THE EVOLUTIONARY RELATIONSHIPS OF CEPHALASPIDEA SL (GASTROPODA, OPISTHOBRANCHIA) - A PHYLOGENETIC ANALYSIS

Cephalaspid opisthobranchs, or ''bubble-shells,'' comprise a diverse g roup of snails commonly considered ''transitional'' between prosobranc hs 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 gener a, produced a data matrix of 47 new and modified-traditional character s. The results present the first phylogenetic hypothesis for shelled o pisthobranchs generated using parsimony-based cladistic methods. The p referred cladogram (length 117, ci 0.50, ri 0.70) has the following to pology: (Outgroup (Acteon, Gegania) (Hydatina ((Ringicula A, Ringicula B) ((Cylindrobulla (Ascobulla, Volvatella)) ((Aplysia, Akera) ((Bulla (Haminoea, Smaragdinella)) (Cylichna (Retusa A, Retusa B) (Acteocina (Scaphander (Philine A, Philine B)))))))))). Non-homoplastic or at lea st strong clade-supportive characters were determined from external an atomy, mantle cavity, and digestive, nervous, and reproductive systems . From the preferred tree topology, the Anaspidea and Sacoglossa (= As coglossa) were confirmed as monophyletic groups, with Cylindrobulla as an unambiguous member of the Sacoglossa. Traditional Cephalaspidea wa s split into two major clades: (a) Acteon, Ringicula, and Hydatina, re moved to the as-yet-unresolved, paraphyletic ''architectibranchs'' or ''lower heterobranchs,'' and (b) the remaining cephalaspids as the mon ophyletic group Cephalaspidea s.s., in sister-group relationship with Anaspidea. Homoplasy was evident in 25 characters, and significant in six, confirming the existence of ''rampant parallelism'' in shelled op isthobranchs. Tree topology suggested several evolutionary scenarios. (a) Formation of the gizzard (most plesiomorphic in Anaspidea) involve d the gizzard plates (many to three) and gizzard spines (present in An aspidea and Bulloidea, lost in Philinoidea). (b) The internal sperm-co nducting duct (''vas deferens'') is presumed homologous with the proso branch external ciliated groove. A second (novel) external groove, loc ated laterally, developed in shelled opisthobranchs, initially for egg transport, and cc-occurs with the internal duct in Sacoglossa. The in ternal duct was lost in Anaspidea and Cephalaspidea s.s., with the ext ernal groove assuming the task of sperm transport. (c) Allosperm stora ge sacks include a proximal receptaculum seminis and distal gametolyti c gland, the latter probably formed from the prosobranch bursa copulat rix. The ''bursa copulatrix'' of sacoglossans is probably secondary. S ome of the ''lower heterobranchs'' may share a proximal ''receptaculum apparatus,'' with the receptaculum and gametolytic gland in tandem ar rangement. (d) A herbivorous diet is presumed plesiomorphic, with carn ivory evolving independently at least five times, associated with diff erent suites of digestive system characters.