The architecture of the hydropolyp, simple though it be, furnishes a long series of variations affecting each part of the body. The greatest variation, however, is seen in the tentacles. As regards number, we find in the aberrant forms _Protohydra_ and _Microhydra_ tentacles entirely absent. In the curious hydroid _Monobrachium_ a single tentacle is present, and the same is the case in _Clathrozoon_; in _Amphibrachium_ and in _Lar_ (fig. 11, A) the polyp bears two tentacles only. The reduction of the tentacles in all these forms may be correlated with their mode of life, and especially with living in a constant current of water, which brings food-particles always from one direction and renders a complete whorl or circle of tentacles unnecessary. Thus _Microhydra_ lives amongst Bryozoa, and appears to utilize the currents produced by these animals. _Protohydra_ occurs in oyster-banks and _Monobrachium_ also grows on the shells of bivalves, and both these hydroids probably fish in the currents produced by the lamellibranchs. _Amphibrachium_ grows in the tissues of a sponge, _Euplectella_, and protrudes its hydranth into the canal-system of the sponge; and _Lar_ grows on the tubes of the worm _Sabella_. With the exception of these forms, reduced for the most part in correlation with a semi-parasitic mode of life, the tentacles are usually numerous. It is rare to find in the polyp a regular, symmetrical disposition of the tentacles as in the medusa. The primitive number of four in a whorl is seen, however, in _Stauridium_ (fig. 2) and _Cladonema_ (Allman [1], pl. xvii.), and in _Clavatella_ each whorl consists regularly of eight (Allman, _loc. cit._ pl. xviii.). As a rule, however, the number in a whorl is irregular. The tentacles may form a single whorl, or more than one; thus in _Corymorpha_ (fig. 3) and _Tubularia_ (fig. 4) there are two circlets; in _Stauridium_ (fig. 2) several; in _Coryne_ and _Cordylophora_ the tentacles are scattered irregularly over the elongated hydranth. As regards form, the tentacles show a number of types, of which the most important are (1) filiform, _i.e._ cylindrical or tapering from base to extremity, as in _Clava_ (fig. 5); (2) capitate, i.e. knobbed at the extremity, as in _Coryne_ (see Allman, loc. cit. pl. iv.); (3) branched, a rare form in the polyp, but seen in _Cladocoryne_ (see Allman, loc. cit. p. 380, fig. 82). Sometimes more than one type of form is found in the same polyp; in _Pennaria_ and _Stauridium_ (fig. 2) the upper whorls are capitate, the lower filiform. Finally, as regards structure, the tentacles may retain their primitive hollow nature, or become solid by obliteration of the axial cavity.