From what has already been stated as the purpose of circulation, we should not expect to find any circulation in those animals which are destitute of a separate digestive cavity; and in such we do not discover a true blood-circulation. But it would appear that no animal is entirely without the power of distributing its food to the parts of the body. In the amœba this is accomplished by the movements of the protoplasmic body, whereby the portions which have enwrapped and dissolved food-particles are blended with the less nourished parts. The "contractile vesicles" of the amœba may also have to do with the distribution of nourishment, though they are usually regarded as respiratory or excretory in function.
Next to this, in simplicity, is the prolongation of the digestive cavity for the distribution of food. This is found in various animals of different classes. The jelly-fish has a system of four canals, radiating from the imperfect stomach, and uniting with a circular canal at the margin of the body. We may regard this as the earliest development of organs for conveying nutriment. A similar condition exists in the anemone; and spiders have prolongations of the stomach in addition to their circulating organs.
| Fig. 5. | Fig. 6. |
True circulation is found only with a complete separation of the digestive cavity from the visceral or general body cavity. In many invertebrates there is simply a flux and reflux of nutritive fluid in this visceral cavity, but no special circulating vessels. This is the condition in the bryozoa, the lowest of mollusks, in the rotifera, and in the larvae of certain myriapods and insects. In these the fluid is more the nature of chyle, and is called the chylaqueous fluid.
The "circulating system is gradually developed as an offset of the visceral cavity." This is shown by the low ascidian mollusks, in
