Tunicates and Lancelets

Early evolutionary steps are explained by models of these two simple and primitive chordates. Exhibition currently closed.

Tunicates are among the most simple chordates and lack the bony vertebrae of their more complex relatives. Also known as ‘sea squirts’, they live on the sea floor and, as their name suggests, pump water through sieves in their body walls, filtering food from seawater. It is not possible to stuff such delicate creatures for display purposes, and so the Halocynthia pyriformis on display in the Museum is made from glass. It is only in their larval stage that they reveal their relationship with fish and other vertebrates. Tunicate larvae are elongated with gill slits like a fish, can swim and bear little resemblance to their adult stage.

Students of biology the world over are familiar with one of the least complex animals in the chordate group – the amphioxus, also known as the lancelet. The lancelet display shows the development of this animal in a series of enlarged wax models, from egg to full-grown adult. Like tunicate larvae, they have no bone or cartilage. The 15 species in the genus Branchiostoma are all small and live half-buried in sea-floor sediments in many parts of the world, but are particularly common in Chinese waters. They filter small organisms from water, which is streamed in through the mouth.