:514 
THE VOYAGE OF H.M.S. CHALLENGER. 
Tliis geuus contains the typical members of the family, with clear soft gelatinous 
colonies, like Macdonald’s Diplosoma rayneri and Giard’s Pseudodidemnum crystallinum 
but it also includes those specie's, like von Drasche’s Diplosoma carnosum, where the 
colony becomes thick and fleshy; in none of them, however, are calcareous spicules ever 
developed in the test. The other characters are merely those of the family, which have 
already been sufficiently discussed. 
In the living condition colonies of this genus are most beautiful objects, and on 
account of their wonderful transparency they allow a great deal of the anatomy of the 
Ascidiozooids to be made out without cutting into the test. The common cloacal 
apertures are also clearly visible in the living and expanded condition of the colony. 
They are circular apertures with very slight lobes or none at all, but often pigmented 
round the margin, and are placed on delicate conical processes from the surface of the 
colony. In spirit-specimens the common cloacal apertures are very rarely visible, and 
the limits of the irregular systems are difficult to determine. 
Six species of Diplosoma are known with certainty. These are Macdonald’s 
Diplosoma rayneri, Giard’s Pseudodidemnum crystallinum, Giard’s Astellium spongi- 
forme, von Drasche’s Diplosoma chamwleon, von Drasche’s Diplosoma carnosum, and the 
new species Diplosoma macdonaldi. Milne-Edwards’ Didemnum gelatinosum- belongs 
to the genus, and may either be the same as Pseudodidemnum crystallinum, Giard, 
or a distinct species. Again, Della Valle’s Pseudodidemnum listerianum may be 
Diplosoma cliameeleon or a distinct species. Jourdain’s Pseudodidemnum zosterarum 
has not yet been described; it may be merely a synonym. Leaving out these doubtful 
species, the genus may be divided as follows : — 
Diplosoma. 
Branchial aperture 
more or less distinctly 
six-lobed. 
I 
Branchial aperturl 
circular, with no lobes 
{Astellmm). 
Colony thin, 
incrustin" 
Larva forms 
two Ascidio- 
zooids. 
Larva forms 
three 
Ascidiozooids. 
D. rayneri. D. crystallinum. 
Colony thick, 
tieshy. 
Test pigmented. 
I 
D. spongiforme. 
Test not 
pigmented. 
D. macdonaldi. 
Colour grey 
or dark 
green. 
D. chamceleon. 
Colour yellow- 
brown. 
I 
D. carnosum-. 
