Vermiform Scolecida, parasitic in insects during a portion of their existence. An imperfectly developed alimentary canal or none. Water-vascular system rudimentary or absent. Sexes distinct.

The 'Gordiacea, or "Hair-worms," are thread-like Scolecids, often singularly like hairs in appearance, which live in the interior of various insects during part of their life. The digestive system is imperfect, an anal aperture being universally wanting. In Mermis, the gullet ends in a blind sac; in Gordius, the digestive tube opens into the body-cavity; and in Sphaerularia, the mouth appears to be wanting. The sexes are in different individuals. In Gordius itself, the embryo is free and aquatic, having a retractile snout armed with hooklets, by means of which it, after a time, bores its way into the tissues of some water-insect, in which it encysts itself. The sexually-mature worms are found in the interior of Orthoptera or Neuroptera; but they leave their hosts and betake themselves again to an aquatic existence for the purpose of laying their eggs. The adult Mermis is found principally in Lepidop-tera; whilst Sphaerularia inhabits the body-cavity of Bumblebees. A form of the Gordiacea has also been found at great depths in the ocean, coiled up beneath the carapace of shrimps (Willemoes-Suhm).

Fig. 119.   Morphology of Acanthocephala. Male of Echinorhynchus angustatus, enlarged about twelve times ; p Proboscis; n Neck ; s Muscular sheath of the proboscis; g Ganglion; b b

Fig. 119. - Morphology of Acanthocephala. Male of Echinorhynchus angustatus, enlarged about twelve times ; p Proboscis; n Neck ; s Muscular sheath of the proboscis; g Ganglion; b b "Lemnisci," sacs connected with the water-vascular system; l Liga-mentum suspensorium ; m m Retractor muscles of the proboscis ; t t Testes; v Vesi-cula seminalis; c c Integument. (After Leuckart.)