PERIPflElUL NERVOUS SYSTEM IN NEMERTINES, 439 
Another question which future investigations will have to 
solve, and which at present I can no more than briefly call 
attention to, is this — How are we to conceive the nervous 
sheath to have gradually become enclosed between the 
muscular layers ? More especially can the difference be- 
tween Carinella and the Schizonemerteans be explained ? 
If I venture to emit an hypothesis at present for 
which I cannot as yet furnish sufficient or conclusive evi- 
dence, and which presented itself to me in studying the 
beautiful investigations of 0. and H. Hertivig, on the struc- 
ture of iheActinice (^JenaischeZeitschrift,’ 1880), it is because 
it appears at the same time to explain the fundamental 
difFerence in the muscular system of different groups of 
Nemerteans, which was first clearly pointed out by Macin- 
tosh. The hypothesis might be briefly formulated thus : the 
external longitudinal muscular layer in Polia, Valencinia, 
and all the Schizonemertini must be regarded as a special 
development of the ectodermal musculature.’^ If this be true 
(and the curious arrangement of the external longitudinal 
muscles in Polia compared with Carinella^ on the one hand, 
with the Schizonemertini, on the other, appears to furnish 
strong arguments in its favour), the wandering inward of the 
nervous system can be more clearly understood, those muscles, 
which from the first were situated exteriorly to it having 
gradually attained this strong development and fascicular 
arrangement. The inner longitudinal and the circular 
muscular layer, as they are present in Carinella and the 
Hoplonemertini, would thus represent the typical mus- 
cular system of Nemertine organisation, whereas the ex- 
terior layer of longitudinal muscles, which is added to it in 
the Schizonemertini, Valencinia^ and Polia, would appear 
to be of accessory character. 
The curious distribution of nervous tissue above described 
is, I believe, of importance towards the solution of two 
problems touching the physiology of these worms. 
These problems are — 1st. The extraordinary restorative 
power of a great number of Nemerteans. 2nd. Their intense 
sensitiveness to outward stimuli, Avhich may even occasion 
spontaneous rupture of the animal into two or more pieces. 
As to the restorative power, it has often been noticed (Pa- 
lyell, Macintosh, Barrois) : and detailed observations are on 
record of severed portions of the trunk having regained a com- 
plete central nervous system — brain, respiratory lobes, and 
eyes included — when only a cylindrical fragment, open at both 
ends, was left. The tail end is always sooner restored, but 
under favorable circumstances the restoration of the head 
