38 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



distance beyond that. It contains in its centre the noto- 

 chord (or urochord, n. ck.) and on the left side of that the 

 nerve or myelon (n.). In section 155 (PI. III. fig. 1) we 

 see the latter greatly enlarged to form the caudal ganglion 

 at the base of the tail. The heart is placed just above the 

 base of the tail (PI. IV. fig. 9, h.). We see a part of it 

 between the stomach and the rectum in PI. II. fig. 5, at h. 

 We now come to the important region, about section 

 170, where the branchial sac or pharynx connnunicates 

 with the exterior by a pair of ventral tubular ciliated 

 openings or stigmata. The series of sections represented 

 by figures 3 to 6 on PI. Ill show the structure of these 

 openings and the passages leading to them. The first 

 change as one approaches the stigmata is seen at a on the 

 right hand side of fig. 3, (section 165) where the lateral 

 edge of the branchial sac bends down ventrally while at 

 the same time the ectoderm on the surface of the body 

 over this region is invaginated, so that only a narrow 

 bridge of tissue is left between the two cavities. The 

 next stage is seen a couple of sections further forward at 

 b in fig. 4, where a few large cells appear in the bridge of 

 tissue and then become arranged, as at c, in two regular 

 rows. These rows of cells draw apart (see d, fig. 5), 

 then additional rows appear till there are in all 4 series of 

 cells with narrow slits between the rows {e. fig. 3). [On 

 account of a slight obliquity in the sections or because of 

 one of the stigmata being slightly further forward than 

 the other, the two sides of the body show different con- 

 ditions in the same section, consequently these four 

 sections figured show us eight or nine different stages 

 {a to i, figs. 3 to 6)] . The middle cells of each row then 

 die out as we trace the sections forwards (see/, fig. 4) 

 leaving merely a set of four pyramidal richly ciliated cells 

 {g. fig. 5) on each wall of the tubular passage which places 



