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In the Hydroids, the organization does not rise above the simple digestive cavity inclosed by the double body-wall; and we might not suspect their relation to the Acalephs, did we not see the Jelly-Fish born from the Hydroid stock.
The excretory system consists of a pair of tubes discharging through the sides of the body-wall, and having each a ciliated, funnel-shaped opening in the perivisceral cavity. These have received the name of nephridia. Through these also the eggs and spermatozoa are discharged. The reproductive organs are modified patches of the peritoneum, or lining of the perivisceral cavity.
Through these the air passes into a system of air-tubes, the tracheæ. There are two main trunks or divisions of the tracheæ just inside the body-wall and a number of shorter connecting trunks.
But it is slower in reaching its full development than the reproductive system. But all the muscles are no longer attached to the stomach; they are beginning to assert their independence, and, in a rude way, to build a body-wall. But they are in many layers, and run in almost all directions.
Between the external transverse and the internal longitudinal layers we often find two muscular layers whose fibres run diagonally. The body is well provided with muscles, but their arrangement is still far from economical or effective. Within the body-wall is the parenchym. This is a spongy mass of connectile tissue in which the other organs are embedded.
When the body-wall contracts the air is forced out of the thin-walled trachea through the spiracles; when the pressure is removed they are refilled by the fresh air rushing in. After a mosquito has been feeding on a man or some other animal it is often so distended that the blood shows rich and red through the thin sides of the walls of the abdomen.
Within the muscles is the perivisceral cavity, and in its central axis the intestine, segmented like the body-wall. The reproductive organs are formed from patches of the lining of the perivisceral cavity, and the reproductive elements, when fully developed, fall into the perivisceral fluid and are carried out by nephridia, just such as we found in the schematic worm.
At a very early stage the gullet-wall joins with the external body-wall in the head of the human embryo, and this is followed by the formation of four clefts, which lead directly into the gullet from without, on the right and left sides of the neck, behind the mouth. These are most interesting embryonic structures.
Other vessels grow into the body-wall and ramify in order to convey blood to it. In addition to the two large vessels of the middle plane there are often two lateral vessels, one to the right and one to the left; as, for instance, in the leech.
But they all agree in certain characters; all take their food and oxygen and carry on excretory processes by osmosis, i.e., through the body-wall; all are capable of some kind of locomotion, some have one or more flagella, others move by a pseudopod movement. Some are capable of moving from cell to cell in the body as do the white blood-corpuscles.
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