BODY FLUIDS AND CIRCULATION 123 



the body fluids of starfish these cells have been recognized as playing an 

 important role in the phagocytic ingestion and elimination of foreign 

 particles, including bacteria. In the perivisceral fluids of echinoderms 

 (Arbacia) there are amoeboid and flagellated phagocytes which take up 

 particulate matter. Three categories of leucocytes are described in the 

 haemolymph of Crustacea. These are explosive corpuscles forming 40-50% 

 of the total; thigmocytes forming 20-30%; and amoebocytes forming the 

 remainder. The thigmocytes, which produce fine processes in shed blood, 

 are mainly responsible for phagocytosis, and ingest particulate matter such 

 as India ink; the amoebocytes are less phagocytic. In lamellibranchs 

 amoebocytes are abundant and are distributed throughout the body, being 

 especially common about the gut. These cells readily take up indigestible 

 particulate matter, such as India ink, carmine granules, etc., injected into 

 the blood stream. Eventually the particle-laden amoebocytes migrate 

 through epithelial layers of the alimentary tract, mantle, pericardium and 

 elsewhere, where they are voided together with their enclosed material. 

 An equally important role of amoebocytes in lamellibranchs is digestion, 

 since these animals do not produce an extracellular protease or lipase. 

 Amoebocytes in the gut take up particulate food material, which is broken 

 down by intracellular enzymes (amylases, proteases and lipases), and made 

 available to the organism in assimilable form (48, 79, 123, 125, 140). 



Peculiar corpuscular elements occur in the body fluids of certain groups. 

 The body fluids of brachiopods and phoronids contain large anucleate 

 bodies termed spindle cells. These are thought to be metamorphosed 

 derivatives of blood corpuscles. Perhaps the most peculiar blood inclusions 

 are small ciliated urns found in the coelomic fluid of sipunculoid worms. 

 Their function is unknown. 



We have touched upon the more important functions of blood cells but 

 they undoubtedly participate in many other metabolic activities. In the 

 succeeding section the functions of amoebocytes in haemostasis are des- 

 cribed, namely closing the wound and initiating blood clotting. 



Blood Coagulation and Haemostasis 



Among animals with circulatory systems, defence mechanisms exist for 

 minimizing loss of blood when a blood vessel or sinus is perforated. In 

 more primitive phyla the ionic composition of the blood is not greatly 

 dissimilar from sea water and the content of organic matter is small. In 

 such animals the loss of relatively large volumes of body fluids is probably 

 not serious metabolically, although there will be interference with the 

 efficiency of hydraulic mechanisms. In higher forms, however, in which 

 the blood constitutes a specialized fluid markedly dissimilar from sea 

 water, the maintenance of a stable internal medium is essential for the 

 continued functioning of the tissues of the body. In these animals con- 

 tinued haemorrhage is disastrous, because of the slump of blood pressure 

 and interference with the metabolic exchanges taking place between the 

 tissue cells and body fluids. 



