1. _The Hydropolyp_ (fig. 1)--The general characters of this organism are described above and in the articles HYDROZOA and POLYP. It is rarely free, but usually fixed and incapable of locomotion. The foot by which it is attached often sends out root-like processes--the _hydrorhiza_ (c). The column (b) is generally long, slender and stalk-like (_hydrocaulus_). Just below the crown of tentacles, however, the body widens out to form a "head," termed, the _hydranth_ (a), containing a stomach-like dilatation of the digestive cavity. On the upper face of the hydranth the crown of tentacles (t) surrounds the peristome, from which rises the conical hypostome, bearing the mouth at its extremity. The general ectoderm covering the surface of the body has entirely lost the cilia present in the earlier larval stages (planula), and may be naked, or clothed in a cuticle or exoskeleton, the perisarc (ps), which in its simplest condition is a chitinous membrane secreted by the ectoderm. The perisarc when present invests the hydrorhiza and hydrocaulus; it may stop short below the hydranth, or it may extend farther. In general there are two types of exoskeleton, characteristic of the two principal divisions of the Hydroidea. In the Gymnoblastea the perisarc either stops below the hydranth, or, if continued on to it, forms a closely-fitting investment extending as a thin cuticle as far as the bases of the tentacles (e.g. _Bimeria_, see G. J. Allman [1],[2] pl. xii. figs, 1 and 3). In the Calyptoblastea the perisarc is always continued above the hydrocaulus, and forms a cup, the hydrangium or hydrotheca (h, t), standing off from the body, into which the hydranth can be retracted for shelter and protection. [Illustration: From Allman's _Gymnoblastic Hydroids_, by permission of the Council of the Ray Society. FIG. 2.--_Stauridium productum_, portion of the colony magnified; p, polyp; rh, hydrorhiza.] [Illustration: FIG. 3.--Diagram of _Corymorpha_. A, A hydriform person giving rise to medusiform persons by budding from the margin of the disk; B, free swimming medusa (_Steenstrupia_ of Forbes) detached from the same, with manubrial genitalia, (_Anthomedusae_) and only one tentacle. (After Allman).]