Quarterly Journal of Microscopical Science, Vol s3-100, 509-527, Copyright © 1959 by Company of Biologists

The Central Nervous System in Larval Acraniates

¹ Marine Biological Laboratory, Plymouth

This paper describes a part of the organization of the spinal cord of the larva of the Acrania, dealing chiefly with those tracts and cell groups that are probably concerned with the control of the swimming pattern.

These observations serve as a basis for the comparison of the organization of the cord of the larva with that of the adult, and with that known for the larvae of the Agnatha and higher vertebrates. Other observations are concerned with the arrangement of the peripheral nervous system, which differs in some respects from that of the adult. The first section deals with the arrangement of the fibre tracts in the cord, and of the cell-bodies which give rise to these fibres.

The form and connexions of the giant cells are then described; it is shown that these cells are equivalent to the Rohon-Beard sensory cells of vertebrate embryos. They send peripheral processes out through dorsal root nerves, but they differ from the vertebrate sensory neurones in their greatly enlarged dendrite field and the longitudinal axon in the cord.

Thirdly, the innervation of the gill musculature is described. A ventro-lateral asymmetrical nerve is found, formed by the junction of fibres from the right anterior dorsal root nerves.

Lastly, the arrangement of the whole system is discussed in relation to the systems found in the larval stages of other primitive chordates.