C (E LENTERA TES. 
49 6 
cance. Wherever these often minute animals settle, they build up great masses of 
rock which may form part of the solid ground of the globe. Although Aristotle 
and his contemporaries recognised the sea- 
anemones as animals, almost two thousand 
years elapsed before corals were considered to 
be related to them. In describing the develop¬ 
ment of a small coral discovered on the Arabian 
coast, and named Monoxenia darwini, Haeckel 
states that the polyp, which is one-eighth of 
an inch long, is of strictly radiate structure, the 
mouth, which lies at the upper end of the 
cylindrical body, being surrounded by eight 
feathered tentacles. It is attached to some 
substratum by means of a flexible disc at the 
opposite end of the body to the mouth. It is 
clear that it has no hard skeleton, as the 
shape of its surface is changeable; and its 
internal structure must be shown by transverse 
and longitudinal sections. The development 
of Monoxenia commences with the egg re¬ 
peatedly dividing into many parts ( C , D, E ). 
This process, which is common throughout the 
animal kingdom and is called egg-segmenta¬ 
tion, in this case proceeds so simply and 
regularly that it ends in the production of a 
hollow sphere enclosed by a single layer of 
cells ( G). Each cell sends out a long cilia or 
whip-like process ( F ) by means of which the 
larva turns about and swims in the body-fluid 
of the parent polyp. One-half of the sphere 
now becomes infolded into the other half {H), 
and forms what is called a gastmda (I, K). The 
Monoxenia darwini (highly magnified). term gastrula has taken a great place in zoology 
in recent years, since the Russian naturalist, 
Kowalevski found that many different classes of animals, in developing from the 
egg, passed through such a stage. Haeckel, generalising from these facts, invented 
his Gastrea theory, according to which all animals in which the gastrula stage 
occurs must have been descended from a common primitive form, Gastrea, which 
has, however, in its simplest form long been extinct, but of which the Ccelenterates 
are the closest modern representatives. 
The gastrula of Monoxenia is of the simplest kind, the infolding being 
complete, and the larva forming a sac, whose walls consist of two layers of cells, 
or germinal layers, an outer ectoderm and an inner endoderm (see section given in 
the illustration). The transition from the flat dish-shape ( H ) to the sac with a 
narrow mouth is at once clear, and the knowledge that all the Ccelenterates proceed 
from a similar larva, and that all the complications of their various systems are 
