Echinoderm gastrula stage:
$$
\begin{array}{c}
\text{Archenteron} \\
\text{---------} \\
\text{( )} \\
\text{---------} \\
\text{Blastocoel}
\end{array}
$$
For the simplified sponge, we can draw a diagram with the choanocyte, amoebocyte, spicules, osculum, and
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