the equator, the trunk 

 is wider than at either pole, and it is often partially divided for some 

 distance at each side of the equator by a deep longitudinal fissure. 



When the nerve- trunk, after being stained with chloride of gold or 

 with osmic acid, is removed from its sinus, it is seen to be enveloped 

 by a thin fibrous sheath. This sheath contains numerous large 

 pigment cells, and has scattered over it irregular masses of protoplasm 

 which have been deposited from the fluid of the neural sinus. 



When the sheath is removed, the trunk is seen to consist of delicate 

 fibres and of fusiform cells. The cells consist of a nucleus and a thin 

 layer of protoplasm, which projects at each end, and terminates in a 

 nerve-fibre. 



The lateral branches of the trunk escape along with and are partly 

 distributed to the pedicels ; the remainder break up into delicate 

 filaments which radiate from the base of the pedicel under the surface 

 epithelium. When one of the large branches already referred to as 

 escaping with the inner row of pedicels is traced through the oral 

 floor, after sending a branch to the foot, it breaks up into delicate 

 fibres, some of which run towards the bases of the adjacent spines 

 and pedicellarias, while others run inwards a short distance towards 

 the oral aperture. 



Either in connexion with, or anatomically independent of, these 

 filaments from the lateral branches of the nerve-trunks, there arises 

 an external plexus lying almost immediately under the surface epithe- 

 lium and extending from the shell to the spines and pedieellariaB. 

 The fibres of this plexus closely resemble the fibres of the lateral 

 branches of the trunk, but generally they are smaller in size and have 



