The Central Nervous System of an Un- 

 known Species of Marine Leach 



Wll.l.IAM A. HILTON 



Tlie little animals from vvliicli this study was made were obtained during the 

 summer of 1920 at Laguna Beach. Two times when a number of Mysis shrimps were 

 brought in with towings these worms were found attached by the posterior sucker to 

 the side of the crustacean. At first it was not clear to which group of animals these 

 small creatures belonged. It was not until a number of the specimens had been cut in 

 series that their nature was learned. Externally they seemed unsegmented, although 

 the body had many circular rings when contracted by reagents, but these rings were 

 evidently not marks of segmentation. Internally at first there also seemed to be little 

 trace of metamerism, but when the nervous system was examined a clearly defined 

 chain of ganglia was evident. 



The mouth is at the base of the large anterior sucker, and it is back of this that 

 the ganglia may be seen. The chief ganglion is the suboesophageal composed of about 

 four parts fused and closely applied to the next ganglion below. The brain or supra- 

 oesophageal ganglion is unimportant; in fact, it is the smallest of all. There are sixteen 

 sinmple ganglia forming the ventral chain back of the suboesophageal and the seven- 

 teenth ganglion or last of the chain. The last center, or the seventeenth, is made up 

 of at least three simple ganglia fused and is the second most important center. It 

 supplies the structures of the large posterior sucker. 



Some of the points of special interest in the nervous system of this creature are; 



1. Lack of true metamerism except in the nervous system. 



2. The large number of simple clearly defined nerve centers. About four centers 

 are represented in the suboesophageal, sixteen separate ganglia and at least three 

 separate centers for the last ganglion. In all then there are at last twenty-three centers 

 in the nervous system. 



3. The small size of the supraoesophageal ganglion or brain. 



4. The large size of the suboesophageal ganglion and the last ganglion. 



5. No special sense organs were located. 



The specimens were from 4-8 mm. in length and, although small, were sexually 

 mature. The identity of the species will be considered at another time. 



(Contribution from the Zoological Laboratory of Pomona College.) 



