210 WILSON. (VoL. II. 
noticeable in older stages such as Fig. 27, which are often com- 
pletely covered by a thin layer of mucus. The mucus cells in- 
crease in size and number, until they become the prominent 
feature of the ectoderm (Figs. 7, 20, 27). In their final condi- 
tion, Fig. 27, they are large, clear sacs, in which a few strands of 
protoplasm may be seen in carefully prepared osmic specimens. 
The nucleus of the original cell in which the sac developed can- 
not be made out in sections. 
The mucus cells develop over the general surface of the larva, 
but are not found at the aboral end. Here their place is taken 
by a slender elongated cell, full of granules which stain even 
deeper with hamatoxylin than does the mucus cell. The gran- 
ular gland cell is easily recognized in its earliest stage, and is 
formed from the embryonic ectoderm cell by a deposition of 
granules throughout the length of the cell. The granular cells, 
though especially grouped at the aboral end of the larva, are 
found here and there over the general surface. 
The remaining cells of the embryonic ectoderm become for 
the most part transformed into slender ‘supporting cells.” 
Thread cells, 7.c., are first noticed in larvz at about the stage 
of Fig. 14. Whether the ectoderm contains any muscle cells, I 
do not know. The larvz can alter their shape to a great ex- 
tent, but the fibres are probably all endodermal. 
For the study of nervous elements, J/anzczna in all stages is a 
very unfavorable subject. Even the ganglion cells which show 
so plainly in sections of actinian larva, I was not able to make 
out in the coral. There is, however, a finely granular stratum at 
the base of the ectoderm, which is very thin over the general 
surface, Fig. 26, but at the aboral end of the swimming larva is 
thick and easily seen, Fig. 20. When the living larva is com- 
pressed, this accumulation of granular substance is very noticea- 
ble. It is perfectly clear, and until I began to section I thought 
it was jelly. When carefully examined, the granular layer in 
this region is found to consist of a mass of fine fibrils. It is 
very probably nervous. The fact that the ccelenterate planula in 
general swims with its aboral end in front, taken together with 
the occurrence of a bunch of long cilia on this end of many 
actinian larve, suggests the existence of some such primitive 
nervous centre as I take this accumulation of granular matter 
to be. 
