2l6 THE EVOLUTION OF MAN. 



traversing the body longitudinally, and terminating in front 

 almost as simply as at the other end : it is a simple medul- 

 lary tube (Plate XI. Fig. 15, m). We found, however, that 

 the rudiment of the same simple medullary tube occurs in 

 the ascidian larva (Plate X. Fig. 5, m) and in the same cha- 

 racteristic position, above the notochord. Moreover, when 

 closely examined a small bladder-like swelling may be seen 

 at the fore end of the medullary tube in these two closely 

 allied animals ; this is the first indication of a separation of 

 the medullary tube into brain (m^ and spinal marrow (m 2 ). 

 When, however, we consider the undeniable relationship of 

 the Ascidia to the rest of the Worms, it is evident that the 

 simple central marrow of the former exactly answers to the 

 simple nerve-ganglion which, in the lower Worms lies above 

 the throat (pharynx), and which has, therefore, long been 

 called the " upper throat ganglion " (ganglion pharyngeum 

 superius). In the Gliding Worms (Turbellaria) the whole 

 nerve system consists merely of this simple ganglion, which is 

 situated on the dorsal side of the body, and from which nerve- 

 threads radiate to the different parts of the body (Fig. 211,</w). 

 This upper throat ganglion of the lower Worms is evidently 

 the rudiment from which the more complex central marrow 

 of the higher animals has developed. An elongation of the 

 upper throat ganglion along the dorsal side gave rise to 

 the medullary tube, which is characteristic of Vertebrates 

 and the young forms of Ascidia alone. On the other hand, 

 in all other animals, the central nerve system has de- 

 veloped in a very different manner from the upper throat 

 ganglion ; in Articulated Animals (Arthropoda) especially, 

 the latter has developed into a throat (pharyngeal) ring, 

 with a ventral marrow ; this is the case, also, in the articu- 



