Results

The Epidermis and "Basement Membrane": The epidermal "channels" (Wineera, 1969) are well shown by phase contrast microscopy of frozen sections (Pl. 1 Fig. 1.EC). They are seen to extend, in many cases, to the bases of epidermal cells, and often appear continuous with small "cavities" situated between the bases of epidermal cells (Pl. 1, Fig. 1, arrow). These cavities are more clearly seen in thin (0.5μ) osmium fixed sections stained with MBAII (Pl. 1 Fig. 2, C). The cells through which subepidermal eosinophil glands open to the surface are devoid of cilia (Pl. 1, Fig. 2, A), but this is not the case in the cells through which subepidermal basiphil glands open.

IN MBAII stained sections, the bases of epidermal cells can be traced by a thin dense line (Pl. 1, Fig. 2, Arrow). Below this line is material of the "basement membrane". Small vacuoles, or spaces are common within this latter structure, as are sections of the ducts of subepidermal glands.

The Muscles: The longitudinal muscle layers differ in size at the dorsal and ventral surfaces. At the dorsal surface the muscle fibres constitute a layer from 5-7μ thick. At the ventral surface this layer is thicker often approaching 25μ. Both in section (Pl. 2, Fig. 1, C) and as seen by dissociation preparations (Pl. 2, Figs. 2 and 3, C) the cell body lies up to away from the contracticle part of the muscle cell, and is joined to it by a thin cytoplasmic connection. In osmium fixed MBAII stained sections the muscle cell body colours a purplish blue while the contractile part colours a light blue grey. The nucleus is often irregularly shaped and contains a characteristic pattern of densely staining chromatin. This pattern is made up of relatively large chromatin clumps situated at regular intervals around the periphery of the nucleus, together with chromatin clumps of similar size in the interior of the nucleus. From circular muscle fibres seen in transverse section, and from dorso-ventral muscle fibres which approach the "basement membrane", fine processes (Pl. 1, Fig. 3, arrow; Pl. 1, Fig. 4, arrow), appear to connect with the "basement membrane" and with other muscle fibres. Muscle cells up to 200μ in length have been seen. Fine processes can sometimes be observed along the length of the muscle cells (Pl. 2, Fig. 2, arrows). The Parenchyma: In the parenchyma the cells fit very closely together. In 0.25μ thick sections many vacuoles of varying size are present. None of these vacuoles can definitely be identified as being extracellular. Also, connective tissue fibres cannot be seen between cells in these thin sections. As well as containing neoblast cells (Pedersen, 1959, 1961a; Wineera, 1969) the parenchyma contains largish cells 10-12μ in diameter. In osmium fixed sections stained with MBAII these cells have cytoplasm page 4 which is partly or wholly composed of moderately dense cytoplasmic strands. The strands are parallel to each other, and are arranged in curvilinear patterns (Pl. 3, Fig. 1, arrows). Other cells of similar size and staining reaction contain numerous vacuoles or pale granules (Pl. 3, Fig. 1, G) and yet others are seen in which both vacuoles and stranded cytoplasm are present. Pale staining large nuclei are present in the parenchyma but it is not possible to identify the limits of the cells to which they belong. Neoblast cells are visible in dissociation preparations, but fixed parenchyma cells (Pedersen, 1961a) have not been positively identified.

The Subepidermal Glands: Basiphil subepidermal glands are easily seen in osmium fixed MBAII stained sections. The cytoplasm is dark blue to purplish blue in colour. These glands open to the surface through the epithelial cells. In the cephalic and caudal regions of the animal the basiphil secretion granules often occur as aggregates in sac like structures within the epidermis.

Eosinophil subepidermal glands are either not stained by the above technique, or are stained a very pale greyish colour. Sometimes they colour with a faint yellowish tinge. In 0.5μ sections their secretion is seen to be made of discrete closely packed granules approximately 1.0μ in diameter. In the epidermis the granules colour a little more deeply, but do not approach the staining intensity of basiphil glands.

Pigment: In all dissociation preparations pigment granules are located in large rounded cells (Pl. 2, Figs. 4A & B, P). They may be few or many in number and may be scattered irregularly throughout the cytoplasm of these cells but often occur in groups several of which may be present in any one pigment cell. In incompletely dissociated cell masses from the dorsal region of the animal, pigment granules occur densely between muscle cells (Pl. 2, Fig 5). In these conditions pigment cell boundaries can not be discerned, but groups of pigment granules may be seen (Pl. 2, Fig 5, arrow).