ABSTRACT

The microstructure of the primary spines in Asthenosoma varium exhibits two principal forms: sharp spines with a poison gland, on the aboral surface; and trumpet-shaped spines on the oral surface. The oral spines have four zones along the shaft with variations in the internal and external microstructure. Each zone exhibits microstructural detail which has also been seen in spines of the Diadematidae (particularly Echinothrix) and the Toxopneustidae (Lytechinus, Sphaerechinus and Toxopneustes). This chapter discusses the microstructural affinities with respect to the phylogeny and evolution of the regular echinoids. It examines gross spine-morphology and structure for differences between spines of a given type which might reflect their position on the test. In the light of these preliminary observations, primary spines of both types are prepared for scanning electron microscope examination. The originality of most of the echinothurid spines lies in the structure of the apical hoofshaped zone, a structure which also occurs in the aspidodiade-matids.