BODY FLUIDS AND CIRCULATION 99 



PUMPING MECHANISMS 



In order to utilize efficiently the fluids present in body cavities and vessels, 

 some circulatory mechanism is necessary. Circulation of body fluids is 

 accomplished in some animals of small size by the action of cilia, or by 

 muscular movements of the body wall. When definite circulatory systems 

 are present, either of the closed or open type, it is found that a rhythmically 

 contractile pumping device is interpolated for keeping the fluid in constant 

 motion. 



Movement of body fluids is often aided greatly by general muscular 

 activity. Peristaltic burrowing and other somatic movements assist in the 

 circulation of coelomic and vascular fluids in polychaetes, sipunculoids 

 and echinoderms. Ciliary tracts in the body cavities of these animals have 

 a similar effect (17, 88, 142). 



Circulatory pumps can be classified as (a) contractile vessels, (b) tubular 

 hearts, (c) chambered hearts, (d) ampullar accessory hearts. In some 

 animals more than one kind of heart is present to suit particular circula- 

 tory needs. To direct the fluid in one direction while the heart is contract- 

 ing, special mechanisms are necessary and these actually take two forms, 

 mechanical and functional. When the heart is provided with valves at its 

 exit and entrance these prevent retrograde flow of blood while the heart is 

 filling or emptying (diastole and systole). In other hearts contraction takes 

 the form of a progressive wave, driving the contained fluid forward. 

 Frequently both mechanisms are employed. 



Contractile Vessels 



These are widely distributed among invertebrates, and are best known 

 in annelids. In Arenicola blood is pumped forward in the dorsal vessel. 

 The circulation follows the course: dorsal vessel -> gastric plexus -> lateral 

 gastric vessel -> lateral heart -» ventral vessel -> sub-intestinal and affer- 

 ent vessels to body wall, nephridia and gills -> intestinal plexus and 

 vessels -> gastric and dorsal vessels. Contractile vessels are the dorsal 

 vessel, the lateral, oesophageal and some nephridial vessels. The two lateral 

 hearts pump blood into the ventral vessel. Contraction begins in the auricle 

 (a thin-walled expansion of the gastric vessel) and proceeds ventrally in 

 the ventricle towards the ventral vessel. There is no relation between 

 contractions of dorsal vessel and hearts, nor any correspondence in the 

 beating of the two hearts. Usually the heart beats faster and more irregu- 

 larly than the dorsal vessel; the auricular region sometimes contracts 

 several times for each ventricular beat (111). 



The blood vessels of many polychaetes contract rhythmically. The 

 vascular system of Nereis contains a contractile dorsal and non-contractile 

 ventral vessel, from each of which segmentally arranged branches extend to 

 capillary plexuses in the body wall, gut and parapodia. Contraction in the 

 dorsal vessel takes the form of a peristaltic wave which begins in the 

 posterior region and travels anteriorly at about 7 mm/sec (17°C). Many of 



