Segmentation and Segmental Differentiation in the Development of the Central Nervous Systems of Leeches and Flies

Abstract

There are two current views regarding the phyletic organization of the arthropods. In one view, all arthropods [chelicerates, crustaceans, and uniramians (onychophorans, myriapods, and hexapods)] belong to a single phylum, arising through a complex evolutionary radiation that occurred some 500 to 600 million years ago; the second view holds that the arthro­ pods are polyphyletic, since embryological features suggest separate evo­ lutionary relationships between polychaetes and crustaceans, and between oligochaetes and uniramians (Anderson 1 973, Gupta 1979). Regardless which of these is correct, the metamerism of annelids and arthropods is generally thought to have arisen in a common ancestor of both groups, and both structural and embryological analyses provide evidence for a close relationship between the two groups. One such analysis of several shared features has led to the proposal of a particular phylogenetic affinity between leeches and insects (Sawyer 1 984). The relative closeness of these relationships has been somewhat compromised, however, by recent cross­ species comparisons of nucleic acid sequences. Metamery is also a feature of the chordates, and, interestingly, sequence analysis of 18s rRNA does not indicate that, among the eucoelomates, annelids and arthropods are