In specimens with the proboscis drawn in, the nerve cord is of course doubled back on itself.
It lies ventral to the nerve cord, and shows no trace of segmentation.
Neural groove: is that furrow in the primitive layer of the embryo in which the nerve cord is formed.
The notochord (fig. 3, 2) is an elastic rod extending along the whole length of the body past the anterior end of the nerve cord.
But Helmholtz readily demonstrated the contrary, showing that the nerve cord is a relatively sluggish message-bearer.
The notochord is enclosed in a thick sheath of connective tissue continuous with a thinner sheath round the nerve cord.