No. 3.] TUBULARIAN HYDROIDS. AIQ 
concerned in the regenerative processes, hence its anatomy and 
histology are of special interest in considering the questions 
presented by the phenomena of regeneration. The outer 
ectodermal layer consists mainly of large cells, among which 
are scattered smaller cnidoblasts ; beneath these there is a 
layer of muscle cells. 
Beneath this muscular layer is found the transparent 
«“ Stiitzlamelle,” or supporting layer, which is strongly devel- 
oped in the Tubularians. 
Lastly, below this layer is found the endodermal layer, 
which consists of large cells possessing nuclei lying near the 
walls, imbedded in a finely granular protoplasm. These cells 
also usually contain red pigment granules ;—— it is owing to the 
presence of this pigment that the pink tint of the hydroids is 
due. On the hydranths the endoderm cells show considerable 
differentiation. According to Hamann (5) the cells of the 
oral part are quite different from those of the middle and basal 
portions. In the first-named region the cells are longer and 
more slender than are the cubical cells in the “stomach”’ por- 
tion ; secreting glandular cells are found in the hypostome 
portion which stain more intensely in carmine than do the 
other cells. The endoderm of tubularian hydranths presents a 
series of folds or “taeniolae,” which are usually four or five in 
number in the hypostome region ; these may branch to form 
eight or ten folds in the stomach region, while below they pass 
into the simple entodermic layer of the stem. 
It is unnecessary, for the present purpose, to enter more 
deeply into the histological details of the cell structure. This 
review of hydroid structure will suffice to show that what at 
first glance appears to be a case of very simple regeneration, 
in the formation of new hydranths from stem portions, is in 
all probability a comparatively complex differentiation of stem 
tissues into those possessing the various functions of absorp- 
tion, secretion, etc. This must be the case, if, as Allman says, 
we are justified in considering the portion of the somatic 
cavity included in the hydranth, as chiefly devoted to digestive 
processes. Before passing on to the experimental results, 
there remains to be described the somatic fluid which fills the 
