In most Heteronemertines these elements are separated by a thin homogeneous basement membrane (fig.
The second is common to all the Heteronemertines, as well as to Polia and Valencinia, and also comprehends three layers, of which, however, two are longitudinal, viz.
In the Heteronemertines there is a layer in which the cutaneous elements are largely represented below the thin basement membrane (fig.
Such a nervous plexus indeed occurs in the body-wall of all Heteronemertines, sometimes even as a comparatively thick layer, situated, as are the nerve stems, between the external longitudinal and the circular muscles (fig.
It stretches forward as far as the brain, and in Carinella is again continued in front of it, whereas in the Heteronemertines the innervation of the anterior extremity of the head, in front of the brain, takes the form of more definite and less numerous branching stems. The presence of this plexus in connexion with the central stems, sending out nervous filaments amongst the muscles, explains the absence, in Pro-, Mesoand Heteronemertines, of separate and distinct peripheral nerve stems springing from the central stems innervating the different organs and body-regions, the only exceptions being the L.N.
In others there is an approximation of the lateral stems towards the median ventral line (Drepanophorus); in a genus of Heteronemertines (Langia), on the other hand, an arrangement occurs by which the longitudinal stems are no longer lateral, but have more or less approached each other dorsally.
Two lateral outgrowths of the foremost portion of the oesophagus, afterwards becoming constricted off, as well as two ingrowths from the epiblast, contribute towards its formation, at least as far as both Metaand Heteronemertines are concerned.
These posterior brain-lobes, which in all Heteronemertines are in direct continuity of tissue with the upper pair of principal lobes, cease to have this intimate connexion in the Metanemertini; and, although still constituted of (I) a ciliated duct, opening out externally, (2) nervous tissue surrounding it, and (3) histological elements distinctly different from the nervous, and most probably directly derived from the oesophageal outgrowths, they are nevertheless here no longer constantly situated behind the upper brain-lobes and directly connected with them, but are found sometimes behind, sometimes beside and sometimes before the brain-lobes.
For the Heteronemertines arguments have been adduced to prove that here they have the physiological significance of a special respiratory apparatus for the central nervous tissue, which in all these forms is strongly charged with haemoglobin.
In all Heteronemertines there is on each side of the head a longitudinal slit of varying length but generally considerable depth, in the bottom of which the dark red brain is very plainly visible by transparency.
With respect to the sense organs of the Nemertines, we find that eyes are of rather constant occurrence, although many Heteronemertines living in the mud appear to be blind.
In most Heteronemertines it is found to be an elongated slit with corrugated borders; in the Metanemertines it is smaller and rounded; in Malacobdella and Akrostomum it, moreover, serves for the extrusion of the proboscis, which emerges by a separate dorsal opening just inside the mouth.
18, in the Heteronemertines the lateral stems, while entirely uniform all through the posterior portion of the body, no longer individually exist in the oesophageal region, but here dissolve themselves into a network of vascular spaces surrounding this portion of the digestive tract.
Even in the larger Heteronemertines these pores are only a few millimetres behind the mouth region.