39G COMPARATIVE ANATOMY. 



can be traced into the anterior larger mass (N). A division of tins 

 anterior mass into three consecutive lobes, produced by an unequal 

 thickening of the wall of the tube, the foremost of which is in 

 Ascidias and Salpee in close connection with the origin of the visual 

 organ, is also to be made out in the Copelata, in which the funda- 

 mental structure sketched out in the Ascidian larva) undergoes a 

 further and permanent complication. In them we find the nervous 

 system made up of an anterior elongated ganglion (Fig. 209, n) 

 which exhibits three dilatations (App. flagellum), and passing back- 

 wards in the form of a chord (n') to the base of the tail, is continued 

 along that organ to its extreme point. At the root of the tail a 

 ganglionic swelling is formed on the chord, and two others follow 

 after this (A. furcata). The first appears to be the more con- 

 stant. This apparatus, in its rudimentary condition a 

 continuous one, as appears from the Ascidians' tadpole, must 

 be regarded as the central-organ of the nervous system. 

 It is noteworthy that in the Copelata, too, it encloses a canal 

 which extends from the foremost ganglion to the ganglion at 

 the root of the tail. The central elementary parts (the nerve- 

 cells) are not, however, equally distributed, but form the 

 ganglia only, to which the rest of the chord serves as commissure. 

 The continuation of the nerve-chord in the tail lies to the left of the 

 chorda, if we consider the two surfaces of the tail (as its relation 

 to the rest of the body requires), as dorsal and ventral respectively. 

 This asymmetry is in the Ascidian larvse either developed late or not 

 at all, so that as the primitive condition we are justified in assuming 

 a median dorsal nerve-chord. The form of the nerve-centre 

 thus determined is one which is in the highest degree noteworthy, 

 since it is not presented in any other division of invertebrate animals, 

 in all of which the prolongation of the central organ takes place in 

 a ventral position. 



Peripheral nerves pass out from the anterior ganglion, branching 

 laterally around the incurrent orifice of the branchial chamber. 

 Others pass backwards to the spiracula. In the tail, nerves pass 

 out from the ganglia, as too in the Ascidian larvse there arc nerve- 

 branches on the caudal portion of the nerve-centre ; terminally the 

 chord gradually breaks up into a number of branches. 



§ 



306. 



The atrophy of the tail, or its complete absence, brings about in 

 connection with the further elaboration of the fore-end of the body 

 by the development of the branchial slits, a considerable change in 

 the nervous centre. In the Ascidians the caudal division of the 

 nerve-centre disappears, and in Pyrosoma and Salpa the embryonic 

 foundation of the organ is limited to the anterior portion which is 

 accordingly more volumiuous in proportion. The three vesicular 

 divisions which still appear as embryonic rudiments in the Salpa? 

 give place to a single ganglion-mass. The Ascidians have this 



