Notes on the morphology of the Tunicata. 533 



tube persists as the tissue of the dorsal raphe, between pharynx 

 and cloaca. All descriptions of the metamorphosis of the Ascidian 

 tadpole say that the whole of its elongated nerve tube degenerates 

 except a little region in front, from which the definitive ganglion 

 and gland arise. It would be well to reinvestigate this matter 

 and see whether the rapheal nerve cord is a remnant of the 

 larval nerve tube which lay in the same region, or if it be a 

 downgrowth from the definitive brain after the metamorphosis. If it 

 be the latter, of course, the ganglia upon the rapheal nerve in some 

 of the Molgulidae can have no metameric significance. I hope to 

 test this point in Molgula manhattensis. The presence of a rapheal 

 nerve in Salpa (cf. page 540) which never has an elongated nervous 

 system in the embryo, would seem to indicate that the rapheal nerve 

 in this and probably also in other Tunicates develops as an outgrowth 

 from the definitive ganglion. 



It would also be of interest to discover if the rapheal duct, in 

 those species where it is found, arises by the metamorphosis of the 

 larval nerve tube in situ, or if it is a downgrowth from the definitive 

 gland. Metamorphosing larvae of Cynthia piriformis or of Cynthia 

 papulosa would probably furnish the best material for this study. I 

 have no favorable material for any species. 



In the descriptive part of this paper I have frequently emphasized 

 the dextral asymmetry of the intersiphonal organs. The ciliated funnel 

 is generally on the right side of the ganglion (cf. Figs. 1, 19, 24, 29 

 and 31), and its horns, when it is horse-shoe-shaped, generally point 

 more or less to the right (cf. Figs. 19, 24 and 31). In those species 

 which have a dorsal gland the duct usually leads up to the right of 

 the ganglion (cf. Figs. 19 and 29). In the posterior region it is 

 usual to find both the rapheal duct and the rapheal nerve, in their 

 origin and the first part of their course, lying to the right of the 

 median plane (cf. Figs. 1, 4, 5, 29 and 39). The cause of this asym- 

 metry may lie in the asymmetry of the nervous system of the tad- 

 pole. At an early stage in development the central nervous system 

 of the Ascidian is a simple straight tube opening in front to the 

 pharynx. At this time it is perfectly bilateral. A little later the 

 sensory vesicle arises as an outgrowth from near the anterior end of 

 the tube. This pushes out to the right side, becoming very large and 

 crowding that portion of the nerve tube which lies behind it. When 

 the ganglion develops from this region behind the sensory vesicle it 

 naturally lies somewhat on the left side of the lumen of the tube owing 



