The fourth category is represented by the Apoda or Caecilians in which, as we have stated above, the male is provided with an intromittent organ.
Amphicoelous (bi-concave) vertebrae are found in the Apoda and in some of the Caudata; opisthocoelous (convexo-concave) vertebrae in the higher Caudata and in the lower Ecaudata; whilst the great majority of the Ecaudata have procoelous (concavo-convex) vertebrae.
In the adult Apoda these arches and the hyoid fuse into three transverse, curved or angular bones (see fig.
The heart is situated quite forward, in the gular or pectoral region, even in those tailed batrachians which have a serpentiform body, whilst in the Apoda (fig.
Although there is much similarity between the Apoda of Africa and of South America, one genus being even common to both parts of the world, the frogs are extremely different, apart from the numerous representatives of the widely distributed genus Bufo.
In the Caudata, external gills (three on each side) persist until the close of the metamorphosis, whilst in the Apoda and Ecaudata they exist only during the earlier periods, being afterwards replaced by internal gills.
They are less marked or more gradual in the Apoda and Caudata than in Ecaudata, in which the stage known as tadpole is very unlike the frog or toad into which it rather suddenly passes (see Tadpole).
Viviparous parturition is known among the Caudata (Salamandra, Spelerpes fuscus), and the Apoda (Dermophis thomensis, Typhlonectes cornpressicauda); also in a little toad (Pseudophryne vivipara) recently discovered in German East Africa (41).
Cope (16) regarded the Apoda as the extremes of a line of degeneration from the Salamanders, with Amphiuma as one of the annectent forms. In the opinion of P. and F.