Jellyfish are mostly water.
From the outside, jellyfish look like squishy, insubstantial blobs, and this is reflected in their structural makeup. Jellyfish are 95 percent water with the rest of them accounting for minerals and proteins. Between their two dermis layers is a gelatinous, water-based substance called mesoglea that contains muscle cells, nerve cells, and structural proteins.
The Haus of Dcaligari 