CILIATED PITS OF STENOSTOMA 237 
The ciliated pit-ganglion is by far the most conspicuous fea- 
ture of the pit. It is located just within. the body and lying 
around the base of the ciliated pit in a cup-like manner. The 
cells of this structure are only indistinctly separated from those 
of the dorsal ganglion or ‘brain’ by a few muscle fibers and have 
the same characteristic, granular nuclei as those of the ‘brain.’ 
Some of these ciliated pit-ganglion cells are seen to send processes 
through the epithelium of the fundus which les in contact with 
the homogeneous body of mucus-like substance. These we take 
to be sensory rods of highly special nature which enable the 
organ to detect very slight changes in its surrounding medium. 
These sensory rods are shown clearly, as dark blue structures, in 
intra vitam staining with Wright’s stain and in many of the 
regular sections. Figure 4 shows such a section. The fact 
that the ganglion cells arise from the epithelial cells which line 
the fundus of the pit also supports the idea that these processes 
are left behind by the cells as they migrate inward to enter into 
formation of the ciliated pit-ganglion. 
Rightful interpretation and appreciation of the above state- 
ments will only be obtained through a study of the origin of the 
ciliated pits. 
THE ORIGIN OF CILIATED PITS 
The origin of the ciliated pits can readily be studied in speci- 
mens which are dividing. The first appearance of the pits 
is seen in two sharp, abrupt depressions of the epidermis, one 
on each side of the animal. From the bottoms of the depressions 
(i.e., the region which will be the fundus of the new pit) the cilia 
disappear. Figure 1 shows an early pit in this stage of develop- 
ment. Ventral to this abruptly lowered region a crowded mass 
of mesenchymal cells is formed which represents the anlage of 
the ‘brain.’! 
1 We are indebted to Prof. Bohmig, through the kindness of Prof. L. von Graff, 
for the following quotation from page 34 of O. and R. Hertwig’s Die Coelom- 
theorie, Jena, 1881. In regard to the Platyhelminthes they say: ‘‘In der Abteil- 
lung stammt wahrscheinlich der motorische Teil der Centralorgane des Nerven- 
systems im Anschlusse und die Muskulatur aus dem Mesenchym, der sensorielle 
Teil im Anschlusse an die Sinnesorgane aus dem Ektoderm.”’ 
