D. ACANTHOCEPHALA, Rud.
Fig. 348a.—The male of Echinorhynchus augustatus. L., lemnisci; T., testicles; P., prostatic glands; P.r., sheath of proboscis, with ganglion; R.r., retractor of sheath of proboscis. 25/1.
Gutless, nematode-like worms that carry at their anterior end a retractile rostrum beset with hooks. In their adult condition they only live in vertebrate animals. During their larval stage they are often parasitic in invertebrate animals.
The Acanthocephala are elongated cylindrical worms, with a rounded posterior end. In some species an annulation is distinctly recognizable; they are, however, not segmented. The size varies according to the species, between about 5 to 10 mm. and 40 to 50 cm.; in general, however, there is a preponderance of the small species. The sexes are separate, and the males can easily be distinguished from the females without examination of the genitalia, as the females are both larger and thicker.
The body wall of Echinorhynchus is limited by a thin cuticle, which is attached inwardly to the hypodermis. In only exceptional cases a syncytium with large nuclei, even in the adult condition, is represented by the hypodermis; and in it fibre systems, the elements of which run in layers in various directions, appear, and it is only towards the interior from these strata of fibres that the nuclei of the hypodermis are found. As a rule, these fibres, at all events the radiary fibres, are regarded as muscles. Hamann describes them as elastic fibres, which lie in a viscid gelatinous connective substance (transformed protoplasm?); a lacune system filled with a granular fluid, the central part of which are two longitudinal lacunes lying at the sides, also belongs to the cutaneous strata, as do the so-called lemnisci, two short, flat organs suspended in the body cavity, and the pedicles of which are attached anteriorly at the border between the rostrum and body; their structure as well as their origin permit them to be traced to the skin (fig. 348A).
Finally, inwardly below the skin there follows a layer of annular, and after these a layer of longitudinal muscles, the structure cells of which remain present in the residues, carrying nuclei. The motor apparatus of the rostrum, the sheath of the rostrum, and the lemnisci also belong to the muscular system. The rostrum represents a finger-shaped hollow process of the cutaneous layer; but, according to Hamann, it originates from the entoderm and passes through the skin secondarily. It is covered by a thin cuticle, and as a rule contains a large number of regularly placed chitinous hooks that adjoin a granular formation tissue. From the base of the rostrum springs a tubular hollow muscle extending into the body cavity; this is the RECEPTACULUM PROBOSCIDIS, from the base of which again bundles of longitudinal muscles originate, which pass along its axis and that of the rostrum itself, and are inserted at the inner surface of its anterior end (RETRACTOR PROBOSCIDIS). These muscles when they contract invaginate the proboscis and draw it into the receptaculum; when reversed they act again as PROTRUSOR PROBOSCIDIS. The whole of the anterior body, however, can be invaginated, and for this purpose there is a muscle that originates from the body wall at a variable distance back, and which is joined to the receptaculum (RETRACTOR RECEPTACULI); there is also a bell-shaped muscle which springs from the body wall behind the lemnisci in rings, and passes forward to the spot of attachment of the lemnisci.