The amoebocytic corpuscles in the circulating fluid of the lamelli-branches, Indonaia caerulia and Parreysia (Parreysia) favidens.

The circulating amoebocytes of the freshwater mussels, Indonaia caerulia and Parreysia favidens have been studied. The total number of amoebocytes is low and variable, and the individual to individual variation in total counts is particularly significant. The amoebocytes are of 3 main types: Acidophils, Large Basophils and Small Basophils. Acidophils contain large acidophilic granules and basophils fine basophilic granules within the cytoplasm. Acidophils are the largest and small basophils the smallest. The greater volume of the cell is occupied by cytoplasm in acidophils and large basophils and by the nucleus in small basophils. Large basophils are the most and small basophils the least abundant. Amoebocytes clump together by means of filipodia. Large basophils possess these protoplasmic processes in the greatest abundance, and partake most actively and in largest numbers in clumping; while small basophils lack filipodia and play an absolutely passive role. Clumping progressively becomes more extensive with passage of time following cardiac injury inflicted during blood collection. I. caerulia appears to be generally more efficient than P. favidens with regard to clumping reaction. The different structural, numerical and behavioural aspects of the amoebocytes have been considered with reference to their probable causative factors and functional import.