almost constant presence of one or more very minute parasites, on the
margins of both scuta, close to the umbones: these are represented, but
rendered darker and therefore more conspicuous than in nature, in the
drawing, Pl. V, fig. 15, which is three times the natural size. I
carelessly dissected one or two specimens, and concluded that they
belonged to some new class or order amongst the Articulata; but did not
at that time even conjecture, that they were Cirripedes. Many months
afterwards, when I had seen in Ibla, that an hermaphrodite could have a
complemental male, I remembered that I had been surprised at the small
size of the vesiculæ seminales in the hermaphrodite _S. vulgare_, so
that I resolved to look with care at these parasites; on doing so, I
soon discovered that they were Cirripedes, for I found that they adhered
by cement, and were furnished with prehensile antennæ, which latter, I
observed with astonishment, agreed in every minute character, and in
size, with those of _S. vulgare_: the importance of this agreement will
not at present be fully appreciated. I also found, that these parasites
were destitute of a mouth and stomach; that consequently they were
short-lived, but that they reached maturity; and that all were males.
Subsequently the five other species of the genus Scalpellum were found
to present more or less closely analogous phenomena. These facts,
together with those given under Ibla (and had it not been for this
latter genus, I never probably should have even struck on the right
track in my investigation,) appear sufficient to justify me, in
provisionally considering the truly wonderful parasites of the several
species of Scalpellum, as Males and Complemental Males. When these
parasites are fully described, will be the proper time to discuss and
weigh the evidence on their sexual relations and nature. I will now
describe the parasite of _S. vulgare_.
_General Appearance._--Shape, flask-like, compressed (Pl. V, fig. 9,
magnified 36 times), with a short neck: the outline is usually
symmetrical, but sometimes is a little distorted on the under side. The
creature is imbedded more than half its length or depth in the
transparent, spine-bearing chitine border of the scutum of the
hermaphrodite. Its length, or longer axis, varies from 10 to 11/400ths;
its breadth, or transverse axis, is 6 to 7/400ths; and its thickness,
for it is much flattened, is only 4/400ths of an inch. On the summit,
there is a fimbriated orifice (_a_), the size of which can rarely be
made out quite distinctly, owing to the extreme thinness of the
membranous edges. A little way beneath the orifice, there are four
little blunt, bristly points (_b_), generally rather more than the
1/1000th of an inch in length; they are rather variable in size, and
seem to be of no functional importance; directly beneath them, there
are four little calcareous beads (as may be known by their dissolving
with effervescence in any acid, and breaking easily under the needle);
these are the 3/2000ths of an inch in their larger external diameter;
they are rather deeply imbedded in the outer integument, and taper a
little downwards ending in a concave terminal point, into which a minute
tubulus enters, like those passing into and through the valves of
ordinary Cirripedia: along the axis of imbedment, they are often
4/2000ths of an inch in length. These calcareous beads or rudimental
valves are seated in pairs, at the two ends of the flattened animal, so
that when the animal is laid on one side, the upper bead in each pair
exactly covers and hides the lower one. The outer integument is composed
of chitine, as may be inferred from boiling caustic potash having no
effect on it; the upper part is thicker than the imbedded portion and is
wrinkled transversely; it is covered with minute spines 4/10,000ths of
an inch in length, either single or in groups of two and three, (Pl. V,
fig. 14.) This outer tunic is lined by corium, sometimes slightly
mottled with dull purple; and this by delicate, longitudinal, striæ-less
muscles, running from the base up to the under edge of the orifice;
these longitudinal muscles are crossed, at least, in the upper part, by
still finer transverse muscles.
