Sense organs are but poorly developed in the Nematoda, as is usual in animals which are, as a rule, either parasitic or live underground. Eyes, consisting of masses of dark pigment with or without a lens, occur in the neighbourhood of the circumoesophageal nerve-ring in some free-living forms. Leuckart described as possible auditory organs certain giant-cells lying near the orifice of the excretory ducts. Later research has shown these cells to have some phagocytic action on the contents of the body-cavity. The chief sense organs are the papillae, of which in _A. megalocephala_ there are two kinds, the lip papillae being distinguished from the genital papillae by the fact that the nerve supplying them ends in a fine point and pierces the cuticle in the former case, whilst in the latter it swells out into an "end-organ," which is always covered by a layer of cuticle, though sometimes by a very thin one. MUSCULAR SYSTEM.—The muscular system is one of the most characteristic features of the Nematoda, both as regards the histology of the muscle-cells and the way in which the cells are arranged. Each muscle-cell is of considerable size, and is of the shape of a somewhat flattened spindle produced into a process near the middle. Each end of the spindle cell is said to be continuous with the fibrils of the sub-cuticular layer.[159] The muscle-cell consists of two portions, a contractile part which lies next the sub-cuticle, and which usually, to some extent, wraps round the second or medullary half. The latter consists of a fibrillar spongioplasm, in the meshes of which lies a clear structureless hyaloplasm. The nucleus always lies in the medullary half. {129}The contractile portion consists of a number of columns, very regularly arranged in two rows and close together, but allowing sufficient space between adjacent columns for fibrils of the spongioplasm to penetrate; and these become continuous with the fibrils of the sub-cuticle, which is thus intimately connected with both nervous and muscular systems. The medullary portion of the cell varies greatly in size; it may stretch far into the body-cavity, which may be thereby almost occluded, or it may be flattened out, leaving a large space around the alimentary canal. At one point, usually about its middle, it is produced into a process, which bends inwards towards the dorsal or ventral nerve-cord, and by means of this process the muscle receives its nerve supply. In most Nematodes there are numerous muscle-cells to be seen in any transverse section, forming a layer within the sub-cuticle, and broken up into four quadrants (Fig. 62) by the projection of the dorsal, ventral, and lateral thickenings of the sub-cuticular tissue. In some genera, however, such as _Oxyuris_, _Strongylus_, _Pelodera_, _Leptodera_, etc., there are but eight muscle-cells in a row, two in each quadrant. Such genera are classed together by Schneider,[160] and termed Meromyarii (_vide_ p. 137).