Using a variety of neuroanatomical and histological techniques, we compare the 
spinal cord and peripheral nerve distribution in the tails of larvae from 
Xenopus laevis and three species of Rana. The relatively large, postsacral 
spinal cord of Xenopus contains abundant motoneurons and their axons. Spinal 
nerves exit from the spinal cord in a regular array, one nerve per myotome, from 
the cervical region to near the end of the tail. Somata of motoneurons 
innervating caudal myotomes are found along the entire length of the tail. In 
contrast, the caudal cord of Rana is reduced to a filum terminale consisting of 
little more than an ependymal tube; spinal nerves to all caudal myotomes leave 
the cord in the sacral region and reach their motor targets via a cauda equina 
and caudal plexus. Motoneuron cell bodies innervating caudal myotomes are found 
only in the sacral region. The Rana larval pattern is similar to that of adult 
frogs and mammals, whereas the Xenopus larval pattern is more like that of 
salamanders and reptiles. These gross neuroanatomical differences are not due to 
differences in the size or developmental stage of the tadpoles, but instead are 
associated with differences in the swimming behavior of the larvae. The presence 
of motoneurons in the caudal spinal cord of Xenopus may provide local 
intermyotomal control within the tail; the elongated topography of the cord 
appears to permit finer, rostral-to-caudal regulation of neuromuscular activity. 
The Rana spinal cord, on the other hand--with motoneurons clustered 
anteriorly--may produce concurrent firing of adjacent ipsilateral myotomes, but 
at the expense of fine intermyotomal regulation. The fact that nerves in the 
tail of Xenopus enter and exit from the spinal cord locally, as opposed to far 
anteriorly as in Rana, means that for tadpoles of the same size, reflex arc 
lengths are many times shorter in Xenopus.