354 A. T. MASTERMAN. 



nected with the central disc of the epistome, and which no 

 doubt subserve the function of attachtnent by suction. Thus, 

 in an indirect way, the front end of the notochord acts as a 

 fulcrum for the epistomial muscles, and before the evolution of 

 the chondroid tissue it would do so directly. 



Lower down (fig. 16, left side of), at the level of the mouth, 

 the notochord is seen as a diverticulum abutting on the nerve- 

 tract and the collar-pore {cp.) ; whilst a little below this 

 (fig. 16, right side of) the pharyngeal cleft is seen to lead 

 forwards and downwards in front of the lower end of the collar- 

 pore {c.p.). This will be referred to later. 



With very little change in character the notochord is con- 

 tinued throughout the length of the pharynx, as a vacuolated 

 longitudinal groove, with a wide lumen in continuity with the 

 pharyngeal cavity. Here and there it is folded or thickened, 

 but this is probably an artefact. Fig. 17 shows very nearly 

 the hind end of the pharynx with the notochords. The mid- 

 dorsal pharyngeal wall is thickened, arched into a ridge, and 

 strongly ciliated, whilst in the mid-ventral line a part of the 

 pharynx tends to become constricted off as a spacious ventral 

 groove. 



Behind this the notochords gradually lose their vacuolated 

 cells, giving place by degrees to the ordinary cells of the 

 pharynx, and a very few sections after this the pharynx passes 

 into the narrow oesophagus. 



In fig. 19 is seen the appearance of the notochord in trans- 

 verse section. The vacuolisation extends throughout, and the 

 nuclei are seen dotted about, especially at the junctions of the 

 vacuoles. 



Amongst those who accept the genetic relationship of Bala - 

 noglossus and Cephalodiscus, I do not think there will 

 be any hesitation in regarding these as a pair of notochords 

 (pleurochords), together homologous with the single notochord 

 of the other Chorda ta. The remarks made concerning the 

 notochords of Actinotrocha apply equally well to these 

 organs, and the homology to each other cannot be doubted, 

 especially as the small differences, such as the presence of a 