_Thorax and Abdomen._--When the external integument is cut open, the
thorax (Pl. V, fig. 13) is found lodged within an inner sack or rather
tube, extending from near the bottom of the animal, up to the external
orifice. The whole thorax is sometimes forced through the orifice, owing
perhaps to the action of the spirits of wine and consequent endosmose,
and is thus well displayed without dissection. The thorax tapers a
little, is much flattened and straight; its length, together with the
terminal abdominal lobe, is about 6/400ths of an inch; it is formed of
very thin, most finely hirsute membrane, transversely wrinkled and so
extensible, that when everted by the internal muscles being seized, it
stretches to twice its former length; in this condition, five transverse
articulations are displayed. The abdominal lobe is smooth, and cannot
be stretched, or turned inside out by pulling the above muscles. On the
thorax, corresponding with the interspaces between the five transverse
articulations, there are four pair of short limbs, but their bases, I
believe, are prolonged across the inner or ventral surface of the
thorax, so as almost to touch each other. These limbs, I believe, have
no articulations, except, perhaps, where united to the thorax. The
anterior or lowest limb, on each side, supports two or sometimes only a
single spine; this pair is rather smaller than the second, and is placed
a little more distant from it, than are the upper pairs from each other.
The second pair differs from the upper two, only in having its three
spines a very little shorter. The two upper or posterior pair exactly
resemble each other; each has two spines on the summit, and a third
seated lower down, on a little notch on the outer side, but with its
point on a level with the others. The points of the spines of the two
upper limbs, stand on a level with the external spines at the end of the
abdomen. All the spines are of excessive tenuity and sharpness; they are
straight, long, and not plumose.
The abdominal lobe is square, and from not being wrinkled, has a
different appearance from the thorax: on each of the posterior angles,
there are three moderately long, very sharp spines, with the tips of the
outer pair bent a little inwards; in the middle between them, there are
two little spines, and a little below and outside these latter, on the
ventral surface, there are two other longer spines with their tips bent
inwards; and again, lower down, two other pair, one beneath the other,
of short spines. Perhaps, the three pair of spines on the ventral
surface, mark the three segments, which are distinct on the abdomen of
the larva in the last stage of its development, in Lepas and other
genera. In the same way, it is probable that the lateral spine on the
notch in each limb, marks the point where, in the larva, there is an
articulation. Altogether, there are seven pairs of spines on the
abdomen, and eleven pairs on the thoracic limbs.
A little way beneath the lower or anterior pair of limbs, the thorax is
abruptly bent, and becomes confluent with the lower internal parts of
the whole animal. Here, the very delicate membrane of chitine which
lines the sack or tube, extending from the external orifice, can be seen
to be continuous, as in all Cirripedes, with the outer tunic of the
thorax. Within the thorax, there are some longitudinal muscles, without
transverse striæ, which, I believe, enter the short limbs, but not the
abdomen, as I infer from the latter not being everted when they are
pulled. At their lower ends these muscles terminate abruptly, and from
being contracted are often a little enlarged. They extend a short way
beneath the lower pair of limbs, and are, I suspect, attached to the
outer integument of the animal, near the base.
After the most careful dissection of very many specimens, and their
examination in many different methods (as by caustic potash, &c.), I can
venture positively to assert that there is no vestige of a mouth, or
masticatory organs, or stomach: I did not see any anus, but I will not
affirm that such does not exist.
In the upper part of the animal, lying under the superficial muscles,
and close beneath the upper line of their attachment, I found in all the
specimens, an eye, of a pointed oval form, rather less than 11/12,000ths
of an inch in diameter, formed of an outer capsule, lined with purple
pigment-cells, and surrounding, as it appeared, a lens. The eye is not
introduced in fig. 9, for I could not see it, except by dissection, and
therefore do not know its exact relative position.
