THE DEVELOPMENT OF ALCYONIUM DIGITATUM. 
69 
and ectoderm cells is brought about by the multiplication of 
very small interstitial cells found at their bases. When very 
young these are even smaller than the nuclei of the adult 
cells. Nematocysts are now formed by the rounded cells 
lying at the outer edge of the ectoderm (Text-fig. 36), these 
cells being also recruited from the interstitial cells (PI. 3^ 
fig. 19, We.). Previous to this many ectodermic interstitial 
-cells give rise to broad columnar mucous cells, i . e . refringent 
cells with a protoplasmic network surrounding the mucus 
which stains deeply with haematoxylin (PI. 3, fig. 2, M. C.). 
At the anterior pole the endodermic tissue forms a deeper 
layer than elsewhere (PI. 3, fig. 1). The ectoderm cells of 
the anterior pole become specially long in the well-grown 
planula, and the mucous cells are particularly numerous. 
Now the larva settles by the broad anterior and aboral pole 
(see general account), and for some hours after fixation the 
still ciliate planula hangs freely in the water, retaining its 
characteristic contractile power. If forcibly detached from 
the mucus plug which fastens the broad anterior end to the 
substratum, it will swim freely again for some time and then 
resettle. The round flat disc formed by the anterior pole 
becomes the base of the new polyp, and hence the nucleus of 
-attachment of a fresh colony. For some time after settling 
the coelenteron still contains much yolky detritus, which is 
gradually used up. The endoderm is similar to that described 
for the late planula, i. e. many cells deep at the fixed aboral 
pole and one layer elsewhere, clumps of non-permanent cells 
clinging to it in places (PI. 4, fig. 24, End. M., and End.). 
Very many round cells arise next to the supporting membrane 
in the ectoderm of the fixed base of the polyp, soon giving it 
a multilayered character (PI. 4, fig. 23, R. G.). Mucous cells 
are still abundant in the ectoderm, but gradually disappear, 
Fig. 35. Ectoderm of stage shown in PI. 3, fig. 16, at time of 
rapid proliferation of cells (temporarily multilayered). Fig. 36. 
Ectoderm and endoderm cells from late planula, showing 
nematocysts. Fig. 37. Sagittal section of newly settled polyp, 
now shrinking in length. The pointed posterior and oral pole 
of the planula is still visible ( X 133). 
