The Central Nervous System of an Un- 
known Species of Marine Leach 
WILLIAM A. HILTON 
The little animals from which 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 subesophageal 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 subeesophageal 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.) 
