Notes on the Didemnidae (Ascidiacea). II. The number of rows of stigmata inDidemnum gelatinosumMilne Edwards and inDidemnum maculosum(Milne Edwards)

Abstract

Milne Edwards (1841) established the genusLeptoclinum for those Didemnidae which possessed a largely expanded common cloacal system. His type of the genus wasL. maculosum which is now placed in the genusDidemnum Savigny (1816). Other species wereL. asperum andL. durum which are now regarded as mere forms ofDidemnum maculosum (e.g. Harant & Vernièeres, 1933); Leptoclinum fulgens which is closely related (see Harant & Vernièeres), if not a mere colour form ofDidemnum maculosum], Leptoclinum listerianum which has been transferred to the genusDiplosoma Macdonald; andLeptoclinum gelatinosum. This last has often been considered synonymous withDiplosoma listerianum, but my reasons for disagreeing with this assumption are elsewhere expressed (Carlisle, 1953). Milne Edwards did not of course describe the course of the sperm duct by whichDiplosoma is distinguished fromDidemnum. In the absence of a description of this diagnostic character we are reduced to a consideration of numerous details of the anatomy of Leptoclinum gelatinosum in order to decide where to place it in the classificatory scheme. When such a point-by-point comparison of shape and curvature of the gut, position of anus, form of atrial and buccal apertures, nature of test, etc., is made, it appears that the one point in which the drawings and descriptions given by Milne Edwards differ betweenL. gelatinosum andDidemnumgelatinosum Mime Edwards (1841) is the presence of a common cloacal system in the former and its absence in the latter. But the presence or absence of a common cloacal system is no longer regarded as a generic distinction (see, for example, van Name, 1945).

Notes on the Didemnidae (Ascidiacea). III. A Comparison ofDidemnum maculosum, D. candidum, D. helgolandicum, andTrididemnum alleni

“…It seems to represent dwarf or young stages of Didemnum candidum. It has already been noted (above and in Carlisle, 1954) that the marginal zooids of D. candidum, and indeed of many species of Didemnum, have only three rows of stigmata and a simplified gut. Moreover, such zooids produce smaller larvae with only three rows of stigmata.…”

“…The majority of zooids of both species have four rows of stigmata, but zooids may be found in most colonies with three or five rows (see also Carlisle, 1954). In D. candidum there are usually six or seven stigmata in a half-row; in D. helgolandicum eight or nine.…”

“…In this species the rays most commonly have a terminal angle of about 45 0 , but they may be rounded, almost square-tipped, or even rodlike. The most common formula (see Carlisle, 1953) is 1, 6, 9, but 1, 6, 12, 1, 4, 8, or even 1, 6, 12, 24 may occur. The variability in spicule form is greater at Plymouth than is illustrated by Van Name (1921) for West Indian colonies.…”

“…The anus is level with the hinder end of the third row of stigmata. The tight S-shaped curve of the rectum of D. helgolandicum is reminiscent of that of Trididemnum niveum (see Carlisle, 1953).…”