158 M. H. de Lacaze-Dutliiers 07i Phoenicurus. 



is flat, rounded at one of its extremities, and drawn out into 

 a point at the other ; sometimes this extremity is simple, 

 sometimes furcate, but whether single or double it is coloured 

 red, which justifies the name of Phoenicurus. 



The two surfaces are very different. One, the most ex- 

 tended, is marked with large black and whitish spots, washed 

 over with a general slight reddish tinge, which is very vari- 

 able in different individuals ] it reminds one of the colouring 

 of the back of certain toads. The other, which is white, is 

 less extended, and margined by the former, which borders it 

 and forms a sort of raised pad all round it. 



The rounded extremity of the body bears an oval pit, 

 pierced in the centre by an orifice. The skin of this pit is 

 fine, smooth, white, and nearly transparent; it is bounded by 

 a pad formed by the extremities of the two surfaces, which 

 differ in colour. 



This is all that we observe in the exterior of the animal. 



When the Phoe.mcurus is in a very lively state it is seen to 

 contract and change its form incessantly ; it twists and inflates 

 itself on the side of the marbled surface, especially towards 

 the rounded extremity, which is then inclined towards the 

 whitish surface, and the body becomes bent. Thus one is led 

 to regard the surface covered with the pattern as the back of 

 the animal, and the white surface as the anterior or abdominal 

 part. 



Led away by this impression I first of all sought the 

 nervous system by opening the PhoRnicurus on the coloured 

 surface, but I found nothing, and it was only by resuming 

 the dissection from the opposite surface that 1 obtained any 

 results and displayed the centres of innervation. 



The most normally constituted nervous system that I have 

 met with presents, as a centre, two ganglia, one left, the other 

 right, united by a long transverse commissure. Jj'rom each 

 of these ganglia issue two principal nerves, a superior one 

 going to the neighbourhood of the mouth, an inferior one 

 descending towards the tail. These centres are distant from 

 one another and nearly lateral ; they are situated at the junc- 

 tion of the anterior third of the body with the inferior two 

 thirds, and thus the four superior and inferior nerves, with 

 the ganglia and the transverse commissure, form an H, of 

 which the branches are of unequal size. These ganglia are 

 small relatively to the size of the animal ; they contain large 

 and not numerous nerve-cells with peculiar characters, which 

 will be referred to hereafter in the histological examination. 



The numerous, transverse secondary nerves i.«sue both from 

 the ganglia and from the large principal nerves to run into 



