Sub-Class--CIRRIPEDIA.
_Crustacea attached by the anterior end of the head, by cement
proceeding from a modified portion of the ovaria; archetype composed
of seventeen segments, with the three first of large size, and almost
always developed into a carapace, not wholly exuviated, and capable of
various movements; antennæ none; eyes rudimentary; mouth prominent,
formed by the partial confluence of the labrum, palpi, mandibles, and
two pairs of maxillæ; thorax attached to the internal sternal surface
of the carapace, generally bearing six pairs of captorial, biramous,
multiarticulated limbs; abdomen generally rudimentary; branchiæ,
when present, attached to the under sides of the carapace; generally
bisexual, when unisexual, males epizoic on the female; penis single,
generally probosciformed, seated at the posterior end of the abdomen;
oviducts none; metamorphoses complex._
Within the memory of many living naturalists, Cirripedes were
universally looked on as belonging to the Molluscous kingdom; nor was
this surprising, considering the fixed condition of their shells, and
the degree of external resemblance between, on the one hand, Lepas and
Teredo, and on the other hand, between Balanus and a Mollusc compounded
of a patella and chiton. It is remarkable that this external false
appearance overbore, even in the mind of Cuvier, his knowledge of their
internal structure, namely, their lateral jaws, articulated appendages,
and regular ganglionic nervous system, which now strike us as such
conclusive evidence of their position in the great Articulate kingdom.
Straus[5] was, I believe, the first who, in 1819, maintained that
Cirripedes were most closely allied to Crustacea. But this view was
disregarded, until J. Vaughan Thompson's[6] capital discovery, in 1830,
of their metamorphoses, since which time, Cirripedes have been almost
universally admitted amongst the Crustaceans. It is well known, that
it is hardly possible to give a definition of this great class, which
shall include every member of it; nevertheless, even if the mature
Cirripede alone be considered, the following characters, viz. the
slight separation of the head and thorax, the latter generally bearing
six pairs of appendages, and the being enclosed in a carapace--together
with the periodical exuviation of the greater part of the external
membranes, would, perhaps, suffice to show that it should be classed
amongst Crustacea.
[5] Mémoires du Muséum d'Histoire Nat., tom. v, p. 381.
[6] Zoological Researches and Illustrations.
But it still remains undecided what rank in this class Cirripedes
should hold. Before briefly discussing this point, it is indispensable
to indicate their essential characters, which I will immediately
attempt. For as long as it remained doubtful which was their anterior
extremity, which the ventral or dorsal surface; as long as the peduncle
was thought by one naturalist to be the legs, by another the abdomen,
in a modified condition, it was hopeless to compare Cirripedes with
ordinary Crustaceans, and assign to them their due rank.
In the larva in the first stage, an eye and two pairs of antennæ are
in process of formation or are developed; here, then, according to
the analogy of all Crustaceans, we have evidence of the existence of
the first three cephalic segments. The mouth always consists of three
pairs of gnathites, and hence again, from analogy, this part may be
inferred to be formed of, and supported on, three other segments;
making thus far six segments. In two Orders out of the three into
which Cirripedes may be divided, namely, in the Abdominalia and Apoda,
eight quite distinct segments succeed the mouth; of these the first
differs slightly from the seven succeeding segments, and may, I think,
be safely considered as forming the seventh (cephalic) segment. The
next seven segments resemble each other in all essential respects, and
are no doubt the normal, seven thoracic segments. These, in both the
above orders, are succeeded by three smaller segments, which differ
in structure from the thoracic segments, and must be abdominal. Hence
we here have, altogether, seventeen segments. It should, however, be
observed that in the two orders just referred to, each includes only
a single species; but I know of no good reason why, on this account,
their structure should be valued the less. In the third order, the
Thoracica, which includes all common Cirripedes, two segments with
their appendages are missing out of the eight that should succeed
the mouth; from the open interval in the pupa, between the mouth and
first pair of natatory legs, and from some other reasons, I believe
that the two missing segments are the seventh and eighth, or last
cephalic and first thoracic segments, and that they have coalesced
close posteriorly to the mouth.[7] In the order Thoracica, the abdomen
is quite rudimentary, though often still bearing caudal appendages;
in the pupa, however, of this order, as in the mature animal of the
two other orders, it is formed of three segments. Hence I conclude
that, notwithstanding the absence of the above two segments with their
appendages in the Thoracica, the archetype Cirripede may be safely said
to be composed of seventeen segments.
[7] This question and the whole subject of the homologies of the
several parts of a Cirripede, will be discussed under the head of
the Metamorphoses of the Balanidæ.
In the classification of Crustacea, the relation and number of the
segments of the different parts of the body, are viewed both by Prof.
