440 
DR. A. A. W. HUBRECHT. 
and brains does not seem to be rare. Although I never 
had occasion to study the phenomenon myself, the descrip- 
tions oiBarrois and Macintosh do not leave any doubt as to 
the way in which it progresses. Three months after the 
fragment was separated from the parent it was again pro- 
vided with the normal brain-lobes and all their accessories, 
which had developed at the anterior ends of the nerve- 
trunks after the cicatrising process at both ends of the frag- 
ment had been concluded. Another fragment was severed 
from the parent animal in September, developed perfect 
spermatozoa and normally deposited them in February next. 
Looking upon the lateral trunks as local thickenings of the 
more indifferent and primitive nerve-sheath in the body-wall, 
these curious facts appear in another light. For if we have 
succeeded in demonstrating that nervous tissue, which per- 
sists in a more primitive stage, the 'cellular elements being 
scattered indiscriminately among the fibrillar, is diffused 
throughout the whole circumference of the body-wall, then 
the appearance of local thickenings in this tissue finally 
leading to the formation of lateral trunks, and eventually of 
brain-lohes sensu strictiori, is less startling than would be 
the reproduction of the central apparatus out of lateral 
trunks, which hitherto have been regarded as essentially 
peripheral. 
I must here lay stress upon the fact that in the Hoplo- 
NEMERTINI, where this primitive^ nerve-sheath has already 
been replaced by separate diverging peripheral branches, 
and, which, therefore, have attained a far more specialised 
stage respecting the structure of their nerve-system, no case 
of similar reproduction of the head and brains has ever yet 
been noticed, as far as I know. 
All such observations were made upon Schizonemertini, 
and I do not believe it will prove a hazardous prophecy to 
predict that later investigations will show that in the armed 
species it really never occurs. At the same time the latter 
die much quicker after having been severely injured than do 
the Schizonemerteans, the families of Ampliiporid<2 and 
TetrastemmidcB enjoying a still more restricted longevity than 
do the family of Nemertidce s. str. 
All this appears to me to be to a great extent explained 
by the above described fundamental differences in the dis- 
tribution of the nervous tissue in the body-wall. 
As to the extreme sensitiveness to outward stimuli, it has 
been noticed by all former observers, and Leuckart applied the 
name of somatotomus to one of the species, because it regu- 
larly broke itself into pieces upon being handled or touched. 
