100 THE BIOLOGY OF MARINE ANIMALS 



the smaller vessels also are contractile, including blindly ending capillaries 

 serving internal organs. In sabellids nearly all vessels, including trunks and 

 blind capillaries, are contractile; the latter fill up with successive lots of 

 blood passing by in the trunk vessels. In these animals peristalsis in the 

 main longitudinal vessels occasionally reverses direction, indicating 

 absence of fixed functional polarization. The blood vessels of worms con- 

 tract at rather slow frequencies, around 5-20 per min (Table 3.3) (38, 

 41, 66, 96). 



In Amphioxus the heart is a simple tube comparable to sinus venosus 

 plus conus arteriosus, and many other vessels are also contractile. A 

 peristaltic contractile wave begins in the hepatic vein and proceeds to 

 sinus venosus (heart), endostyle artery (ventral aorta), bulbils and afferent 

 branchial arteries. Velocity of propagation is slow, around 0-3 mm/sec. 

 Contractile activity is rather irregular and after several heart beats there 

 may be irregular pauses. In general, the glomus (a buccal plexus) and 

 subintestinal vein beat with twice the frequency of the heart (Table 3.3). 

 Antiperistaltic waves have also been observed (120, 137, 138). 



The heart of brachiopods is a contractile muscular vessel lying above the 

 gut. Rhythmically pulsatile vessels occur in the mantle of the oyster 

 (Ostrea), where they drive blood into the circumpallial artery. Certain of 

 the haemal (lacunar) vessels on the intestine of holothurians are spon- 

 taneously contractile. The beat is rather irregular and slow (2-10 per min) 

 in different species (Table 3.3) (61, 109). 



Chambered Hearts 



Molluscs. These occur in vertebrates and molluscs. In the latter group 

 they range in complexity from the rudimentary heart of scaphopods, 

 connected by ostia with venous sinuses, to the highly developed organs of 

 cephalopods. Typically the heart consists of two auricles (one in certain 

 gastropods, four in tetrabranch cephalopods), opening into a ventricle. 

 Auricles are receiving chambers with slight musculature; the ventricle is 

 more muscular and strongly contractile. Cardiac muscles are sometimes 

 striated (Murex, Octopus, etc.). Guarding the aperture between auricles 

 and ventricles is an A.V. or semilunar valve. In lamellibranchs valves are 

 also present at the origin of the aortae, and prevent blood from being 

 driven backwards into the heart when the foot or siphons suddenly con- 

 tract. Blood is carried away from the heart by anterior and posterior 

 aortae, and is returned by veins, differently organized in the various groups. 



In lamellibranchs and gastropods with open circulations the blood 

 passes into lacunae and venous sinuses. The branchial circulation takes 

 its origin largely from the renal sinus. Not all lamellibranchs have a com- 

 plete branchial circulation; in Mytilus, for example, some of the venous 

 blood returns to the heart without passing through the gills. In cephalo- 

 pods the venous return from the peripheral capillary network involves 

 vena cava, abdominal and pallial veins, which lead into a pair of branchial 

 hearts. These are muscular dilatations serving to pump blood through the 



