Diagram of the structure of a medusa; the ectoderm is left clear, the endoderm is dotted, the mesogloea is shaded black; a-b, principal axis (see Hydrozoa); to the left of this line the section is supposed to pass through an inter-radius (I.R.); to the right through a radius (R).
The ectoderm beneath each fold becomes detached from the surface of the basal plate, and both it and the mesogloea are folded conformably with the endoderm.
As the name jelly-fish implies, the mesogloea is greatly developed and abundant in quantity.
The ectoderm is indicated by close hatching, the endoderm by light hatching, the mesogloea by thick black lines, the horny skeleton of the pneumatophore and sail by dotting.
In many cases, as, for example, in the Medusae or jelly-fish, the mesogloea may be so thick as to constitute the chief part of the body in bulk and weight.
Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termed mesogloea, secreted by the cell-layers of the body-wall; the mesogloea may be a very thin layer, or may reach a fair thickness, and then sometimes contains skeletal elements formed by cells which have migrated into it from the ectoderm.
The corallum shaded with dots, the mesogloea represented by a thick line.
The mesogloea is in itself an inert non-cellular secretion, but the immigration of muscular and other cells into its substance, from both ectoderm and endoderm, gives it in many cases a strong resemblance to the mesoderm of Triploblastica, - a resemblance which, while probably superficial, may yet serve to indicate the path of evolution of the mesoderm.
In Actinia equina the mesogloea consists of fine fibres imbedded in a homogeneous matrix, and between the fibres are minute branched or spindle-shaped cells.
The mesogloea is greatly developed in them and they are often of very tough consistency.