Go to National Library of New Zealand Te Puna Mātauranga o Aotearoa
Volume 70, 1940-41
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The Mesoderm.

The majority of the mesoderm, excluding the muscles, consists of a highly vacuolated storage tissue, rather similar in appearance to the fatty tissue of higher animals. The cells are large and their boundaries are indistinct, but the nucleus appears to be placed at one edge. In sections these cells always appear as vacuoles. Running through them is a system of thin-walled canals, which seem to communicate with the enteron, and can be traced by the injection of the gut by a pipette. Only by these means can these canals be distinguished from storage cell vacuoles, but they are quite different in appearance from the excretory canals, which are surrounded by an epithelium of ciliated cells.

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The muscles and their non-contractile terminal portions occupy a considerable portion of the space between ectoderm and endoderm. The muscle is made up of several fibres, the terminal ones being non-contractile and drawn out into long thread-like processes which mingle with others from associated muscles. The contractile portions are elongated and twisted on themselves like strands of a rope. The muscles may be classified into the following groups:—

1. Circular, lying just below the epidermis. These are oval in cross-section and are finer than other types. The non-contractile portions curve down and interlock with those of the next group, to lie just below the contractile portions.

2. Longitudinal, lying just below the circular muscles, their non-contractile portions mingling with those of the latter. There are generally four to six layers, and they are exceedingly thick on the ventral surface.

3. Oblique muscles form an open network through the body, running from the region of the circular muscles to that of the muscles surrounding the enteron.

4. Enteric group of muscles surround the diverticula of the enteron, and appear to facilitate the circulation of fluids.

These muscles groups can best be distinguished by polarised light, in which the systems show up one by one as an unstained section is examined.

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