188 Recent Progress in our Knowledge of 
the primordial stomach or intestine cavity, which opens outward on 
one pole of the axis by a simple orifice — the primordial mouth, and 
whose walls are composed of two layers, the endoderm or inner 
germ lamella, and the ectoderm or outer germ lamella. 
This larval form has now been shown by the researches of 
Haeckel, Kowalevsky, Kay Lankester, and others, to occur in 
members of all the six higher primary groups of the animal king- 
dom ; and Haeckel, in conformity with what he has called the 
biogenetic fundamental law* — the recapitulation of ancestral forms 
in the course of the development of the individual — had already in 
a former work t concluded in favour of a common descent of all the 
six higher types from a single unknown ancestral form which must 
have been constructed essentially like the Grastrula, and to which he 
gives the name of Gastrma. 
From this common descent the Protozoa alone are excluded, 
these not having yet attained to the formation of germ lamellae or 
of a true intestinal cavity. 
He regards this difference between the development of the 
Protozoa and that of all the other animal types as so important, 
that he founds thereon a fundamental division of the whole animal 
kingdom into two great primary sections — the Protozoa and the 
Metazoa. The former never undergo segmentation, never develop 
germ lamellae, and never possess a true intestinal cavity ; the latter, 
which include all the other types of the animal kingdom, present a 
true segmentation of the egg-cell, have all two primary germ 
lamellae— endoderm and ectoderm — a true intestine formed from the 
endoderm, and a true epidermis from the ectoderm ; they all pass 
through the form of the gastrula, or an embryonic form capable of 
being immediately deduced from it, and (hypothetically) are all 
descended from a Gastraea. 
The only Metazoa which in their existing condition have no 
intestine are the low worm-groups — Coestoda and Acanthocephala ; 
but these form only an apparent exception, for the loss of their 
intestinal canal is a secondary occurrence caused by parasitism, and 
Haeckel regards them as having descended from worms in which 
the intestine was present. 
Several years ago Haeckel united into a separate kingdom, 
under the name of Protista, certain low organisms, some of which 
had been previously placed among the Protozoa, while others had 
been assigned to the vegetable kingdom. To this neutral group 
he refers the Monera, the Flagellatae, the Catallactas, the Laby- 
rinthuleae, the Micromycetae, and the Acytariae and Eadiolariae. 
After the elimination of these there remain as geuuine Protozoa 
the Amoibinae, the Gregarinae, the Acinetae, and, above all, the 
true Infusoria or Ciliata. 
* ' Gcncrcllc Moi'phologie.' f * Die Kalkschwamme.' 