_Generative System._--The contents of the animal, between the sack
containing the thorax and the outer integuments, and directly under the
thorax, varied much in condition: in young and lately attached specimens
the whole consisted of a pulpy mass with numerous oil-globules; in other
specimens, apparently more mature, there were vast numbers of cells,
sometimes cohering in sheets, about 3/10,000ths of an inch in diameter,
and having darkish granular centres; these I believe to be the testes,
for in a specimen presently to be mentioned, in which the vesicula
seminalis was gorged with spermatozoa, I found adhering to its outside,
a mass of cells of exactly the same diameter, but now empty and
transparent instead of having brownish centres. Lastly, in several other
specimens, at the very bottom of the sack-formed animal, there was a
brownish, pear-shaped bag, of different sizes in different individuals,
and occasionally broader even than the thorax. This bag contained either
pulpy matter, or a great mass of spermatozoa. Before being disturbed,
these spermatozoa lay parallel to each other in flocks, and they yielded
to the needle in a peculiar manner, so that I found (having had
experience with these bodies in living Cirripedia) I could almost tell
before examination under the compound microscope, whether or not I
should see spermatozoa. Many had distinct heads,[52] which were two or
three times as broad as the filamentary bodies; the latter when placed
between glass were the 1/20,000th of an inch in diameter. I compared
these spermatozoa with others taken out of the vesiculæ seminales of the
individual hermaphrodite _S. vulgare_, to which the parasite was
attached, and could not perceive the slightest difference in them. The
brownish pear-shaped bag, or vesicula seminalis, the coat of which seems
fibrous, could sometimes be distinctly traced, sending a chord or
prolongation far up the thorax: at the end of the abdominal lobe, no
doubt there is an orifice; and this, I believe, I once distinguished.
Owing to this chord, the bag often adheres to the thorax, when the
latter is dissected out of the general integuments; in this condition, I
twice clearly made out that it was single: in one other specimen,
however, there appeared to be two small vesiculæ seminales. By using a
condenser and very brilliant light, the outline of the vesicula
seminalis could sometimes be distinguished before dissection, at the
bottom of the sack-formed animal; and such was the case in the specimen
drawn in fig. 9.
[52] I do not understand the development of the spermatozoa in
Cirripedia: in a recent Chthamalus and Balanus, I found the
greater number had a little filament in front of the head or
nodular enlargement, which latter varied in size and in shape
from globular to that of a spindle. The filament before the head,
also, varied in proportional length; it did not project in
exactly the same straight line with the hinder part, and some of
the spermatozoa were entirely without this filament in
front;--such is the case with the spermatozoa here described.
Although I have dissected, at least, thirty specimens, taken at
different times of the year, and from different localities, and when
many of the specimens were mature and ready for the impregnation of ova,
as clearly shown by the presence of innumerable spermatozoa, I have
never seen even a trace of an ovum or ovaria.
_Antennæ and Attachment._--The prehensile antennæ (Pl. V, fig. 10), are
seated a little above the very base of the sack-like animal; and this
might have been expected from the antennæ in the larva, being seated on
the ventral surface, not at the very extremity of the head. By a very
strong light, they can sometimes just be seen whilst the parasite is
attached to the hermaphrodite (the scutum of the latter having been
cleaned on the under side), and are thus represented in fig. 9. They are
formed of thicker membrane than the general integument of the body: the
second segment, or disc, is pointed and hoof-like; when seen in profile
(fig. 11), the upper convex surface has a uniform slope with the upper
surface of the basal segment; it is furnished with a single backward
pointing spine, attached, I believe, on the under side, nearly opposite
the articulation of the ultimate segment: at the apex, there are some
excessively minute hairs or down. The ultimate segment projects
rectangularly outwards as usual, and has on its inner side, rather
beneath the middle, a conspicuous notch (fig. 12), which bears two or
three long, non-plumose spines; on the summit there are three or four
rather shorter spines. On the outside of the great basal segment there
is a single spine curving backwards. The importance of the following
measurements (in fractions of an inch) will hereafter be seen.
Length of whole organ, from end of disc to the further }
margin of the oblique basal articulation } 38-39/6000
Length of whole organ, to the inner margin of the oblique}
basal articulation } 1/6000
Breadth of basal segment, measured half-way between the }
basal and second articulations,--the limb being viewed }
from vertically above } 8/6000
Length of hoof-like disc, measured from the apex to the }
middle of the articulation with the basal segment } 9-10/6000
Breadth of ditto 5/6000
Length of ultimate segment 6/6000
Breadth of ultimate segment beneath the notch 7/20000
Breadth of ultimate segment above the notch 5/20000
I did not see the cement-ducts, which, perhaps, was owing to the corium
extending from the inside of the whole animal some way into the antennæ,
thus rendering them rather less transparent than in common Cirripedes.
