OPISTHOBRANCHIA
Opisthobranchia
Opisthobranchs are a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia, but no longer considered to represent a monophyletic grouping. Euopisthobranchia is a revised collection of opisthobranchs that is monophyletic, but this group leaves out some "traditional" opisthobranchs.The above text is a snippet from Wikipedia: Opisthobranchia
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