Circulation of Body Fluids 545 



of Limidus contracts nearly simultaneously."- A wave of contraction occurs 

 in some insect tubular hearts. 



Pulsating vessels. Blood vessels which contract with peristaltic waves are 

 widely distributed. In annelids many blood vessels show rhythmic peristaltic 

 waves. In the earthworm the contraction waves in the dorsal vessel pass from 

 the posterior to the anterior end at a speed of about 20 millimeters per second 

 and waves are repeated at the rate of about 15-20 per minute."*-^ The lateral 

 vessels of the earthworm, usually referred to as the hearts, contract at rates 

 differing from that of the dorsal vessel and each heart beats at its own rate, 

 although the two hearts of a segment tend to contract together. In the leech, 

 instead of a dorsal vessel, there are two lateral vessels which contract alter- 

 nately. In Nereis the dorsal vessel contracts rhythmically at a somewhat 

 slower rate than that in the earthworm. The circulation in Arenicola has 

 been extensively studied. '^'*' '*• ^'**^ The blood flows forward in the dorsal ves- 

 sel, passing on each side by the gastric plexus to the lateral gastric vessel, 

 and thence to the lateral heart, which pumps to the ventral vessel. The area 

 connecting the lateral gastric vessel to the heart beats before the heart, and 

 sometimes beats several times during a single contraction of the heart. The 

 dorsal vessel, and the lateral, esophageal, and some nephridial vessels are 

 contractile; the ventral vessel and posterior gastric vessels are not. In general, 

 the contractions of the annelid pulsating vessels are not as regular as the 

 beats of chambered hearts. 



The distinction between tubular hearts and contractile vessels breaks 

 down for some insects, particularly dipteran larvae. In Amphioxus many ves- 

 sels are contractile and the wave of contraction spreads slowly; the heart is 

 little more than "sinus venosus" and "conus arteriosus."^^*' 



Ampidlar Hearts. In several animal groups there are accessory organs for 

 propelling blood through peripheral channels. The branchial hearts of the 

 cephalopods consist of a spongy tissue of faintly striated endothelial cells 

 surrounding many small vessels.^^^ These hearts are- interposed between the 

 systemic veins and the gill arteries. 



Among insects accessory hearts are very common, particularly at the base 

 of antennae, at the attachment of wings, and in legs.^-^^ Insect accessory 

 hearts consist of muscle fibers arranged in various patterns and serve to keep 

 the hemolymph moving in the open circulation. 



Fishes, amphibians, and reptiles have lymph hearts which are contractile 

 enlargements of lymph vessels and which tend to force lymph into the veins. 

 Lymph enters veins at many points and not through thoracic ducts as in mam- 

 mals and birds. The lymph hearts are composed of striated, non-anastomo- 

 sing fibers and may have valves which prevent a backflow of lymph. In some 

 amphibians destruction of all the lymph hearts is fatal, probably because 

 of accumulation of lymph in tissue spaces. ^''^ 



The myxinoid cyclostomes have several vein hearts which are valved but 

 not contractile; a lateral cartilage can be pushed against the vein by muscles 

 controlled by the spinal cord."'' 



Pacemakers, Cardiac rhythms, unlike respiratory rhythms, usually origi- 

 nate within the active organ itself rather than in extrinsic nerve centers. 

 The nature and location of the pacemaker tissues differ in various groups 



