mesoglea
noun
me·so·glea ˌme-zə-ˈglē-ə 
ˌmē-,
-sə-
variants or less commonly mesogloea
: a gelatinous substance between the endoderm and ectoderm of sponges or cnidarians
Example Sentences
Recent Examples on the Web Covered in silicone to replicate the jellyfish’s wavy, bioluminescent mesoglea — the jelly, basically — the Cyro weighs a staggering 170 pounds, all thanks to a five-year grant from the Office of Naval Research. 
Spencer Ackerman, WIRED, 28 Mar. 2013
Spencer Ackerman, WIRED, 28 Mar. 2013 Word History
Etymology
New Latin, from mes- + Late Greek gloia, glia glue — more at clay
First Known Use
1886, in the meaning defined above