156 EEPOEX— 1888. 



numerous long wavy lines, which according to Schultze indicates a 

 lamellar structure, and this in turn is followed by a still thicker layer of 

 fiuely granular substance containing large oval nuclei similar to that first 

 described, but excavated and tunnelled to such an extent as to receive 

 the name of ' Schwammkorper ' from an early observer. These layers 

 pass quite insensibly into one another, the striated appeai-ance often 

 occurring on the trabeculis bounding the vacuolations, and the oval nuclei 

 occasionally occurring amidst the sinuous lineations of the central por- 

 tions. The homologue would appear to be the intermediary nucleated 

 layer in the plate of the torpedo in which Krause describes the presence 

 of transverse linear markings. As Robin has shown, the electrical 

 apparatus is supplied exclusively by spinal nerves, but a point of great 

 interest, suggested to me by Dr. Meyer, and which I hope some day to 

 investigate, is the question as to where the fibres originate. There is 

 strong reason to believe that they do not spring from the spinal column, 

 but have their source in the brain. 



In observing the movements of the torpedo and of the skates in the 

 Aquarium I was struck with the difference in their modes of locomotion. 

 The torpedo employs the tail with its powerful muscles to discharge the 

 function of a propeller, while the skate makes use only of the pectoral 

 fins, the tail remaining quite rigid and evidently serving to some extent 

 •as a rudder. This fact appears to me worthy of remark, and brings into 

 prominence the importance of the organ in the skate, for it is inconceiv- 

 able that a structure involving for its accommodation the transference of 

 the entire function of locomotion to a difiFerent system of muscles should 

 not subserve some very important function. 



I frequently endeavoured to obtain a ' shock ' from the skate by 

 grasping the tails of living specimens, but although I have repeatedly 

 made an attempt both with the smaller specimens I got at Naples and 

 with the much larger forms which I secured when recently superintend- 

 ing the work of the ' Garland ' for the Fishery Board for Scotland, I 

 never succeeded in experiencing a perceptible discharge. 



But besides giving attention to the structure of the electric organs in 

 the torpedo and in the skate I devoted a considerable amount of time to 

 the search for homologous organs among their nearest allies. This 

 appeared to be necessary in order to throw light on the obscure problem 

 as to how the transformation of a muscular into an electric organ could 

 have been effected — a transformation which the researches of Babuchin, 

 and more recently of Ewart, prove beyond a doubt. It is clear that the 

 discovery of an imperfectly developed or of an abortive organ in any 

 member of a neighbouring group would aid greatly in the elucidation of 

 the question. 



First of all I examined the organ in the skate, which M'Donnell de- 

 scribed and regarded as the homologue of the electric organ in the torpedo, 

 h.is work receiving a welcome recognition from Mr. Darwin ; but I find it 

 is present in a well-developed condition also in the torpedo, along with 

 the electric organ, and exists more or less perfectly in other Elasmo- 

 branchs, e.g., in SqxMtina angelus and Mustelus laevis. Indeed, it is no 

 other than the thymus gland, and its structure has no characters in 

 common with electric organs. I studied it both in a fresh condition and 

 by means of sections, and find it both in its intimate structure and in 

 the nature of its contents to present a close resemblance to the thyroid, 

 so that it evidently performs a similar function. 