That the ducts and cement-glands exist, is certain, for the antennæ in
every case were enveloped in a little irregular mass or capsule of the
usual, brown, transparent, laminated cement. When several of these
parasites were attached close together, the cement ran up between them.
I may here state, that I found on one Scalpellum, three males very
lately attached, and not as yet imbedded in the chitine border; they
were white, opaque, pulpy, and full of oily globules; the lower part was
considerably more pointed, and extended further beyond the prehensile
antennæ, than in the older and imbedded specimens. There were distinct
remnants of two great reddish-brown eyes, showing that in this respect
the larvæ of the male in their last stage of development, are
characterised like the larvæ of other Lepadidæ. The male larva would,
probably, be a little larger than the male itself; but yet compared with
the larva in the earliest stage, there can have been unusually little
increase of size during the several intermediate metamorphoses; I judge
of this from the dimensions of the larva of the hermaphrodite in the
first stage, namely, 9/400ths of an inch, exactly the size of some of
the smaller males. In the allied genus Ibla, the increase is also less
than is usual, namely, from 15/1000ths of an inch, the diameter of the
ovum, to only 25/1000ths of an inch, the length of the boat-shaped
larva, just before its final metamorphosis.
_Habits and Concluding Remarks._--The males are imbedded in the spinose
chitine border of the occludent margin of the scuta, exactly over an
oblique fold or notch (fig. 15 _á_ _a_), close by the umbo. This fold
has no direct relation to the males, but being present is taken
advantage of by them; for it occurs in the young hermaphrodite, before
the attachment of the males, and in species of the genus in which the
males are attached to other parts. It occurs, also, in fossil species of
Pollicipes, and in these it seems caused by the upper inner part of the
valve being rendered more and more prominent during growth: in the
present species, I suspect, its origin is connected with the formation
of a ridge bounding the outer side of the pit for the adductor scutorum
muscle: we shall see in the next species, that this fold is of the
highest importance in relation to the position of the Males. The
transparent chitine border of the scuta is broad, and fills up the fold
in the shell, so that the outline of the occludent margin is not
affected by it: in the drawing (fig. 9) some of the inner layers of
chitine (_e_ _e_), which dipped into and filled up the fold, have been
removed, that the lower part of the animal might be more plainly
exhibited. The chitine bears numerous spines of various lengths, which
must afford some protection to the males, rudely arranged in lines,
parallel to the edge of the valve, indicating the successively-formed
layers of chitine; each spine has a fine, tortuous tubulus connecting
its base with the underlying corium. The extreme outer edge of the
border is thin, forming a kind of lip, close beneath which the delicate
tunic lining the sack is attached. During continued growth, the valve is
added to in thickness, and so is the chitine border, and likewise in
breadth. It appears that the larva of the male must attach itself on the
under side of this border, on the edge of the tunic of the sack, and
that by the action of the cement, the corium beneath is killed (as I
believe always is the case with other parasitic Cirripedia), whereas on
both sides, the chitine continues to be added to, so that the male,
excepting the upper and always projecting portion, becomes imbedded at
first laterally, and ultimately all round: I have seen specimens in
several different stages of imbedment. Hence, in old specimens, with a
thick and broad chitine border, it might and does come to pass that one
male is imbedded (the valve being laid flat) directly beneath another.
