Lysosomes in digestive gland of mussels maintained in 18◦C sea-water for 24 h appeared as sparse and small cytoplasmic vesicles reactive for GUSB. Instead, after maintenance in 26◦C seawa-ter reactive lysosomes were more conspicuous (Fig. 2B and F).Likewise, lysosomes were more evident at 26◦C than at 18◦Cin Cd exposed mussels (Fig. 2D and H). Lysosomal enlargementwas accompanied by evident thinning of the digestive epithelium(Fig. 2C and G).