The lateral-line sensory system in the caecilian family, Ichthyophiidae (Amphibia: Gymnophiona)

Abstract

The sense organs commonly known to anatomists as the lateral-line or neuromast system developed in early aquatic Chordata (Ostracoderms) and continued to appear in subsequently developed aquatic groups. It was a sensory system believed to serve the organism in adjusting to an aquatic environment. When an aquatic animal having such a system changes to a terrestrial environment, the system seemingly no longer functions and tends to disappear. This is true of certain species of fish that assume a semiterres-trial habitat (Symbranchidae). It is also true of many Amphibia that have a free-swimming larval period after hatching and which, at transformation from larval to adult form, usually assume a terrestrial habitat. The system is present in numerous Caudata during the larval period. One species of the genus Taricha is said to lose the system on transformation to a land habitat. Later in life when it again resumes the aquatic habitat the system reappears and presumably again functions! Most Salientia that have a free-swimming larval period seek a terrestrial habitat on transformation and the system is lost. However, in certain species that are, as adults, semiaquatic, the system may be partially retained (Rami hexadactyla and R. cyanophlyctis). Others may remain permanently aquatic and retain more of the system (Xenopits). The neuromast organs of the lateral-line system are usually described as groups of sensory cells, each having a hairlike process at its free end and surrounded by supporting cells. These organs are arranged in series that follow the nerves. They are in pi ts or depressions in the skin which are usually connected by canals filled with a mucous secretion. The series are symmetrically arranged on the two sides of head and body. In most fishes there is a series crossing the head that connects the two sides. If this is present in the caecilians, it is not obvious externally.