122 THE BIOLOGY OF MARINE ANIMALS 



or of other ions can be osmotically balanced by addition of some neutral 

 substance such as sucrose. Replacement of all Na + by sucrose usually 

 results in reduction of heart rate, and in these circumstances there is prob- 

 ably some loss of electrolytes from the cells. Na + generally has a stimula- 

 tive action on pacemakers and contractility. In both molluscs and arthro- 

 pods K> appears to stimulate the pacemakers (myal and neural, respec- 

 tively). Ca^ '■ ion may affect the contractile process as well as the pace- 

 maker, the exact effect differing in various groups. Mg ++ appears to be 

 essential for rhythmic beating in only certain groups, especially gastropods 

 and lamellibranchs. In these animals alterations of Ca + + and Mg ++ may 

 have analogous effects, but the ions are not interchangeable. In general, 

 it appears that the nervous pacemaker of the arthropod heart is more 

 sensitive to changes in the K + /Ca ++ ratio than the myogenic molluscan 

 heart. Changes from normal in the hydrogen ion concentration of the bath- 

 ing fluid are also deleterious to continued functioning, and perfusates are 

 usually buffered to normal pH to secure optimal activity. Ionic effects 

 have been less studied in fish heart than in invertebrates and higher chor- 

 dates and are deserving of more attention. 



BLOODS CELLS 



The blood, haemolymph and coelomic fluids of animals usually contain 

 formed elements of various kinds. These are chromocytes and leucocytes, 

 the latter including amoebocytes, lymphocytes, phagocytes, thrombocytes, 

 thigmocytes, motile cells etc. In the accompanying table (3.4) are given 

 some figures for cell counts of certain animals, and data for erythrocytes 

 are summarized in Table 4.9. Probably the most striking feature of these 

 figures is the variation, amounting in some species to 50 % or more (16, 99). 



There is no doubt that corpuscular numbers fluctuate greatly under 

 various conditions : during growth, seasonally, in response to stress, disease, 

 infection and with short- and long-term cycles of organic activity. The 

 figures for Limu/us, young and adult, are illustrative of ontogenetic 

 changes. Maluf (83) relates the large variation in corpuscular numbers in 

 crustaceans to fluctuations in blood volume, which is known to alter during 

 the moult cycle (p. 48). Seasonal changes in erythrocyte numbers do take 

 place in certain teleosts (69). 



Chromocytes are coloured blood cells often containing respiratory 

 blood pigments. Erythrocytes containing haemoglobin are found in vascu- 

 lar and coelomic fluids of many animals (Table 4.8). Certain cells in the 

 coelomic fluid of echinoids (elaeocytes) contain naphthoquinone pigments. 

 The blood of pycnogonids (Anoplodactylus) contains numerous nucleated 

 corpuscles, pink to purple in colour, of unknown function. Vanadium 

 chromogens are enclosed in blood corpuscles (vanadocytes) in certain 

 ascidians (Table 4.11). The role of respiratory pigments in oxygen trans- 

 port is described in the following chapter (p. 172) (47, 80, 132). 



Phagocytic amoebocytes are of common occurrence in blood and coelo- 

 mic fluids of animals. Ever since Metchnikoff discovered phagocytes in 



