THE GYMNOTID EELS OF TROPICAL AMERICA. 163 
found only in the posterior half of the fish. The large organ of each side is more 
or less quadrant shaped in cross-section, and is of greatest diameter about a centi- 
meter back of its origin. It tapers gradually back of this point becoming more 
nearly circular in cross-section until it disappears a few centimeters from the end 
of the tail. It lies on each side of the hemal spine above the anal fin musculature 
and below the muscle ventralis. In the region of its maximum size the top of each 
organ is on a level with the vertebral centra, but as the caudal end is approached, 
the dorsal portion of each organ lies more and more ventrad. 
The organ of Sachs consists of a series of bundles of fibers which resemble both 
muscle and electric tissue. From the middle of the body to the caudal end of the 
large electric organs, the organs of Sachs lie on the dorso-lateral surface of the 
latter, just below the muscle ventralis. The bundles of this organ wrap around the 
large electric organ obliquely in a latero-ventral direction. They extend farther 
ventrad as the caudal extremities of the large organs are neared. They finally 
close over the ends of these. The organs of Sachs increase in diameter caudad. 
The organs of Hunter are triangular in cross-section and much smaller than 
either of the other two pairs of organs. They are in the anal fin region and le 
between the muscles pinnalis analis externalis and the muscles pinnalis analis 
internalis. Dorsally they are separated from the large organ by the remnants of 
the muscles lateralis imus. They taper off as their caudal ends are approached 
and terminate a few centimeters in front of the ends of the large organs. Plate 
XIX, figs. 21, 22, 23 represent cross-sections of the electric eel showing these points. 
Both the large organs and the organs of Hunter are composed of plates of tissue 
which run parallel to the large axis of the fish. In the large organs these plates 
are more or less arched ventrally in cross-section. In the small organs they are 
almost flat. The number of these plates seem to be rather constant in each organ, 
regardless of the size of the fish. Bois-Reymond (in Sachs, Zitteraal, p. 32) gives 
the following table: 
Observer. Body Length. Plates in Large Organ. | Plates in Small Organ. 
SHANE o.cuie 6c UOC OR UCN Oe on eerie 31 cm. 30 14-19 
TOR 3.5 pio bib o-oo SERCO OE eeO Eee CR ene 48.5 32 17 
[Rah DeLee heen 6 oiite ols Sas evn stasis 68.5 32 13 
ETL CCE ee cee eesti are sto oonle cto oue « 71 35 15 
Kupffer and Keferstein.................. 120 31 Not given 
TahimiloY lrs bo co daa poo Rae OO coeIaaae Not given 36 20 
SAGs 6 ons Coged ne Diet nt an nee ee | Not given | 30 14-19 
According to Sachs, who confirmed in general the work of Pacini, the large 
electric organs are made up of minute units about .14 mm. broad, which lie at right 
angles to the long axis of the plates. Each unit is divided near the center by a 
vertical partition. On the anterior face of this are several papillee which do not 
reach the wall of the unit. On the posterior face are fewer papillae which reach 
out to the wall of the unit. Between the latter are several minute papille. It is 
