There are few adequate descriptions of British members of the ascidian family Didemnidae and little attention has been given to the larvae. In the autumn of 1947 a didemnid was collected from the intertidal zone in the Menai Strait, North Wales. This ascidian agrees superficially with Leptoclinum maculosum Edwards, as figured by Milne Edwards (1842) and by Alder & Hancock (1905–1912), although differing somewhat from the diagnosis of Didemnum maculosum (Edwards) given by Harant & Vernières (1933). It is closer anatomically to the D.fulgens (Edwards) of Harant & Vernières, but differs in colour. In most features the colony and zooid closely resemble the descriptions given by Michaelsen (1923) for his species D. helgolandicum Michaelsen, and it is probable that this is the species to which it should be assigned. Nevertheless, the name D. maculosum has been retained since it is under that name that it has probably appeared in most faunistic lists.

Until recently all accounts of didemnid larvae have described three anterior adhesive organs. This was the number given by Giard (1872) for Diplosoma gelatinosum Edwards, and by Lahille (1890) for Trididemnum tenerum Verrill and Polysyncraton lacazei Giard. Berrill (1947), however, has described a new species of didemnid, Trididemnum alleni Berrill, the larva of which has only two anterior adhesive organs, a condition not previously found in any ascidian larva.

The larva (Fig. IA) of the colonies found in the Menai Strait resembles that of T. alleni in many features. The trunk is about 0.35 mm. long from the tip of the adhesive organs to the base of the tail.