Archidistoma is a genus created for a species discovered and described briefly by Garstang in 1891 as A. aggregatum. It has been found at different times only in the Plymouth region, the original locality, attached to rock surfaces in a few feet of water. Recently, however, it has been found near Cape Hatteras on the eastern American coast (Van Name, 1945), suggesting either harbour to harbour transportation by ship bottom, or that its inconspicuous appearance has obscured a wide distribution.

This form is of considerable interest as it represents a type less specialized in zooid structure, nature of tadpole, and method of budding than any merosomatous ascidian except the diazonids. Diazonids are oviparous and have the simpler development and tadpole associated with small eggs, although there is a more elaborate branchial sac correlated with the relatively large size of the mature zooid.

Colony

The colony consists of a brownish tough matrix forming a thin encrustation on stones and shells, with individual zooids (a few millimetres long), partly embedded in the test but for the most part extending freely in clumps usually of three or four. Sand grains and round particles adhere to or become buried within the test, making this form the most inconspicuous of ascidians.

Structure of Zooid

The zooid is divisible into a thorax and a long slender abdomen (Fig. 1). There is no postabdomen, nor is there hypertrophy of the ventral test vessels, thereby segregating Archidistoma as a type from that of the synoicids (polyclinids) and the clavelinids respectively.