126 THE BIOLOGY OF MARINE ANIMALS 



Blood platelets are replaced in lower vertebrates by nucleated spindle 

 cells (thrombocytes). Plasma concentrations are about 2,000 thrombo- 

 cytes/mm 3 of blood (trout). Thrombocytes also show a tendency to 

 agglutinate and disintegrate in shed blood, and probably release a thrombo- 

 plastic factor concerned in the formation of thrombin (22). 



Coagulation can be retarded or prevented in shed vertebrate blood by 

 the use of waxed collecting vessels which retard agglutination and disinte- 

 gration of formed elements, and by the addition of Na citrate or K 

 oxalate, which reduce the Ca ++ ion content of the blood. The erythrocytes 

 of fish blood are comparatively fragile, and are progressively haemolysed 

 by K oxalate; 5 % haemolysis occurs in teleost blood with 0-1 % K oxalate 

 and complete haemolysis with 0-3 %. Clotting is also inhibited by heparin, 

 a naturally occurring polysaccharide (mucoitin-polysulphate-ester) in 

 vertebrates and by hirudin, an anti-coagulant from the leech (10, 144, 

 145). 



Invertebrates. In the majority of invertebrates with definite circulatory 

 systems blood coagulation is effected solely by the formation of a cellular 

 thrombus, and it is only in arthropods (certain crustaceans and insects) 

 that a fibrin-clot is formed. Earlier work dealing with haemostasis in 

 different invertebrates is reviewed in Winterstein's Handbuch der verg- 

 leichenden Physio logie (1925, Bd 1). 



In polychaetes, echinoderms and molluscs coagulation is a cellular 

 process resulting from aggregation of amoebocytes at the wound. In 

 extravasated perivisceral fluid of sea urchins and other echinoderms, the 

 amoebocytes tend to collect and send out long processes which contract 

 and form fibres. Apparently a protein material is liberated from the white 

 cells and this quickly forms a retractile clot. Free calcium and some factor 

 released from injured tissues are necessary for clotting, which can be pre- 

 vented by addition of citrate and oxalate. In molluscs a similar situation 

 is found. Blood corpuscles of Cardium, for example, adhere to foreign 

 surfaces and to one another when blood is collected, and a structureless 

 Plasmodium of agglutinated cells is formed, which closely resembles a 

 fibrin-clot. The cellular agglutinations of gastropods possess little haemo- 

 static value. The haemolymph of Limulus lacks fibrinogen, and clotting is 

 initiated by cellular aggregation. This is followed by the formation of a 

 gelatinous mass or pseudo-clot. Aggregation times are 6-20 sec, and gela- 

 tion times (first appearance of a gel) are 24-55 sec. Clot retraction (synere- 

 sis) subsequently occurs. Gelation is ascribed to the liberation of cell- 

 fibrin (14, 32, 48, 51). 



In crustaceans the situation is more complex. Coagulation in many 

 species is due largely or solely to aggregation of blood cells, and there is 

 no subsequent gelation of the plasma, e.g. Cancer pagurus, Maia squinado, 

 etc. A true fibrin-clot, however, is formed by many species. This is rather 

 weak in Carcinus maenas and Palaemon serratus. Firm fibrin-clots are formed 

 by lobster blood (Palinura, Astacura), isopods (Ligia) and by Gammarus 

 locusta. Gelation of the plasma is said to take place in two stages in the 



