The metamorphosis of the jaws and their muscles in the Toad, <i>Bufo regularis</i> Reuss, correlated with the changes in the animal's feeding habits

Abstract

SUMMARY. In the tadpoles of Bufo regularis it has been found that: A commissura quadrato‐cranialis anterior and an ascending process are present; both of these structures are of common occurrence in all other Salientia and help in joining the quadrate to the skull. There is no larval otic process (a primitive character), but instead there is a fibrous ligament, the quadrato‐otic ligament. The processus muscularis quadrati is attached to the lamina orbitonasalis by a bridge of cartilage which is here called the “commissura quadrato‐orbitalis”. The infrarostral cartilage and Meckel's cartilage are connected by a cartilaginous bar, which is here called the “commissura intramandibularis”. The suprarostral cartilage consists of a median and two lateral lobes. This is a primitive character. There is neither a M. l. m. anterior lateralis (as in Ascaphus truei ), nor a M. l. m. subexternus (as in Bufo bufo ). The M. l. m. externus consists of two separate parts, anterior and posterior , the former is inserted on the lower jaw, the latter on the suprarostral cartilage. The M. mandibulo‐labialis is inserted on the lower lip only, and has no connection to the upper lip. The important changes which take place during metamorphosis are: All three larval processes joining the quadrate to the skull break down and disappear. The otic process develops exclusively from the posterior half of the quadrate. The pseudo‐basal process develops as an outgrowth from the anteroventral surface of the auditory capsule. A quadrate bone develops on the articular part of the quadrate cartilage and fuses with the quadratojugal, which develops on the ligamentum cornu quadratum laterale. The M. mandibulo‐labialis fuses with the M. intermandibularis posterior . The two parts of the M. l. m. externus fuse with each other. The MM. orbitohyoideus, suspensoriohyoideus and the suspensorioquadrato‐ , and hyo‐angularis fuse and give rise to the M. depressor mandibulae . The MM. l. m. posterior superficialis and profundus remain separate throughout life. The M. l. m. anterior lateralis is differentiated from the M. l. m. anterior articularis late during metamorphosis. The jaw muscles show a drop in their absolute lengths as well as in volume during metamorphosis, after which they begin to grow again. The lower jaw keeps on increasing in length until the adult condition is attained.