184 Eecent Progress in our Knowledge of 
liquid which has transuded through the intestinal wall, and which 
may be called chyle, or blood in the wider sense of the v^ ord. 
Haeckel has thus taken, I believe, the true view of the intes- 
tinal and body cavities of animals. He had already advocated it in 
his work on the Calcareous Sponges. It necessarily involves a 
belief in the homological identity of organization between very 
distant groups of the animal kingdom, a belief which all recent 
embryological research has only tended to confirm. 
It follows from this view that the cavity of the Coelenterata 
would represent an intestinal cavity only, while a true body cavity 
would be here entirely absent. This way of regarding the cavity 
of the Coelenterata is at variance with the conclusions of most other 
anatomists who regard the coelenterate cavity as representing a true 
body cavity, or a body and intestinal cavity combined. I had 
myself long entertained the generally accepted opinion that the 
cavity of the Coelenterata represents a body cavity. I must, how- 
ever, now give my adhesion to the doctrine here advocated by 
Haeckel, and regard the proper body cavity of the higher animals 
as having no representative in the Coelenterata. I believe that this 
is supported both by the facts of development and by the structure 
of the mature animal. Indeed, the body cavity first shows itself, 
as Haeckel has pointed out, in the higher worms, and is thence 
carried into the higher groups of the animal kingdom. 
If such be the real nature of a true intestinal cavity and of a 
true body cavity, it is plain that neither the one nor the other can 
exist in the Infusoria, for there is here nothing which can be com- 
pared with either the endoderm or the ectoderm. 
The whole, then, of the alleged chyme of the Infusoria is 
nothing more than the internal soft protoplasm of the body. It is 
quite the same as in Amoeba and many other unicellular animals. 
The peculiar currents which have been long noticed in the 
endoplasm of many Infusoria must be placed in the same category 
with the rotation of the protoplasm observed in many organic cells. 
Yon Siebold, indeed, had already compared the endoplasm currents 
of the Infusoria to the well-known rotation of the protoplasm in the 
cells of Chara. 
The presence of a mouth and anal orifice in the ciliate Infusoria 
has been urged as an argument against the unicellular nature of 
these organisms. The so-called mouth and anus, however, admit 
of a comparison not in a morphological but only in a ]physiological 
sense with the mouth and anus of higher animals. They are 
simple lacunae in the firm exoplasm, and have, according to Haeckel, 
no higher morphological value than the " pore canals " in the wall 
of many animal and plant cells, or the micropyle in that of many 
ogg-colls. Kolliker had already compared them to the excretory 
canal of unicellular glands. Since, therefore, they do not admit of 
