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organs, electric, torpedo, gymnotus and columns

STRUCTURAL The general structure of the organs in Torpedo had been described in 1678 by Lorenzini (70), and a detailed account was published in 1835 by John Hunter (es), but the anatomy of the electric organs in general was not given much consideration by investigators until the discovery, by Bilharz, in 1857 (1m), of the electrical plates in Gymnotus. Ballowitz worked out the fine anatomy of the electric organs of Torpedo, Gymno tus, and the Rajiche, to the latter of which he gave the name of the "pseudoelectric fishes," and to the organs the name of the "pseudo electric organs," inasmuch as their structure resembled that of other electric organs, while their function was not then known. Ogneff (75) and Max Schultze (85) made careful and accurate observations of the innervation of these organs and of the finer details of the nerve-endings of the same.

The peculiar structure common to all electric fishes is the semi transparent material, of the consistency of jelly, which lies between and around the electroplaxes and in which lie the connective-tissue septa and numerous nerve-branches. In Torpedo these are two large masses, concave on the inner border and divided by septa into hexago nal columns, the ends of which lie under the dorsal and ventral surfaces of the skin. In Torpedo occiclentalis there are 1,969 columns, in Tor pedo ocellata but 450 (Gotch so).

In Raja there are two very slender organs, tapering at both ends and lying one on each side of the spinal column. These organs are formed from and lie embedded in the muscles of the tail.

In Gymnotus there are four such organs, two dorsal and two ventral, of which the dorsal are the larger. The organs are divided into col umns which run variable distances from the cephalic end to the tip of the tail of the fish, which may be from 8 to 10 feet in length.

Mormyrus has four organs formed from and replacing the posterior regions of the muscles of the tail. The long diameter is parallel to the spinal column, and with the general position very like that of the organs in Gymnotus, though not extending so far into the anterior end of the body. In Gymnarchus there are eight cylindrical organs, four on each side, embedded in the muscle-tissue of the tail, as close to the median bony parts as a little connective tissue in between will permit.

The organ of Malopterurus is peculiarly situated in the thickened skin and is imperfectly divided into halves by a connective-tissue sep tum on the ventral surface. It enwraps the whole body except the

fins and the head.

The finer anatomy of the electric organs differs considerably in the different groups. In Malopterurus there is no obvious arrangement into columns, the organ being composed of lozenge-shaped electro plaxes, dove-tailed into each other and surrounded by the character istic jelly-like material. Each half of the organ is innervated by a single gigantic nerve-cell, with but one axis cylinder, the branches of which reach all of the somewhat over 72,000 compartments in each half of the organ and are estimated to have a peripheral distribution unsurpassed by that of any other single nerve-cell (Gotch so).

In all the other electric fishes the organs are divided into columns, themselves subdivided at regular intervals by connective-tissue septa and fixed at their peripheral edges by the boundary wall. Each of these compartments contains a protoplasmic mass, the electroplax, in which the efferent nerves end, while the rest of the space is occupied by the transparent material. The essential elements are much the same in all. The protoplasmic mass with its supporting fibers, cross-striations, and nerve terminations is called the electric plate, electric disk, or elec troplax. The nerves on coming in contact with the electroplaxes branch in a characteristic dichotomous manner in most of the groups.

In Raja batis a band of parallel wavy fibers crosses the transparent area, still possessing the optical properties common to ordinary stri ated muscle-fibers and representing the vestiges of the cross-striation of the voluntary muscle-fibers, from which, according to Ewart and Engelman (az and ii), the electric plate has been derived. This layer will be termed the striated layer for convenience in this paper, although the striations are no longer wholly comparable to those of muscle tissue and although they have almost entirely disappeared in Gymnotus and Torpedo, the strongest of the electric fishes. In Torpedo there is a faint fibrillation in each electroplax during its early embryonic state, which is completely resorbed during development. In Gymnotus and in Malopterurus no indication of the striations is visible even in the earliest stages yet seen. It is therefore evident that the loss of definite striation is correlated with the gain in electrical power and it is possible that the evolution of the electric organs may follow the loss of the original muscle striations.