CIRCULATORY CHANNELS 



several kinds of weakly differentiated cells, known as a 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 intercellular spaces, which are united 

 into a large connecting system that extends throughout the body 

 (Fig. 134). 



This system of spaces is filled 

 with a fluid and this fluid carries 

 the digested food materials, the oxy- 

 gen supply for internal cells, the 

 combustion products, and in every 

 other way acts as a simple blood. 

 This is the undifferentiated and un- 

 organized form of blood-vessel sys- 

 tem, and a sort of circulation must 

 inevitably take place as a result of 

 the ordinary movements of the ani- 

 mal's body. This grade of structure 

 is to be seen in a number of the 

 lower and simpler animal forms and 

 sometimes as an accessory apparatus 

 to several grades of complete blood- 

 channel systems. 



Structure of the Blood-Vessel Walls in the Nemertean Worm, Cere- 

 bratulus. The location of the blood vessels is a morphological matter. 

 They can be found for our study in the connective tissue around the di- 

 gestive tract, particularly a large thick- walled vessel between the oesopha- 

 gus and the rhynchoccel. A branch of this that runs circularly around 

 the digestive tract will be studied in longitudinal sections (Fig. 135). 

 This vessel is lined with an endothelial layer of cells that are very thin 

 and delicate and can only be observed to advantage in well-hardened 

 material. Otherwise one is liable to confound their nuclei with those of 

 the blood corpuscles. When the vessel is contracted, as it is in many 

 fixations, this layer is thrown into longitudinal folds, and the nuclei 

 usually lie in that part of the fold that projects into the lumen. The 

 individual cells are elongate, as are also their nuclei, and the reticulum of 

 cytoplasm has longitudinal, drawn-out meshes that give the cell body a 

 striated appearance in the section we are examining. 



The endothelium rests upon a very weak basement membrane of 

 so little substance that it seems to be a mere boundary in the thinner 

 vessels, but of appreciable thickness in the larger arteries. In some fixa- 

 tions it seems to be radially striated, but this appearance is probably due 

 to fine longitudinal folds and the contact of the endothelium. Directly 



FIG. 134. Body tissue from a small flat- 

 worm to show the intercellular clefts (ch.) 

 which act as circulatory channels. X 700. 



