132 THE VASCULAR SYSTEM 



These form the so-called blood glands, and in them the blood is 

 renovated by the removal of some of its old parts and the addition 

 of other new ones or both." 



" The internal tissue of a Turbellarian worm is a loose aggregate 

 of several kinds of weakly differentiated cells, known as parenchyme. 

 These cells do not touch each other at all points, but are connected by 

 strands, and in consequence there may be easily seen between them a 

 great many spaces, known as the intracellular spaces, which are united 

 into a large connecting system that extends throughout the body. This 

 system of spaces is filled with a fluid, and this fluid carries the digested 

 food materials, the oxygen supply for internal cells, the combustion 

 products, and in every other way acts as a simple blood. This is the 

 undifferentiated and unorganised form of blood-vessel system, and a 

 sort of circulation must inevitably take place as a result of the ordinary 

 movements of the animaVs body. This grade of structure is to be 

 seen in a number of the lower and simpler animal forms and some- 

 times as an accessory apparatus to several grades of complete blood- 

 channel systems." In the several typical forms studied by Dahlgren 

 and Kepner, the walls of the blood channels show a strong analogy 

 based on the physiological (which are here mechanical) needs of 

 the vessels. The blood fluid must be confined to the channels, 

 and this is usually done by the single inner layer of cells, the intima. 

 In some forms the intima is formed not by the cells themselves 

 but by a cuticle which is the product of these cells (lobster, &c.). 

 " The intima may alone confine the blood stream, or if the pressure 

 is too great, it may be reinforced by the connective tissue cells 

 that immediately surround it. These cells develop their connective 

 tissue as fibrils or plates or webs with which they bind and hold the 

 vessel intact when the blood presses on its walls. Again, these 

 primitive mesoderm cells may develop into muscle cells that sur- 

 round the channel and by their contractile strength cause it to 

 pulsate and drive the blood on its course. The arrangement of 

 these three classes of tissues to form the wall of the vessel falls, 

 naturally, into layers, the so-called coats of the blood vessels. 

 Each kind of coat usually has a particular position with reference to 

 the lumen. This position, however, is sometimes changed in the several 

 groups for no apparent reason. All these cells and the tissues that 

 they form were probably not cells that were bound in the course 

 of their development to become so specialised, but as far as can 

 be told, they were such of the connective tissue cells as happened 



