The hypostome of the hydropolyp may be small, or, on the other hand, as in _Eudendrium_ (Allman, loc. cit. pls. xiii., xiv.), large and trumpet-shaped. In the curious polyp _Myriothela_ the body of the polyp is differentiated into nutritive and reproductive portions. [Illustration: FIG. 4.--Diagram of _Tubularia indivisa_. A single hydriform person a bearing a stalk carrying numerous degenerate medusiform persons or sporosacs b. (After Allman.)] _Histology._--The ectoderm of the hydropolyp is chiefly sensory, contractile and protective in function. It may also be glandular in places. It consists of two regions, an external epithelial layer and a more internal sub-epithelial layer. The epithelial layer consists of (1) so-called "indifferent" cells secreting the perisarc or cuticle and modified to form glandular cells in places; for example, the adhesive cells in the foot. (2) Sensory cells, which may be fairly numerous in places, especially on the tentacles, but which occur always scattered and isolated, never aggregated to form sense-organs as in the medusa. (3) Contractile or myo-epithelial cells, with the cell prolonged at the base into a contractile muscle-fibre (fig. 6, B). In the hydropolyp the ectodermal muscle-fibres are always directed longitudinally. Belonging primarily to the epithelial layer, the muscular cells may become secondarily sub-epithelial. [Illustration: From Allman's _Gymnoblastic Hydroids_, by permission of the Council of the Ray Society. FIG. 5.--Colonies of _Clava_. A, _Clava squamata_, magnified. B, _C. multicornis_, natural size; p, _polyp_; _gon_, gonophores; rh, hydrorhiza.] The sub-epithelial layer consists primarily of the so-called interstitial cells, lodged between the narrowed basal portions of the epithelial cells. From them are developed two distinct types of histological elements; the genital cells and the cnidoblasts or mother-cells of the nematocysts. The sub-epithelial layer thus primarily constituted may be recruited by immigration from without of other elements, more especially by nervous (ganglion) cells and muscle-cells derived from the epithelial layer. In its fullest development, therefore, the sub-epithelial layer consists of four classes of cell-elements. [Illustration: FIG. 6 A.--Portion of the body-wall of _Hydra_, showing ectoderm cells above, separated by "structureless lamella" from three flagellate endoderm cells below. The latter are vacuolated, and contain each a nucleus and several dark granules. In the middle ectoderm cell are seen a nucleus and three nematocysts, with trigger hairs projecting beyond the cuticle. A large nematocyst, with everted thread, is seen in the right-hand ectodermal cell. (After F. E. Schulze.)]