Anatomy.—The body of the mature forms can usually be divided into three sections—the proboscis, the neck, and the trunk, but the middle region is not always discernible. The proboscis is armed with rings of hooks (Fig. 93) arranged in longitudinal rows; they are usually of two kinds, but in E. proteus of three. They have a certain specific value, but not much stress can be laid on the number of rings, e.g. in E. angustatus the number varies from eight to twenty-four. The recurved hooks serve to fasten the parasite very firmly to the tissues of the host. The proboscis is hollow and retractile; it can be withdrawn into the body by means of muscles attached internally to its tip. It does not, however, pass straight into the body-cavity, but is retracted into a special cavity—the proboscis sheath—with a double muscular wall. The proboscis sheath may perhaps be looked upon as a septum, such as is found in some of the Nematomorpha, dividing the body-cavity into two parts. It is inserted into the body-wall at the junction of the neck and trunk or of the proboscis and trunk. In addition to the muscles which withdraw the proboscis into its sheath, there are two retractors running from the outside of the sheath to the body-wall; these serve to retract the whole sheath and its contents into the body-cavity of the trunk.
The structure of the skin is essentially like that of Nematodes, but the details are much more complicated. The whole body is covered by a thin cuticle secreted by the epidermis, which, as in the other groups, breaks down and forms a syncytium called the sub-cuticle. The minute fibrils which penetrate this layer are much more definitely arranged than in Nematodes; the largest of them run from without inwards, others run concentrically round the body. Large oval or spherical nuclei are scattered in the sub-cuticle, which is further honeycombed by a number of lacunae or spaces which are described below.
Fig. 93.—A, Five specimens of Echinorhynchus acus Rud. attached to a piece of intestinal wall, × 4; B, the proboscis of one still more highly magnified.
Within the sub-cuticular layer is found a sheath of circularly-arranged muscle-fibres, and within this again a sheath of longitudinal muscles which do not extend into the proboscis; this inner layer lines the body-cavity, there being no epithelium within it. In their minute structure the muscle-cells resemble those of Nematodes.
The canals in the sub-cuticle form a very curious system of anastomosing spaces, in which a clear fluid containing fat globules circulates. The extent to which the system is developed varies in different species, but in all there is a pair of longitudinal canals which are situated laterally, and which give off the subsidiary channels in their course. The above description applies to the lacunar spaces in the skin of the trunk; those of the proboscis are quite distinct, and there is no communication between the two sets of spaces; in fact, the sub-cuticle in which the lacunae are formed is not continuous across the line of junction of the proboscis and the neck, or, when the latter is absent, of the proboscis and the trunk, but it is interrupted by the ingrowth of a thin ring of cuticle which reaches down to the muscular layers (Fig. 94).
Fig. 94.—A longitudinal section through the anterior end of Echinorhynchus haeruca Rud. (From Hamann.) a, The proboscis not fully expanded; b, proboscis-sheath; c, retractor muscles of the proboscis; d, cerebral ganglion; e, retinaculum enclosing a nerve; f, one of the retractors of the sheath; g, a lemniscus; h, one of the spaces in the sub-cuticular tissue; i, longitudinal muscular layer; j, circular muscular layer; k, line of division between the sub-cuticular tissue of the trunk and that of the proboscis with the lemnisci.
All the spaces in the skin of the proboscis open ultimately into a circular canal situated round its base; on each side the canal opens into a sac-like structure which extends through the body-cavity towards the posterior end of the animal. These two lateral diverticula are termed the lemnisci. They have always attracted considerable attention from the workers at the group, and numerous functions have from time to time been attributed to them. They are more or less hollow, and their walls consist of sub-cuticular tissue surrounded with a scanty muscular coat; they contain the same fluid as the lacunae of the skin of the proboscis, with which they are placed in communication by means of the circular canal; and it seems most probable that, as Hamann[[216]] suggests, they act as reservoirs into which the lacunar fluid retires when the proboscis is retracted, and which, by means of the contractions of their muscular coat, force the fluid into the lacunae when the proboscis is everted, and thus aid in its protrusion.
The parasitic habits of Echinorhynchus have had a deeper influence on the structure of the body than is the case with the Nematoda. All traces of an alimentary canal have disappeared, and the animals live entirely by the imbibition through the skin of the already elaborated fluids of their hosts. The power of absorbing fluids is shown by the fact that they swell up and become tense when placed in fresh water.