The tentacles border a broad, flattened peristome, from the centre of which arises the hypostome with the mouth at its extremity; the hypostome is at first low, but soon becomes a projecting, chimney-like tube.
In Cunina and allied genera the actinula, formed in the manner described, has a hypostome of great length, quite disproportionate to the size of the body, and is further endowed with the power of producing buds from a stolon arising from the aboral side of the body.
Thus the body becomes umbrellashaped, the concave side representing the peristome, and the convex side the column, of the polyp. Hence the tentacles are found at the edge of the umbrella, and the hypostome forms usually a projecting tube, with the mouth at the extremity, forming the manubrium or handle of the umbrella.
The ectoderm loses entirely the ciliation which it had in the planula and actinula stages and commonly secretes on its external surface a protective or supporting investment, the perisarc. Contrasting with this, the anthopolyp is generally of s q uat form, the diameter often exceeding the height; the peristome is wide, a hypostome is lacking, and the ectoderm, or so much of it as is exposed, i.e.
The hypostome of the hydropolyp may be small, or, on the other hand, as in Eudendrium (Allman, loc. cit.
The capitulum, with its associated structures, is sometimes called the rostrum, whereas sometimes the term rostrum is restricted to the hypostome alone.
The palpi and probe or hypostome are attached to a movable sclerite or horny plate called the capitulum.
In the latter the single pair of antennae springing up from each side of the camerostome or hypostome or upper lip-lobe are seen.
The parasitic actinula is found attached to the proboscis of the medusa; it thrusts its greatly elongated hypostome into the mouth of the medusa and nourishes itself upon the food in the digestive cavity of its host.
It is by means of the hypostome that ticks pierce the integument and firmly adhere to the host whose blood they suck for food.