1888.] 



The Electric Organ of the Shite. 



309 



study of its structure and development is likely to throw considerable 

 light on the nature of electric organs generally, and also on the 

 structure of the motor plates of muscles. While B. batis may reach 

 a length of over 180 cm., B. radiata seldom measures more than 

 45 cm. from tip to tip, and is thus only about half the size of a large 

 B. circularis. In B. radiata the electric organ is absolutely and rela- 

 tively extremely small. In B. batis the electric organ may be 60 cm. in 

 length and 7 cm. in circumference at the centre, and extend from 

 the skin to the vertebral column, but in an adult B. radiata the organ 

 is seldom over 13 cm. in length and 8 mm. in circumference, and the 

 posterior two-thirds is confined to a narrow cleft between the skin 

 and the great lateral muscles of the tail. Further, the organ of 

 B. radiata consists of minute shallow cups, which only remotely 

 resemble the large well-formed electric cups of B. circularis. In the 

 latter species the various layers of the electric cup are readily com- 

 parable to the more important layers of the electric disk of B. batis, 

 but in B. radiata the electric cup is little more than a muscular fibre, 

 with one end expanded and slightly excavated to support a greatly 

 enlarged motor plate, in which terminate numerous nerve-fibres. 

 The striated layer of B. batis and B. circularis, which consists of cha- 

 racteristic lamellae having an extremely complex arrangement, is 

 entirely absent from R. radiata, the electric layer is indistinct, and 

 instead of a thick richly nucleated cortex, the cup is merely invested 

 by a slightly thickened sarcolemma. Further, the tissue forming 

 the shallow, thick-walled cup, both in its appearance and con- 

 sistency, closely resembles an ordinary muscular fibre, while the long 

 stem usually remains distinctly striated to its termination. 



In the second part of the paper an account is given of the develop- 

 ment of the electric cups of B. radiata. It is shown that the rate of 

 development compared with B. circularis, but more especially with 

 B. batis, is extremely slow. The young B. radiata is nearly double the 

 size of the B. batis embryo before the muscular fibres reach the " club " 

 stage, and the long nearly uniform clubs, instead of at once developing 

 into rudimentary cups as is the case in B. batis, assume the form of 

 large Indian clubs. When the young skate reaches a length of about 

 35 cm., the long secondary (Indian) clubs begin to expand anteriorly, 

 and this expansion continues until a fairly well moulded cup, mounted 

 on a long delicate stem is produced. But the process of conversion is 

 scarcely completed when the skate has reached a length of 40 cm., 

 i.e., when it has nearly reached its full size, for in the species radiata 

 a length of 50 cm. is seldom if ever attained. 



The cup-stage having been eventually reached, the stem, which for 

 a time may still increase in length, is often compressed by two or 

 more cups being closely applied together, and part of the rim of the 

 cup may be slightly everted or projected forwards, but even in the 



