The ultrastructure of the musculature of Lymnaea stagnalis was studied. Each of the six muscle systems of the body wall, previously distinguished in an anatomical study, has its own type of smooth muscle, characterised by the size and number of the myofilaments, number of mitochondria and distribution of smooth endoplasmic reticulum. The visceral musculature comprises both smooth and striated muscle. Cross-striated muscle is found in the heart and proximal aorta, obliquely striated muscle in the buccal mass, gizzard and vas deferens. Myofibroblasts and myoendothelial cells were also distinguished. On the basis of the observations it is concluded that striated muscles in L. stagnalis contract and relax more rapidly and have a higher endurance than smooth muscles, but that the latter can contract over a wider range. Among smooth muscles the head retractor muscle contracts most rapidly, the shell muscle is most powerful and the diagonal muscle is slow to contract but has a relatively high endurance. The latter muscle, together with the horizontal foot muscle, plays a major role in maintenance of the hydrostatic skeleton. A model for the organisation of the smooth muscles is deduced from the ultrastructural observations. It implies that the myosin-paramyosin filaments change their actin filament partners during contraction. This agrees with a model deduced for other smooth muscles on the basis of physiological experiments and X-ray diffraction analysis.

The development of lymphoid and nonlymphoid cells in Peyer's patches (PP) of the rat was investigated using light microscopical methods (routine histological techniques, enzyme histochemistry and immunohistochemistry). In newborn rats PP were mainly populated by T lymphocytes and Ia-positive nonlymphoid cells, which most likely are interdigitating cells. At about 12 days after birth the B and T cells were localized in defined regions, the follicular (FA) and interfollicular area (IFA), respectively. Compartmentalization within the FA started about 14 days after birth. The first signs of the development of secondary follicles were seen from about 18 days onward. PP obtained their mature structure at about 4 weeks after birth. It is suggested that after PP had developed fully, cells having cytoplasmic IgA migrate via the high endothelial venules (HEV) to the lamina propria of the intestine; cIgM and IgG cells seem to develop locally within the FA.

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