$$ 123, 124. 
CHAPTERS IIl. 
THE TURBELLARIA. 
135 
AND IV. 
NERVOUS SYSTEM AND ORGANS OF SENSE. 
§ 123. 
N 
The nervous system with the Turbellaria, is quite indistinct, for it has 
not yet been observed in the small species, and in the larger ones its dis- 
position is yet doubtful. 
A double ganglion in the cervical region appears 
to form its central part, and from this nerves pass off in different direc- 
tions. 
§ 124. 
Among the organs of sense, those of vision are the most developed with 
very many species. 
The red, brown, or black spots on the anterior extremity, two or 
more in number, are not always simple pigment cells, but may be regard- 
ed as eyes, for they have a cornea, —a light-refracting ‘body surrounded 
with pigment, and a nerve-bulb.® 
As to the sense of touch, no special tactile organs have yet been found, 
but. the whole surface reacts sensitively from the lightest contact; and this 
sonuidility appears particularly prominent at the anterior extremity, which, 
with many Dendrocoéli, is furnished with lobular and other appendages. 
1 Ehrenberg has seen two disconnected ganglia 
with Planaria lactea (Abhand. d. Berl. Akad. 
1835, p. 243). With other Dendrocoéli, as with 
Planocera sargassicola, and pellucida, these two 
ganglia are blended into one; at least, the organ 
which Mertens has here described as a heart, has 
exactly the appearance of two united ganglia (loc. 
cit. Taf. I. fig. 6, Taf. II. fig. 3, m. or Isis 1836, Taf. 
IX. fig. 3, c. m.). The light pulsations which this 
author affirms to have here observed, are perhaps, 
as Ehrenberg has supposed (loc. cit. p. 244), due to 
the contractions of neighboring organs. According 
to Schulze (loc. cit. p. 39), with Planaria torva, 
the double central ganglion gives off two nerves, 
which pass backwards on both sides of the intes- 
tine. : 
This double ganglion, situated in the cervical re- 
gion, and the nervous filaments which it gives off, 
have been demonstrated by Quatrefages (loc. cit. 
p. 172, Pl. IV.-VI.). 
1 Most commonly there are two eye-dots. With 
Pl ‘a, and Leptoplana, there are many which 
are grouped together, and with Polycelis nigra, 
the whole anterior part of the body is covered with 
them. In many small species, they appear to be 
of a simple pigmentary nature. 
2 With Planaria lactea, there is, between the 
cornea-like bulging of the skin, and a semilunar, 
pigment layer, a small, conical, transparent body, 
corresponding exactly to a crystalline lens; see 
Ehrenberg, loc. cit. p. 248, and Schulze, loc. cit. 
* [§ 124, note 2.) See also Leidy (Proc. Acad. 
Nat. Sc. Philad. III. 1848, p. 248) on the eye-specks 
of Phagocata gracilis, a sub-genus made by him 
from Planaria, and Schmidt (Die Rhabdocoél 
Strudelwiirmer, &., p. 7, and Neue Beitrage Zur 
Naturgesch, der Wiirmer, &., p. 11). Both of 
these observers agree in considering these parts in 
p. 37. With Monocelis, these organs are very 
remarkable, being composed of two eyes blended 
into one, and the simple and spherical ball of the 
eye is filled, according to Orsted, with a transpa- 
rent vitreous body, in which two conical crystalline 
lenses are buried with their apices pointing in- 
wards (loc. cit. p. 6, 56, Taf. I. fig. 1, 2, and in 
the text, fig. 10). Orsted has distinctly seen two 
optic nerves passing laterally to this organ. It is 
quite remarkable that with one of the three known 
species, the Monocelis unipunctata, the eye is 
entirely without pigment. Ehrenberg affirms that 
he has observed with Polycelis, many star-like 
ganglia in the middle of the anterior part of the 
body, which are for the long row of eye-dots (loc. 
cit. p. 243). 
For the eyes of the marine Planariae, see also 
Quatrefages, loc. cit. p. 178, Pl. III. The organ 
which with Monocoelis has been taken for an eye 
by Orsted, appears to be, according to the re- 
searches of Frey and Leuckart (Beitr. p. 83, Taf. 
I. fig. 18), an auditory organ. That which Orsted 
regarded a vitreous body, is an otolite, and his two 
crystalline lenses, are two semicircular prolonga- 
tions attached loop-like to the otolite. Frey and 
Leuckart are also convinced that Convoluta par- 
adoza Orst.,hasa single auditive capsule, situated 
on the median line of the cervical region, and con- 
taining an otolite which floats in a lilac-colored 
fluid ; see Beitr. loc. cit. p. 82, Taf. I. fig. 17.* 
8 There are contractile and antenniform append- 
question as visual organs. Schmidt has often 
failed to find anything like an otolite ; but, on the 
other hand, has often found with various Derosto- 
mum a complete visual apparatus. This point, 
therefore, is still tled, unless, as Schmidt in- 
geniously suggests, it may be that one organ serves 
the functions of two separate senses. — Ep. 
