The polypites which digest the food are vermiform double sacs communicating at one end by a valve with the canal in the body of the animal; and at the free end they are prolonged into a mouth with an everted lip, and the digesting apparatus lies in the centre. Each polypite is supplied with food by its own fishing-line descending from a point close to where the polypite is fixed to the long axis. It is a long, tubular, branched tentacle, each branch ending in a coloured, pear-shaped, or fusiform battery of thread-cells with their stings. A gelatinous plate is placed on the upper side of the common axis immediately over the isolated groups, to protect and separate them. Such are some of the most general characters of the family Diphyidæ: the Praya diphys has something peculiar to itself. In the Praya, each individual group has a swimming-bell of its own adjacent to the polypite, and lying parallel to the axis of the animal, with its mouth turned backwards. It is connected by tubes both with the general central canal, and with a helmet-shaped protecting plate. On the other side of the polypite, there is a tuft of vermiform buds with spiral terminations, bristled with thread-cells. From the centre of this tuft a tentacle, or fishing-line, descends with numerous branches, the whole forming a tubular system connected with the common canal in the axis. Each of the branches of the tentacle terminates in a vermilion-coloured tendril, coiled up into a minute capsule. The inside of the tendril is not only bristled with the points of sabre-shaped darts, but it conceals a filament crowded with thread-cells. On the slightest touch, the tendril stretches out like a corkscrew of red coral, and every dart springs forth. Such is, more or less, the complicated structure of the offensive and defensive weapons of many of this order of oceanic Hydrozoa, which appear to the naked eye as merely brightly-coloured points. The use of these tentacles, or fishing-lines, is the same in all; they seize, kill, and carry their victims to the mouth of the polypite by contracting their long lines. In the Praya, the groups are individualized in the highest degree consistent with union; for, when the animal is at rest, each of the individual groups, amounting to thirty or forty, swims about by means of its little bell independent of the rest. Their motions can be compared to nothing but a troop of jugglers performing gymnastic exercises round a cord represented by the common body of the animal; except for adherence to which the life and will of each group are so perfectly independent, that the mutual dependence of the whole is only seen when the common trunk contracts to bring all its appendages towards the two principal bells, which then begin to move.[25]