148 HISTOLOGY 



invagination is taken in 133, F, by small additional extensions from 

 the three remaining invaginations. 



Such is the fundamental idea of the circulatory system and its rela- 

 tions to the system of invaginations of the original surface of the 

 animal body. We shall now consider it apart from its relations to 

 the differentiation of the surface functions and merely as an agent of 

 distribution. 



The circulatory channels appear in their simplest form, both taxi- 

 nomically and ontogenetically, as one or more irregular spaces in the 

 inner or mesodermal regions of the animal body. These spaces have 

 no boundaries other than the mesodermal cells among which they lie, 

 and some of these cells become detached and float freely in the lymph 

 fluid that fills the lumen. Later, most of these mesodermal cells be- 

 come specialized into connective-tissue cells, while many of them, in 

 the neighborhood of the blood vessels, become modified to form 

 the specialized walls of these spaces. Those in the fluid of the lumen 

 may become blood corpuscles or they may also join in forming the 

 walls. At such a period the circulatory space is not to be distin- 

 guished from other body cavities that may afterwards become separate 

 or partly separate from it. 



A low form of a specialization of this primary circulatory space is its 

 enlargement into one or more long, continuous cavities extending the 

 length of the body and into the limb or appendages. The blood is driven 

 about in this set of channels by the movements of the general muscula- 

 ture of the body, and in some cases it exhibits a special rhythmic move- 

 ment, passing first toward the anterior part of the body and then in the 

 opposite direction. This cavity also, since it occupies the greater part of 

 the body and contains the organs, must be looked upon as the body-cavity 

 or ccelom. 



This development, in some animals, consists of the growth of a part 

 of the early ccelomic cavity into a long, tube-like channel with many 

 branches and a more or less definite wall. In this tube, which may be a 

 closed circuit or only a partial circuit, the blood acquires a continuous 

 movement, being pumped through the system usually in one direction. 

 At this point in its history the blood -channel system is usually separated 

 more or less completely from a remaining portion of the ccelom, which 

 we shall call the body-cavity. Many other differentiations and separa- 

 tions from these cavities occur, as the cavities in connection with the hearts 

 and the nephridial organs, the secondary reproductive organs, and the 

 lymphatic system and its various modifications. In the higher animals 

 the blood -channel system arises de novo as a series of clefts in mesodermal 

 tissue. There may be different regions of a circulatory system which are 

 separate and contain different kinds of circulating media. 



