238 W. HAROLD LEIGH-SHARPE 



angle of 180°, and whose apex, from pointing inwards, becomes 

 directed outwards, also propping the oviduct open, and dis- 

 tending it. Posterior to the signal, but remaining on the oppo- 

 site face, is a knife-blade-like structure, the sentinel, so keen is 

 whose edge, that, in spite of an epidermis over it, I repeatedlj^ 

 cut my finger on it. The oviduct is thus distended and stimu- 

 lated in three directions. A pseudosiphon is present on the 

 inner border of the concavity, serving partly as a sheath for the 

 sentinel, when not in the erect condition. A shield is present, 

 not so well developed as in K. blanda, and covered by the signal 

 in figure 16. 



The greater part of the year 1920 was spent in the electrical 

 stimulation of pithed skate. Various species were used, in- 

 cluding both R. circularis and R. clavata, but the results in the 

 latter case were more conclusive owing to less difficulty in iso- 

 lating the nerve twigs. 



Previous practice in dissection revealed (fig. 17) that the 

 siphon (*S) and its contained gland are innervated by a posterior 

 branch of the 52nd2 spinal nerve, and that the clasper is inner- 

 vated by a posterior branch of the 54th spinal nerve. 



The animal figured is 84 cm. long. The spinal cord was se^^ered 

 just behind the medulla and the brain destroyed; it was also 

 necessary to divide the cord again just anterior to the 50th 

 spinal nerve to prevent any electrical reflux agitating the pec- 

 toral fins. The results point to the conclusion that the sugges- 

 tions tentatively put forward in Memoir II, page 371, are not 

 tenable for this species. This does not directly prove any 

 point, since erection is of a different type in the two species, and 

 the circumstance that the claspers of R. circularis taken im- 

 mediately after copulation were both in a state of erection may 

 well point to the fact that that phenomenon is there due to a 

 chemical stimulus. 



The results of electrical stimulation were as follows: 



1. Stimulation of the posterior branch of the 52nd spinal 

 nerve causes, first, secretion from some or nearly all of the 

 papillae of the gland followed bj^ a slow undulating contraction of 



2 The basis of this count is that of fish having thirteen spinal nerves in the 

 anterior group of the brachial plexus. 



