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The Indo-West Pacific watering pot shell Penicillus philippinensis (Penicillidae) is essentially amyarian, that is, the posterior adductor and pedal retractor muscles are lost, their anterior equivalents vestigial. In addition to a small group of pallial retractor muscles arising from the pallial line, as is typical of other penicillids, this bivalve is connected to its adventitious tube dorso-laterally by a saddle-shaped array of papillae that previses the more elaborate system in the southern Australian Kendrickiana veitchi. That species and P. philippinensis also differ from other penicillids in that their siphons are capable of only limited retraction into the tube. Their extension in both species is largely by hydraulic means, the complex pallial musculature acting antagonistically with extensive blood-filled haemocoels. P. philippinensis can also be separated from other penicillids by a number of anatomical characters. For example, some taxa such as K. veitchi and Nipponoclava gigantea (but not Foegia novaezeJandiae) have vestigIal posterior pedal retractor muscles and associated pericardial proprioreceptors but P. philippinensis does not. Like other penicillids, however, P. philippinensis has a muscular pedal disc whereas K. veitchi does not. A trend in the Penicillidae towards even greater specialization for life inside an adventitious tube has arguably culminated in the unique muscular system of K. veitchi. P. philippinensis indicates how this may have been achieved.

records of the western australian museum

The functional morphology of Penicillus philippinensis (Anomalodesmata: Clavagelloidea: Penicillidae) and the evolution of an unique muscular system in the Bivalvia