Early development of the thorax and the nervous system of the brine shrimp Artemia
The early development of the thorax and the nervous system of Artemia is described and compared with that of other arthropods. The thoracic segments of Artemia develop in an antero-posterior sequence, as pairs of mesoderm bands are segregated off from the growth zone in the posterior of the larva. These mesoderm bands, with their associated ectoderm grow and differentiate gradually, each segment eventually bears a pair of ventral appendages called phyllopods, and a pair of mechanoreceptive setae dorsally. Seven stages are designated in a description of the external development of the segment, with specific reference to the morphogenesis of the phyllopods. Two pairs of terminal pioneer neurons located near the posterior tip of the larva are described. Their axons grow out over uninnervated tissue to pioneer firstly the longitudinal connective pathways. These axons then branch medially in an antero-posterior sequence as segment development progresses to pioneer the commissures. The posterior commisure pathway is completed before the anterior one in each segment. The two dorsal nerves in the posterior of each segment are the first peripheral nerves to be pioneered to the C. N. S., by axons from the pairs of neurons that innervate each dorsal seta. Ultrastructural descriptions are given of the terminal and dorsal pioneer neuron cell bodies, axons and associated structures. The dorsal seta neurons have a sensory as well as a pioneering function, and the terminal pioneer neurons may also be derived from the neurons of sensory receptors. The ultrastructure of the dorsal setae is uniquely simple compared to other crustacean mechanoreceptors. In the ventral cord the location of the early pioneer neuron cell bodies and the pathways pioneered by their axons are different and far less complex than in decapod crustacea or insects.