THE BLOOD AND ITS CIRCULATION. 



465 



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 no't 

 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 amoeba this is accomplished by the movements of 

 the protoplasmic body, whereby the portions Avhich have enwrapped 

 and dissolved food-particles are blend'ed with the less nourished parts. 

 The " contractile vesicles " of the amoeba 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, radi- 

 ating from the imperfect stomach, and uniting with a circular canal 

 at the margin of the body. We may regard this as the earliest devel- 

 opment 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. 



Circulation of the Spider Mtgale Blondii. Fig. 5, The stomach, with its caeca and the 

 remainder of the alimentary canai, with the liver and Malpighian tubes. Fig. 6. Heart and 

 arterial vessels. 



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 



VOL. XIX. 30 



