Frenzel’s Mesozoon Salinella. 475 
direct stimulus to it. The great caducity of endoderm cells 
(and of gland-cells altogether) is a character of very general 
occurrence. 
Were we able, says Frenzel, “still to regard the latter (7. e. 
the intestinal cells) at all events as Protozoon cells, this view 
would be absolutely inadmissible for the former, the cells of 
the mesoderm and ectoderm .. . .””. Ido not at all see why. 
The intestinal cells with intra-cellular digestion correspond to 
holophytic Protozoa; the rest of the cells of the body corre- 
spond partly to saprophytic Protozoa, because, thanks to the 
labours of other cells, they need only to feed, but not to digest 
their food; in part, however, the body-cells (especially those 
of the mesenchyma) are likewise holophytic Protozoa, and 
remain so even when the intestinal cells have long lost the 
faculty of intra-cellular digestion. In more primitive cases 
the intestinal cells themselves digest ; they subsequently lose _ 
this faculty, and henceforth expend their energies in the pro- 
duction of digestive juices; the latter, however, are not sufl- 
cient for the digestion of the food-matter, and the wandering 
amoeboid cells have to assist more or less with their power of 
intra-cellular digestion. The intestinal cells continue to be 
Protozoa, with which other body-cells, likewise corresponding 
to Protozoa, live together in a kind of symbiosis: their 
services to one another are reciprocal, so that their functions 
are consolidated into a physiological whole. Not only do the 
intestinal cells feed the rest, but a large portion of the latter 
also make provision for the intestinal cells: oxygen is in the 
widest sense food, just as much as albumen, fat, and carbo- 
hydrates. 
I believe that I have sufficiently demonstrated in the fore- 
going that it is precisely the physiology of digestion which 
causes least difficulties in deriving the Metazoa from the Pro- 
tozoa; but also the other “gulf” between Protozoa and 
Metazoa, which Frenzel likewise emphasizes, and which is 
occasioned by the multilaminar character of the Metazoa, 
appears to us less great when we take into consideration the 
following facts. 
As a single-layered multicellular animal we are now also 
acquainted with Salinella, besides Volvox. The next stage, 
with the representatives of which we have closer acquaintance 
as adult animals, already consists—to leave T’richoplax ad- 
herens out of consideration—of three layers, since in them a 
mesoderm, or, better, mesenchyma, is already present between 
ectoderm and endoderm ; for of animals which in the adult 
stage also would correspond to the typical Gastrula, and con- 
sist merely of ectoderm and endoderm, we have no knowledge. 
