MORPHOLOGY OF ANIMALS 



fluid or plasma within which floating cells or corpuscles 

 are almost invariably present. With the increasing com- 

 plexity of this system the walls become thicker by increase 

 of the muscular or connective tissue coats, and in certain 

 parts of the system the vessels become larger. The mus- 

 cular walls may be pulsatile throughout the entire length 

 of a vessel, or this function may be limited to a small por- 

 tion of a large vessel, which is then known as a heart; 

 even in the highest animals the heart is only a differentia- 

 tion of a simple pulsatile blood vessel. The vessels leading 

 away from the heart are arteries, those through which the 

 blood flows back to the heart are veins, while the small 

 thin-walled vessels connecting these two, and through the 

 walls of which the plasma escapes into the tissues, are 

 capillaries. In annelids there is a large longitudinal vessel 

 on the dorsal side and another on the ventral side of the 

 body, which are connected in each somite by commissural 

 vessels. The dorsal vessel is pulsatile along its whole 

 length, and peristaltic contraction waves can be seen in 

 a living worm to pass from its posterior to its anterior 

 end ; correspondingly the blood flows forward in the dor- 

 sal vessel, down through the commissural vessels into the 

 ventral one, and then backward through the latter to the 

 posterior portion of the body, where the blood ascends 

 through the commissural vessels to the dorsal vessel, after 

 which the same circuit is repeated. Throughout this whole 

 course the blood flows through vessels with definite walls, 

 and therefore the circulation is said to be closed. 



In mollusks and arthropods a heart is present which is 

 more complicated and more complete than in annelids. In 



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