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). {176}[Illustration: 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, {177}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.