Note
Cephalaspid opisthobranchs, or “bubble-shells,” comprise a diverse group of snails commonly considered “transitional” between prosobranchs and “higher gastropods.” Comparative morphological investigations at gross, light, and scanning electron microscopic levels, involving 20 taxa of cephalaspids and related shelled opisthobranchs in 16 genera, produced a data matrix of 47 new and modified-traditional characters. The results present the first phylogenetic hypothesis for shelled opisthobranchs generated using parsimony-based cladistics methods.