The nervous system consists of a cluster of ganglia situated at the base of the rostrum, from which three nerves pass towards the front and two towards the back. No sensory organs are known.
The excretory organs, according to Kaiser, lie at the upper border of the ductus ejaculatorius in the male and at the so-called bell in the female. Here they represent the long-known villous tufts, placed on disc-like cushions. In each of the cylindrical villi—which terminate blindly towards the body cavity—there lies a cilium, which springs from the membrane lining the villus, and which lies in a space cavity of the villus, which ultimately proceeds as a little canal. There are three canals discharging into the uterus that serve to conduct the excretory materials from the body cavity; special glandular cells corresponding to the terminal cells of the Platyhelminths, at the commencement of the system, are not present in the Acanthocephala.
Sexual Organs.
(a) Male Organs.—The greatest part of the male genital apparatus is contained in a muscular sheath—the ligament—which originates at the posterior end of the receptaculum proboscidis, passes along longitudinally through the body cavity, and is inserted at the posterior end of the worm. The two oval testicles usually lie one behind the other; their vasa efferentia unite sooner or later into a vas deferens which passes backwards, and finally terminates in the penis; the terminal portion of the conducting apparatus is surrounded by six large glandular cells (prostatic glands) the excretory ducts of which open into the vas deferens. The penis itself is placed at the base of a bell-shaped invagination of the posterior end, the bursa, which is everted during copulation.
Fig. 348b.—Anterior portion of the female apparatus of Echinorhynchus acus. On the left seen from behind, on the right seen from the front. F, inferior orifice of the bell; B, bell; Lig, ligament; M, mouth of bell; Ut, uterus. Magnified. (After Wagener.)
(b) Female Organs.—There are only two ovaries present in the ligament during the larval stage. During the course of growth they divide into accumulations of cells (placentulæ, loose or floating ovaries), which finally cause the ligament to burst and they thus attain the body cavity. Thence a peculiarly constructed apparatus finally conveys the eggs out. This apparatus consists of the uterine bell and vagina, the latter discharging at the posterior extremity of the body. The bell is a muscular canal provided with apertures at both the anterior and posterior extremities. Its interior space is in direct communication with the body cavity, and the anterior orifice takes up all materials floating in the cavity—egg-balls, mature and immature eggs—and pushes these further backwards. The continuation of the bell lumen is now narrowed by a number of large cells in such a manner that only bodies of a certain form can pass through this tract and attain the uterus; everything else is conveyed back into the body cavity through the posterior opening of the bell.
The eggs are already fertilized in the body cavity, and in this position go through their development to the formation of the embryo. Completely developed eggs are surrounded by three shells, and are generally fusiform. The eggs agglomerate in masses in the uterus until they are finally deposited through the vagina and vulva. For the further development, the transmission of the eggs into an intermediary host—usually a crustacean or an insect—is necessary; the metamorphosis is very complicated; but this transmission may be very easily effected artificially by feeding suitable crustaceans (Asellus, Gammarus, etc.) with the eggs of Acanthocephala; this being the only method of inducing the larva to hatch out so that its structure may be studied. The larva appears in the form of an elongated, somewhat bent body, at the stumpy anterior end of which there is a crown of hooks or spines, whereas the posterior end is pointed. Especial retractors draw in the hook-beset anterior surface, and an elastic cushion beneath them jerks them forward again when required. In the middle of the body a roundish heap of small cells is seen, from which the entire body of the Echinorhynchus originates, even to the cutaneous layer; the latter is also the larval skin in which the small Echinorhynchus gradually grows. The development of all the organs takes place within the intermediary host, and the parasite only needs to be imported into the terminal host to attain the adult stage after a certain growth. In some cases, however, a second intermediary host is utilized.
Species of Acanthocephala only occur exceptionally in human beings.