FRIAPDLUS AND HALICRYPTUS. 
209 
cord and an oesophageal ring. Both in Priapulus and Hali- 
cryptus the nervous system lies entirely in the ectoderm — a 
condition which is of rare occurrence, but which has, as far as 
I know, been likewise noticed in a few Annelids — for example, 
Hesione and Ovenia. 
Ou the body proper the position of the cord is well-marked 
externally by a shallow groove running along the ventral surface, 
whose two sides are slightly raised, while two of the spike- 
bearing ribs indicate its continuation on the proboscis. The 
nerve- cord is not continued into the tail appendage (Schwanz- 
anhang), but ends at the posterior part of the body in a con- 
siderable swelling. Anteriorly it divides into two branches 
which surround the oesophagus. Its position is here again 
indicated externally by a very deep groove (fig. 8, g.). 
Although it appears in cross-sections as if swellings existed 
in the cord at regular intervals, I believe this to be merely 
due to the powerful contractions of the annular muscles, 
allowing the cord to bulge out slightly in the intervening 
spaces. In the oesophageal ring, however, a real thickening, 
already observed by Saenger, exists dorsally. As regards the 
size of the nerve-cord in the ring as compared with that in the 
body, the diameter in the former is about two to three times 
as great. 
All previous observers state that the nervous system lies 
immediately under the hypodermis, between it and the annular 
muscles. In reality, however, it is placed within the hypo- 
dermis-, the ganglionic cells being simply modified hypodermic 
cells and the fibrils their processes. As the hypodermis 
approaches the cord its cells become elongated just as we have 
seen before in the case of the spikes, and ultimately they swell 
up, becoming modified into ganglionic cells (figs. 6, 7, 8). 
Internally the cells of the hypodermis send out numerous 
processes. These are well seen in the long cells close to the 
large mass of nerve-fibres in the body (fig. 6, h.p.). The dorsal 
part of the cord is wholly taken up by the nerve-fibres (figs. 
6, 7, S,/.), and on each side ventrally we find a cluster of 
ganglionic cells (fig. 6, g. c.). A similar arrangement has been 
