MUSEUM OF COMPARATIVE ZOOLOGY. OF 
cells of coelomic epithelium. Eisig (’87, p. 752) has already clearly ex- 
pressed how, in view of the many cases of high excretory activity of 
peritoneal and blood cells demonstrated by him, “ kiinftighin bei der 
Beurtheilung gewisser Zelleneinschliisse erst genau festzustellen sein wird, 
ob man est mit von aussen aufgenommenen (gefressenen), oder aber mit 
von der Zelle ausgeschiedenen Producten zu thun habe.” 
A criterion for judging this matter may be found, in the first place, I 
believe, in this: that the products of exeretion increase with the activi- 
ties of the cells, and are thrown out, usually in the shape of concrements, 
either from the cell or with the cell into the cwlom ; whereas bodies 
taken in from without for digestion decrease with the activities of the 
region. In the second place, vacuoles are less characteristic of excretory 
tissue than of imbibitory. But vacuoles are the important feature of the 
reticulated cells in Paludicella, and the highly refractive bodies are less 
constant phenomena. As for the latter, they are not found in the later 
stages, nor in the earliest. Moreover, these bodies differ from excretion 
concrements in this, that they are always transparent, often almost indis- 
cernible in the vacuole, except by their higher refractiveness, and there 
is no sharp demarcation between cases of vacuoles filled by such bodies 
and those the contents of which are less highly refractive. ‘The degree 
of refractiveness is variable, at one end of the series grading off into the 
undifferentiated fluid of the vacuole. What significance is to be assigned 
to these highly refractive bodies in the vacuoles? There are two reasons 
why I do not believe that they represent solid food particles devoured 
as such by the mesodermal cells. First, I do not find such highly 
refractive bodies lying loose in the body cavity before the stage at which 
they first appear in the cells; and, secondly, one can find all gradations 
between less highly refractive vacuoles and highly refractive ones (which 
I have assumed to be entirely filled by one highly refractive body), and 
between the latter and vacuoles containing a small body surrounded by 
a broad, clear area. I believe, therefore, that the vacuoles are rather 
cavities filled with chemically different nutritive fluids, which are acted 
upon differently by the reagent. 
I have assumed that the contents of the vacuoles represent material 
taken up from the body cavity, because it seemed most reasonable to 
look there for the source of their supply. ‘The ectoderm is covered on 
its outer surface by an apparently continuous cuticula, so that food 
cannot be gained from the outside world directly. It is, moreover, not 
unreasonable to suppose that some of the products of digestion elabo- 
rated by the adult polypides of the colony pass through the wall of the 
