Some Remarks on the Nervous Systems 

 of Two Sea- Urchins 



WIILIAM A. HILTON 



The largest and smallest species of sea-urchins occurring at 

 Laguna Beach are the materials for this study. Lytechimis ana- 

 mesus H. L. C, a centimeter or less in diameter, were sectioned 

 while the radial nerves from Struiiyyloceutrotiis franciscaniis A. Ag., 

 of fifteen times this diameter were studied. 



In sections of the smaller species it was possible to trace the chief 

 branches of the nervous system. The long radial nerves, with their 

 side branches to the tube feet and the branches to the large spines, 

 with the ganglion-like rings about the bases of the spines, were easily 

 found, also the branches from the circumoral nerve ring to the 

 intestines in the region of Aristotle's lantern. Here stands fused 

 with epithelial cells of the intestine. The general parts of the ner- 

 vous system, such as described and figured by Delage and Heroward, 

 '03, were found. The radial and circumoral bands of nervous tissue 

 as is well known, resemble those of the superficial radial and cir- 

 cumoral strand of starfish very closely, but the deep system Is poorly 

 represented. The superficial plexus was clearly seen as a whole, 

 only parts were made out such as ganglion-like rings at the bases 

 of the larger spines, a section of one of which is shown in Fig. 4. 

 From the radial nerves lateral branches were easily followed to 

 the tube feet. Fig. 2 is a cross-section of a radial nerve, in which 

 a branch on the right is shown just as it enters a tube foot. The 

 radial nerves are thickest In the more central portions, thinner at 

 the oral and especially at the aboral end. A longitudinal section of 

 one of the radial nerves of the smaller sea-urchin Is shown In Fig. 1. 

 The oral end Is below and at the left, the aboral above at the right. 

 Fig. 2 is a cross-section of two-thirds of one of the radial nerves 

 near Its central portion, and Fig. 3 Is a cross-section of a portion of 

 a radial nerve near one end. These figures are from the smaller 

 sea-urchin, but enlarged more than Fig. 1. 



The structure of the nerve bands seems a little more complex 

 than those of starfish, in that the nerve cells are more modified and 



