Most, if not all, Anura and some Urodela have a voice produced by the larynx, which, especially in the Anura, is provided with a complicated cartilaginous and muscular apparatus and with vocal cords. The voice of the Urodela is at the best a feeble squeak. The females of the Anura are either mute or they produce a mere grunt, but that of many males is very loud, and, moreover, in many species it is intensified by vocal sacs which act as resonators. These sacs are diverticula of the lining of the mouth-cavity, and bulge out the outer skin and the muscles, chiefly the mylo-hyoid, of the throat. The nostrils and the mouth are firmly closed during the croaking. "The sacs are called internal when they are covered by the unmodified gular integument, however much this may be distended; external when their membrane projects through slits at the sides of the throat, as in Rana esculenta (Fig. 52, p. [269]), or when the skin is thinned and converted into a bladder-like pouch, as in Hyla arborea."[[24]] These sacs exhibit many modifications. They may be unpaired and median, and open by two slits into the mouth, on either side below the tongue; in Bufo one of the slits or openings, either the right or the left, is obliterated. They may be paired and symmetrical, and open one on each side of the head, below and near the posterior angle of the jaws. These modifications differ in closely allied species. They reach their greatest complication in Rhinoderma and in some of the Cystignathidae by extending far back beneath the skin into the wide lymphatic spaces. In Rhinoderma they are put to the unique use of nurseries for the young (see p. [228]). Leptodactylus typhonius has a very distinct pair of outer vocal sacs and a well-marked unpaired sac which extends into the belly and communicates with each outer sac. Several species of Paludicola, e.g. P. fuscomaculata and P. signifera, have a similar arrangement, in addition to an unpaired gular sac which can be inflated independently of the rest (see Fig. 45, p. [220]).
Urino-Genital Organs
The kidneys and the male generative glands are still intimately connected with each other. The general plan is as follows:–
The kidneys consist of a large number of glomeruli, produced by the coiled segmental tubes, each of which is composed of a nephrostome or funnel opening into the body-cavity, a Malpighian body and an efferent canal. The latter combine to form the segmental duct which opens into the cloaca. The testes, composed of a large number of sperm-producing glands, are drained by transverse canals which combine into a longitudinal canal, and this again sends off numerous efferent canals which open into the efferent canals of the kidney, so that the segmental duct (Leydig's duct of many authors) conveys both sperma and urine.
Fig. 7.–Diagrammatic representation of modifications of the urino-genital ducts. 1, 2, Male and female Newt; 3, a tubule of the kidney; 4, male Rana; 5, male Bufo; 6, male Bombinator; 7, male Discoglossus; 8, male Alytes. a, Artery entering, and producing a coil in, the Malpighian body, M; B, Bidder's organ; ef.s.c, efferent segmental canal; F.B, fat-body; gl, glomerulus; K, kidney; l.c.c, longitudinal collecting canal; M, Malpighian body; Md, Müllerian duct; N, nephrostome; O, ovary; Ov, oviduct; s.d, segmental duct; T, testis; Ur, ureter; V.d, vas deferens; V.s, vesicula seminalis.
In the female the network of transverse and longitudinal canals, which originally connect the generative glands with the kidney's efferent canals, is deduced in so far as the connection is interrupted and the vestiges of the transverse canals are no longer functional. The eggs fall into the body-cavity and are caught up by the ostium or inner abdominal opening of a special duct, the oviduct (Müllerian duct of many authors). Vestiges, more or less complete, of these oviducts persist in the males of most Amphibia.
This general scheme presents some modifications in the various groups of Amphibia.
The Apoda retain the most primitive conditions. The kidneys are still long and narrow, and the glomeruli are, at least in the anterior part of the organ, still strictly segmental, agreeing in number and position, each with a vertebral segment; later, the number of the glomeruli is greatly increased, and the former agreement becomes quite disturbed. The generative glands still retain their segmental arrangement, but they are restricted to a much shorter region than the kidneys. In the male Apoda a considerable portion of the cloaca can be everted by special muscles, and acts as an intromittent organ. Both sexes possess a ventral urinary bladder.
In the Urodela both kidneys and testes are much concentrated, the testes especially have lost all outward appearance of segmentation, and their efferent canals, connecting them with the longitudinal collecting canal, are much reduced in numbers. The greater portion of the kidneys, at least their anterior half, has all the appearance of a degenerating organ and is on the way to losing its urinary function, although it still possesses Malpighian bodies and complete ducts; the main function of the latter is now the conveyance of the sperma. In the Perennibranchiata, and in some others, e.g. Spelerpes variegatus, the longitudinal collecting canal, between testis and kidney, is sometimes suppressed, a very simple, but pseudo-primitive arrangement. A urinary bladder is present. The cloaca is not eversible.