I have examined a great number of specimens from various localities,
taken at different times of the year,--some dozen specimens from
Cornwall,[53] and several from unknown localities in various
collections; some from Ireland, from the Shetland Islands, from Norway,
and from near Naples. Every one of these specimens, with the exception
of some of the Neapolitan ones, had parasitic males attached to them: I
must also except very young specimens, on which they never occur. On a
Cornish specimen, with a capitulum a little more than one fifth of an
inch in length, it may be mentioned as unusual that there were three
males. In young specimens there is generally one male on each scutum,
but sometimes there are two, and sometimes none on one side. In large
old Cornish specimens I have counted on the two sides together, six,
seven, and eight males, and in one Irish specimen no less than ten,
seven all close together on one valve and three on the other, but I do
not suppose that all these were alive at the same time. In the
Neapolitan specimens, however, which are the largest that I have seen,
there was in no case more than two; and out of seven or eight
specimens, four had not any male; so that it would appear there is
something in this locality hostile to the development of the parasitic
males. I have noticed only one instance (that given in fig. 9) in which
the males were imbedded a little way apart; generally they touch each
other, and are cemented together: where there are several males, they
occur at different levels, as measured from the under or upper surface
of the chitine border: in one instance of four males adhering to one
valve, I distinctly perceived that the lowest one was white, pulpy, and
recently attached; the two above, which were placed close together and
between the same laminæ of chitine, were mature; and the third still
higher up, was dead, empty, transparent, and half decayed: in some other
instances, I have found the uppermost parasites dead, and, together with
the surrounding chitine, partially worn away.
[53] I am greatly indebted to Mr. Peach for his unwearied
kindness in procuring me fresh specimens. Mr. W. Thompson allowed
me to dissect one, possessing particular interest, out of his
three Irish specimens. Professor Forbes procured me a specimen
from the Shetland Islands, and Professor Steenstrup was so kind
to take pains to send me some Scandinavian specimens.
The larva of the male must have a different instinct from the larva of
the hermaphrodite; for the latter attaches itself head downwards to a
coralline, whilst the male larva crawling on the scuta of the
hermaphrodite, discovers, I presume by eye-sight, the fold in the shell
beneath the translucent border of chitine, and there invariably attaches
itself. Its object in choosing this particular spot, I believe, simply
is that the depth or thickness of the chitine is there greater, and
sufficient for its imbedment, which would hardly be the case elsewhere.
This parasite has, as we have seen, no mouth or stomach, and indeed,
considering its fixed position and the non-prehensile condition of its
limbs or cirri, a mouth would have been of no service to it, without it
had been extraordinarily elongated. The male must live on the
nourishment acquired during its locomotive larval condition; and its
life no doubt is short, but yet not very short, as I infer from the
depth to which mature specimens are buried in the chitine border. The
full development of the spermatozoa consumes, I suppose, some
considerable lapse of time. The thorax and limbs, though furnished with
muscles, are obviously, as already remarked, of no use for prehension;
these parts serve, probably, to defend the little creature, when its eye
announces the passing shadow of some enemy, and for this purpose they
are well adapted from the extreme sharpness of the spines. The thorax,
into which I traced the vesicula seminalis, no doubt also serves for the
emission and first direction of the spermatozoa; and hence, perhaps, its
singularly extensible structure. I have already remarked, that in
specimens preserved in spirits, the thorax is often largely protruded,
and bent down at right angles to the orifice. I presume this is caused
by endosmose; nevertheless it deserves notice, that it was in these
protruded specimens that the vesicula seminalis was most conspicuously
gorged with spermatozoa. I suspect the longitudinal and transverse
muscles lining the upper part of the outer integuments of the whole
animal, can be of little use to the creature, without it be to aid in
the protrusion of the thorax, and perhaps in the violent expulsion of
the spermatozoa, thus causing them to reach the ovigerous lamellæ within
the sack of the hermaphrodite. It is also probable, that the action of
the cirri of the hermaphrodite, would tend to draw inwards the
spermatozoa in the right direction. In one specimen, the spermatozoa in
the hermaphrodite and in the male were mature at the same time; in
another this was not the case; and as the males, apparently, become
attached at all periods of the year, this want of coincidence in
maturity must often occur. Can the males retain their spermatozoa, till
told by some instinct, that the ova in the sack of the often fecundated
hermaphrodite are ready for impregnation; or are the spermatozoa
sometimes wasted, as must annually happen with such incalculable
quantities of the pollen of many dioecious plants?