1917 Pomona College Journal <>t Entomology and Zoology, I ol. l\. No, I. Decembei 



Some Remarks on the Nervous Systems 

 of Two Sea-Urchins 



Willi. Wl \. HILTON 



rhe largest and smallest species <>t sea-urchins occurring ;ii 

 Laguna Beach are the materials lor this study. Lyt echinus ana- 

 mesus 11. I.. C, a centimeter or less in diameter, were sectioned 

 while (he radial nen es I rom Strongylocentrotus franciscanus A. A.g., 

 ol fifteen times this diameter were studied. 



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

 branches ol the nervous system. I he 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 ol tlu' spmes, were easil) 

 found, also the branches I rom the circumoral nerve ring to the 

 intestines in the region ol Aristotle's lantern. Mere stainls fused 

 with epithelial cells ol the intestine. I he genera] parts ol the ner- 

 vous system, such as described and figured by Delage ami I [eroward, 

 '03, were found. The radial ami eireumoral bands ol nervous tissue 

 as is well known, resemble those ol the superficial radial ami eir- 

 eumoral strand ol starfish very closely, but the deep system is poorly 

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

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

 ol the larger spines, a section ol one ol which is shown in Fig. A. 

 From the radial nerves lateral branches were easily followed to 

 the tube leet. Fig. 2 is a cross-section ol a radial nerve, in which 

 a branch on tlu- right is shown just as it enters a tube loot. The 

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

 the oral and especially at tlu- aboral end. A longitudinal section ol 

 one ol the radial nerves ol the smaller sea-urehin is shown in Fig. I . 

 Hie oral c\m.\ is below and at the left, the aboral above at the right. 

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

 near its central portion, and Fig. 3 is a eross-seetion ol a portion ol 

 a radial nerve near one end. These figures an- I rom the smaller 

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



The structure ol the nerve bands seems a little more- complex 

 than those ol starfish, in that the nerve cells are more modilied ami 



