200 
THE. YOYAGE OF H.M.S. CHALLENGER 
(2) as a subgenus or section of the wider Aplidium, and corresponding to the present 
genus. Giard considered this group of species as characterised by the six-lobed 
branchial aperture and the short post-abdomen. The first of these characters, however, 
is shared by nearly all of the Polyclinidse, and characters derived from the post-abdomen 
are somewhat unreliable on account of the share which that part of the body takes in 
gemmation in the Polyclinidm and its consequent changes in size. 
Von Drasche apparently felt this objection, for while he retained Giard’s subgenus, 
he added another character to its diagnosis, viz., that the atrial aperture is placed 
on the dorsal edge far behind the branchial, and has either no atrial languet or only 
a very small one. He figures a new species {Aplidium asperum, von Drasche) in which 
this feature is well seen, and seems to regard this amended definition of Aplidium as 
perfectly satisfactory. I am afraid, however, that it is little better than it was before. 
In the first place the type species of the genus Amaroucium (a genus which is most 
closely allied to Aplidium) has the atrial aperture placed far back on the dorsal edge ; 
it is, however, provided with a large languet. And, secondly, the exact position of 
the atrial aperture varies considerably, not only in the different species of Aplidium, but 
even in the different Ascidiozooids of one colony, and in some cases it is found in an 
intermediate position which might either be described as dorsal or anterior. 
In the Table on p. 152, von Drasche’s characteristic has been made use of in dis- 
tinguishing Aplidium, as I believe that it is somewhat more reliable than the length of 
the post-abdomen, but in any particular case where it failed to indicate the genus, 
then, on account of the six-lobed branchial aperture, the longitudinally folded stomach 
and the sessile post-abdomen, Aplidium would be thrown (see Table, p. 152) along 
with Amaroucium and Sigillina, The extremely long and attenuated post-abdomen 
of the latter genus, and its distinctly six-lobed atrial aperture, would sufficiently distinguish 
it from Aplidium, and consequently the only risk of confusion would be with Amaroucium. 
After a careful consideration of all the known species of both genera I find that there is 
no single reliable character which will separate the two groups in all their species. 
Consequently in such a case it would be necessary to make use of a combination of 
characters, of which the following are the most important : — 
Aplidium. 
Colony not pedunculated, Ascidio- 
zooids not very long, atrial aper- 
ture placed dorsally and usually 
with no atrial languet, post- 
abdomen shorter than thorax 
and abdomen. 
Amaroucium. 
Colony often pedunculated or elon- 
gated vertically, Ascidiozooids 
long, atrial aperture anterior or 
slightly dorsal, provided with 
a large atrial languet, post- 
abdomen usually long. 
