108 DE W. A. HEEDMAN ON 



ends, and hence the anterior end of the endostyle and the dorsal spirals come 

 to be situated in the 2nd intermuscular space. 



The aperture in the prebranchial zone is small, and leads into a funnel- 

 shaped cavity continuous with the neural duct (PI. XIX. fig. 9). At the posterior 

 narrower end of this cavity, the flat cells lining the duct become gradually 

 cubical and then low columnar, and bear each a long cilium which projects into 

 the centre of the cavity, and is directed posteriorly. This funnel-shaped 

 cavity is apparently merely the aperture of the neural duct. I have searched in 

 vain for any trace of a sensory apparatus. In several specimens I have suc- 

 ceeded in tracing one of the smaller nerves given off from the anterior end 

 of the ganglion in its entire course forwards (see PI. XVIII. fig. 12, n). It 

 runs alongside the duct and close to it, but passes the funnel-shaped cavity 

 upon its left side without giving off any branch, and continues its way 

 anteriorly to supply the lobes around the branchial aperture. 



The heart is situated on the ventral surface of the posterior end of the 

 branchial sac, just between the termination of the endostyle and the oesopha- 

 geal aperture and in the posterior part of the 4th intermuscular space 

 (PI. XIX. fig. 10, h). In chromic acid specimens the transverse muscle bands 

 of the wall of the heart were well shown (see PI. XIX. fig. 11), but each band 

 appears to me to be composed of a large number of very fine fibres placed side 

 by side, and not of one fibre only as supposed by Keferstein and Ehlers.* 



The alimentary canal, omitting the pharynx or branchial sac, which has 

 been already considered, consists of oesophagus, stomach, and intestine, and 

 forms a curved tube, lying mainly in the 5th and 6th intermuscular spaces 

 (PI. XVIII. fig. 4). 



The oesophageal aperture is placed at the posterior end of the branchial sac 

 in the middle line, and nearer to the ventral than to the dorsal surface. It 

 lies in the membranous area prolonged back from the region around the 

 posterior extremity of the endostyle (PI. XIX. fig. 10), and is surrounded 

 laterally and dorsally by the posterior end of the ventral series of stigmata. 

 This is a notable point, since it is usual in the Ascidiacea for the oesophageal 

 aperture to be placed on the dorsal edge of the sac, and invariably so amongst 

 Ascidiae Simplices, in some of which it is placed nearer to the anterior than 

 to the posterior end of the dorsal edge. 



The oesophageal aperture is surrounded by a membranous rim, which on its 

 left anterior edge is continued forwards to join the posterior extremity of the 

 endostyle, while at its other end, after surrounding the aperture (see PL XIX. 

 fig. 10, mb), it is continued as a spiral ridge into the cavity of the oesophagus. 

 The oesophagus is short, and leads downwards and backwards to the anterior 

 end of the large irregularly quadrangular stomach (PL XX. fig. 1, st). From 



* Loc. cit., p. 58. 



