ON THE DIPLOCHORDA. 291 



gloss us, and the lophophoral segment (already sometimes 

 termed the "collar") of Cephalodiscus and Rhabdo- 

 pleura. The whole segment may well be termed in all these 

 animals the " mesomere " (see below). 



3. The trunk, extending as an elongated cylindrical portion 

 backwards from the mesomere, and bearing terminally the 

 perianal band and the anus. It will probably be a matter of 

 no great difficulty to show that this '' trunk " segment (" meta- 

 mere'^) is homologous with the trunk of Balanoglossus, 

 Cephalodiscus, and Rhabdopleura. 



The three segments are sufficiently well defined externally, 

 but the arrangement of the internal organs, such as coeloniic 

 cavities, is even more marked, and corresponds exactly with 

 the divisions here indicated. 



There are three prominent ciliated bauds, the pre-oral (or 

 prototroch), the collar-band following the line of the tentacles 

 (mesotroch), and the trunk band (perianal or metatroch) 

 surrounding the anus. Each is defined not only by the pre- 

 sence of very long cilia, but of elongated and densely aggre- 

 gated epithelial ectodermal cells bearing them, and well 

 supplied with nerve-tracts. 



Of the three the perianal band is the most prominent, and 

 functions as the locomotor organ of the larva. 



The other general relations of the three divisions are 

 diagrammatically shown in PI. 22. figs. 47 and 50. 



The Organs of the Epiblast. 



The dorsal and ventral surfaces of the pre-oral hood are 

 sharply separated by the pre-oral band of cilia and its thickened 

 cells covering a nerve-ring. The whole dorsal surface is covered 

 with thick glandular epiblast cells, which bear minute cilia 

 (PI. 19, fig. 6, and PI. 20, figs. 18, 19, 20, 28, and 29). Each cell 

 is elongated, with a nucleus near its middle. In some parts 

 the longest cells are found in the mid-dorsal part, but in others 

 the part next to the ciliated band is thickened. 



In the small specimens referred to above, this very thick 

 glandular epiblast is more conspicuous than in the larger 



