282 Proceedings of Royal Society of Edinburgh. [sess. 
contain the sexual elements, and the great elongation of these may- 
be correlated with the burrowing habit. The nervous system is 
still in connection with the ectoderm, the main ganglion dorsal to 
the mesomeres, and a post-oral ring gives off a ventral cord. A 
dorsal cord is superadded along the hypertrophied metameres. 
In the protomere is the proboscis-vesicle, which may he the 
Fig. 8. — Larval Balanoglossus 
(after Bateson). 
Fig. 2.— -Diagrammatic hori- 
zontal section through 
young Balanoglossus. 
much-modified primitive excretory organ, * and the sub-neural sinus 
or heart, connected with a primitive vascular system. 
The facts of ontogeny bear out our contention. 
Bateson’s larva has three distinct segments, each with its ciliated 
band, one pre-oral and two post-oral, and the coelomic cavities of 
these arise, as archenteric diverticula, or by a slightly modified 
process. In Tornaria the three bands — the prototroch, mesotroch, 
and metatroch — can be discerned, and the development of the five 
coelomic elements appears to be reducible to the type of archenteric 
diverticula. The full significance of Tornaria will be discussed 
* Q. J. M. S., August 1897, “ On the Diplochorda.” 
