ACASTA SULCATA. _Lamarck._ Animaux sans Vertèbres, 1818.
---- ---- _Deshayes_, in Guerin, Magasin de Zoologie, 1831, Tab. 24.
_Carino-lateral parietes about one sixth of width of lateral parietes:
inner surface of the parietes generally ribbed strongly: basis with the
edge strongly crenated: orifice of shell rather small: tergum with the
spur generally truncated and nearly half as wide as valve._
_Var._ (_a_) (fig. 2 _b_): _with the walls externally ribbed_.
_Var._ (_b_) (fig. 2 _a_): _with small membrane-covered clefts
between the edges of the walls, close above the basal cup_.
_Hab._--Sydney, Port Fairy, Moreton Bay in lat. 27° S., New South
Wales; Southern Australia; and, according to Lamarck, Western
Australia, in lat. 25° S. Mus. Brit., Cuming, Bowerbank, &c.
I am almost ashamed to admit this species, so small are its differences
compared with _A. spongites_; yet I think that it probably is a
distinct form.
In general appearance and character this species comes very near to
_A. spongites_. As in the latter, the parietes of the carino-lateral
compartments are narrow.[104] The orifice seems always to be smaller.
Internally, the parietes are generally much more strongly ribbed,
and the edges of the basal cup more plainly crenated. The sheath is
generally coloured of a brighter pink, sometimes tinged with orange.
The average largest specimens (from .3 to .5 of an inch in basal
diameter) are a little larger than the largest European specimens:
I have seen one specimen from Moreton Bay .4 in basal diameter, and
from the basal cup being very deep, actually .75 in height. The
_scutum_ has the articular ridge not so prominent and not so abruptly
cut off at the lower end, as in _A. spongites_: on the other hand,
the adductor ridge seems rather more prominent; but these differences
are trifling. In the _tergum_ the breadth of the spur (fig. 2 _c_, 2
_d_) varies in specimens taken out of the same branch of sponge; some
can hardly be distinguished from the same valves in _A. spongites_,
but generally the spur is broader and squarer.
[104] In Mr. Cuming's collection there is a specimen, from Sydney,
which I fully believe to be the present species, but cannot be
positively sure, as the opercular valves have been lost, which is
very remarkable from the walls of the carino-lateral compartments
being reduced to the thickness of a mere thread, not one twentieth
of the width of the lateral compartments; we here see the structure
of _Acasta sporillus_ prefigured.
This species presents some remarkable varieties: in one specimen,
from Sydney, the parietes were externally ribbed longitudinally
(fig. 2 _b_), the ribs being roughened with minute points, giving to
the shell an elegant appearance; and this without doubt is the _A.
sulcata_ of Lamarck, procured by Peron at Shark's Bay, lat. 25° S.,
on the opposite of the Australian continent: some specimens from Port
Lincoln, in South Australia, were not ribbed, only smoothly striated
in longitudinal lines; although both sets of specimens had almost
smooth scuta, and were thus different from common specimens, yet
there could be no doubt, from their similarity in all other points,
that they did not differ specifically from them, though the latter
had their scuta striated, but not their parietes. Hence we see that
there is no relation between the striæ on the parietes and on the
scuta. The Port Lincoln specimens, and some others, were remarkable
from the radii not extending down to the basal cup, a minute cleft,
covered only by membrane, being thus left along the sutures, low
down between the parietes (fig. 2 _a_); we shall see this singular
structure strongly developed in _A. perforata_. Owing apparently to
these clefts, the edge of the basal cup, exhibited traces of six
knob-like teeth, like those characteristic of _A. glans_.
_Cirri._--The cirri resemble those of _A. spongites_, with the
exception that the segments on the posterior pairs bear only
three main pairs of spines. With respect to the anterior ramus of
the fourth pair of cirri, the following very singular facts were
observed:--in a specimen from New South Wales (_var._ with the tergum
having a narrow spur), on two or three of the lower, but not on the
lowest, segments, the front margin was produced or developed into two
or three minute, thick teeth, slightly curved like hooks downwards:
in other specimens from New South Wales (_var._ with the tergum
having a broad spur, and inhabiting the same branch of sponge with
the last-mentioned variety), there was no trace of these teeth. But
again, in two other specimens with the tergum having a broad spur
(collected by different persons, near Sydney), and in another from
South Australia, this structure was carried to an extreme, for in
these (as represented, Pl. 29, fig. 2) there were beautifully formed
teeth on the fourteen lower segments (the twelve upper being without
them), and likewise on the upper segment of the pedicel. These teeth
are graduated in size on each segment; they are admirably adapted for
securing any prey; and, in fact, they convert each segment into a
mandible-like organ. On the segments, on which these teeth are well
developed, some of the regular pairs of spines are aborted.
_Diagnosis._--Finally, this species, if it be, as I believe,
distinct, differs from _A. spongites_ only in the internal surface of
the parietes being more strongly ribbed and brighter coloured,--in
the edge of the basal cup being more plainly crenated,--in the
articular ridge of the scutum being of a different shape,--in the
spur of the tergum being often broader,--in the segments of the
posterior cirri having only three pairs of main spines,--and, lastly,
in the occasional presence of the hook-like teeth on the anterior
ramus of the fourth cirrus.