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THE VOYAGE OF H.M.S. CHALLENGER. 
Didemnum, Savigny. 
Didemnum, Savigny, M4moires, 1816. In part. 
Didemnum, Giard, Recherches, &c., Archives d. Zool. exper., t.' i. p. 617, 1872. 
Trididemnum, Della Valle, Nuovi Contribuzione, &c., 1881. 
Didemnum; von Drascbe, Die Synascidien, &c., 1883. 
non Didemnium, Kowalevsky, Archiv f. mikrosk, Anat., Bd. x., 1874. 
Colony usually thick and fleshy, rarely thin and incrusting. 
Ascidiozooids with the atrial aperture on the dorsal edge of the thorax, often 
placed far back. Atrial siphon lobed or simple. No atrial languet present. 
Test gelatinous or cartilaginous, usually not very hard or stifi'. Calcareous 
spicules usually present. 
Branchial Sac with three rows of stigmata. 
This genus, as formed by Savigny, would include all the Didemnidae with the 
exception of Eucoelium. Giard, however, has restricted its use by employing Milne- 
Edwards’ genus Leptoclinum for the thin incrusting forms and retaining Didemnum for 
the more massive species of the family. It was more fully and correctly characterised 
recently by von Drasche, and it is used here in the same restricted sense, except that I do 
not consider the number of rows of stigmata in the branchial sac of so much importance 
as von Drasche does. 
Della Valle’s generic title Trididemnum seems unnecessary, and ought to lapse. 
Sufficient information has not yet been given in regard to his new species Trididemnum 
henda, but, so far as can be made out from his figures, it seems to agree in all essential 
points with a typical Didemnum. If it should happen to be a species forming a thin 
incrusting colony, it might be convenient to split up Didemnum and apply Della Valle’s 
name Trididemnum to species with thin colonies and three rows of stigmata, while the 
thicker species would remain under Didemnum. 
The genus Didemnum as used here is characterised by having thick, massive, and 
usually fleshy colonies in which the test is not so hard and stifl" as it is in most species 
of Leptoclinum. Calcareous spicules are as a rule present, but they are not in great 
abundance throughout the whole test. The ordinary arrangement is that the spicules 
are numerous in the superficial layers of test, and are scarce or even absent in the 
deeper parts. In Didemnum inarmatum, von Drasche, Didemnum tortuosum, von 
Drasche, and Didemnum {?) inerme, Herdman (see below, p. 265) there are no spicules 
present in the test. 
The Ascidiozooids are characterised by having the atrial aperture placed far back on 
the dorsal edge of the thorax in place of being at the anterior end. As a rule there is 
an atrial siphon, and no atrial languet is present. 
