106 THE BIOLOGY OF MARINE ANIMALS 



ing a blood volume of 75 c.c. and a cardiac rate of 100 beats per min can 

 turn over its entire blood volume in 3-8 min. The heart is the main pump- 

 ing mechanism in decapod crustaceans, but body movements assist to some 

 extent in propelling blood through the gills and back to the heart (20). 



In solitary ascidians such as Ciona the heart is a simple /^-shaped tube. 

 The walls are composed of curious muscle fibres, differentiated into an 



Fig. 3.10. Myogram of the Heart of Palinurus, Stimu ed with 

 Induction Shocks at Various Stages of the Cardi Cycle 



Stimulus ineffective at the beginning of systole (a), but effective during diastole 

 (b, c, d). (From Carlson, 1906.) 



inner striated, and an outer sarcoplasmic portion. There is no endothelium. 

 Periodically, the heart reverses the direction of its beat, at the same time 

 reversing the course of circulation about the body. Alternation of direction 

 of heart beat is characteristic of all tunicates — salps, pyrosomae and 

 ascidians. Blood pressure is very low, around 2 mm Hg in Ascidia (36, 90). 



Ampullar Hearts 



Accessory devices for propelling blood through peripheral channels 

 sometimes take the form of contractile ampullae. In cephalopods the 

 branchial hearts are booster devices which drive systemic venous blood 

 through the gills towards the systemic heart. The walls consist of spongy 

 tissue lined with faintly striated endothelial cells; exit and entrance are 

 guarded by sets of valves. The two branchial hearts contract simultane- 

 ously and rhythmically (121, 127). 



In lancelets there a re small contractile bulbils at the bases of the gill 

 bars. Lymph vessels of fishes sometimes bear contractile lymph hearts 

 which drive lymph into the veins. In the tail of Angui/Ia, for example, 

 there is a lymph heart which opens into the caudal vein. Lymph hearts are 

 composed of striped anastomosing muscle fibres, and are provided with 

 valves to prevent reflex of blood or lymph. Evidence exists that activity of 

 the lymph heart is controlled by the c.n.s. (central nervous system): it 

 stops beating when the spinal cord is destroyed, and its frequency is 

 altered by stimulation of the cord. 



Heart Rates. Some representative data on heart rates of different 

 animals are presented in Table 3.3. In general the frequencies of con- 

 tractile vessels in such animals as polychaetes and holothurians are rather 

 low, often less than 10 per min. The hearts of sluggish animals beat at 

 slower rates than those of more active forms (e.g. lamellibranchs versus 

 cephalopods). Heart rates are related to general metabolism and are an 



