FIG. 8.--Vacuolated Endoderm Cells of cartilaginous consistence from the axis of the tentacle of a Medusa (_Cunina_).] The endoderm of the polyp is typically a flagellated epithelium of large cells (fig. 6), from the bases of which arise contractile muscular processes lying in the plane of the transverse section of the body. In different parts of the coelenteron the endoderm may be of three principal types--(1) digestive endoderm, the primitive type, with cells of large size and considerably vacuolated, found in the hydranth; some of these cells may become special glandular cells, without flagella or contractile processes; (2) circulatory endoderm, without vacuoles and without basal contractile processes, found in the hydrorhiza and hydrocaulus; (3) supporting endoderm (fig. 8), seen in solid tentacles as a row of cubical vacuolated cells, occupying the axis of the tentacle, greatly resembling notochordal tissue, particularly that of _Amphioxus_ at a certain stage of development; as a fourth variety of endodermal cells excretory cells should perhaps be reckoned, as seen in the pores in the foot of _Hydra_ and elsewhere (cf. C. Chun, HYDROZOA [1], pp. 314, 315). The mesogloea in the hydropolyp is a thin elastic layer, in which may be lodged the muscular fibres and ganglion cells mentioned above, but which never contains any connective tissue or skeletogenous cells or any other kind of special mesogloeal corpuscles. [Illustration: From Allman's _Gymnoblastic Hydroids_, by permission of the Council of the Ray Society. FIG. 9.--Colony of _Hydractinia echinata_, growing on the Shell of a Whelk. Natural size.] [Illustration: From Allman's _Gymnoblastic Hydroids_, by permission of the Council of the Ray Society. FIG. 10.--Polyps from a Colony of _Hydractinia_, magnified. dz, dactylozoid; gz, gastrozoid: b, blastostyle; gon, gonophores; rh, hydrorhiza.]