Milne Edwards[8] and Mr. Dana,[9] as of the highest importance.
I may premise that both these authors divide the Crustacea into
Podophthalmia, Edriophthalmia, and Entomostraca; Milne Edwards making
a fourth legion, the Branchiopoda, and another division, including
Limulus, of equal value to the above four legions altogether; whereas
Dana sinks Limulus and the Branchiopoda under his Entomostraca. As
far as concerns our present discussion on Cirripedes, the first
three divisions, as valued by Dana, will best serve as standards
of comparison; but it is not unimportant to our present purpose,
as showing how difficult it is to weigh the value of the higher
divisions of a Class, to observe the wide difference in opinion of two
naturalists, so eminent for their knowledge of the class in question
and for their high abilities.
[8] Annales des Sciences Nat., tom. xviii, p. 120, 1852.
[9] Crustacea: 'United States Exploring Expedition,' p. 1395, 1852.
In the order Thoracica, including all common Cirripedes, the
cephalic and thoracic segments are as much confounded together (but
with coalescence and abortion of two middle segments) as in most
Podophthalmia; but in the two other orders, the cephalic and thoracic
segments are as distinct as in the Edriophthalmia. The number of the
segments, however, which _strictly_ appertain to the anterior part of
the head and mouth, being only six, is an Entomostracan character; on
the other hand, the first pair of cirri in the Thoracica, has some
claim from their position, apparent functions, and separation from the
succeeding pairs, to be said to belong to the mouth; on which view, the
first nine segments would, in function, be cephalic, as in the highest
Crustaceans. The fewness of the segments of the abdomen, and their not
bearing in two of the orders appendages, is an Entomostracan character.
Cirripedes are permanently attached, even before their final
metamorphosis, by a tissue or cement, first debouching through the
second pair of antennæ, and, subsequently, in most cases, through
special orifices, penetrating the anterior part of the head; this
cement proceeds from glands, which certainly are modified portions of
the ovarian system. This fact I consider of the highest classificatory
importance, for it is absolutely the one single character common to all
Cirripedes, besides such as show only that these animals belong to the
articulated kingdom, and are Crustaceans. No structure of this kind has
hitherto been observed in any other member of the class or kingdom.
Even in the Suctorial Entomostracans, which become immoveably attached
to the fish on which they prey, the males are free; and the means of
attachment, as far as known, are quite different.
Both the first and second pairs of antennæ are absent in the mature
animal; for the three terminal segments of the antennæ of the pupa,
which may always be found cemented under the centre of the surface of
attachment, are functionless, after maturity. The eyes are rudimentary,
and are singular from being seated far from the anterior extremity of
the head. In their rudimentary state, and in the absence of antennæ, we
have characters common with certain Suctorial Entomostracans; and this
similarity apparently arises from the fixed condition of the animals
of both groups.
The carapace, which covers the dorsal surface of the larva in the first
stage, in the last larval or pupal stage is developed so as to enclose,
like a bivalve shell, the whole body. In the mature Cirripede, the
whole external covering, whether shell and operculum, or capitulum and
peduncle, can be conclusively shown to be the carapace of the pupa,
modified. In thus enclosing the mouth and whole body, the modified
carapace resembles that of several Entomostracans; but in being
apparently formed (as I hope hereafter to show) by the development
of the third segment of the head, and in consisting generally of
distinct sclerodermic plates, arranged in an imbricated order, there
is, I think, a closer resemblance to the same part in some of the
Podophthalmia. The carapace, or portions of the carapace, being capable
of other movements, besides merely opening and shutting, differs, I
believe, from that of all other Crustaceans; as it likewise does[10] in
the greater part not being periodically moulted.
[10] The carapace, however, of the Isaura, a Branchiopod, according
to M. Joly ('Annales des Sc. Nat.,' 2 ser. vol. xvii, p. 293), is
not moulted.
Moreover, in all Cirripedes there is another striking peculiarity
connected with these parts, namely, the exclusive attachment of the
whole thorax or included body to the internal ventral or sternal
surface of the carapace and head. In the pupa, the thorax, as in all
Crustaceans, opens into, and is continuously united with, the large
anterior part of the head; but from the singular fact that the thorax
of the young Cirripede is developed not within the thorax, but within
the head of the pupa (Pl. 30, fig. 2), with its longitudinal axis
placed at right angles to that which it held in the pupal condition
(the mouth and the whole exterior being developed conformably with
that of the pupa), it comes to pass after the metamorphosis, that the
Cirripede is, as it were, internally cut in twain (compare Pl. 25, fig.
1, and Pl. 30, figs. 2 and 3). Thus it is, as will hereafter be more
fully explained, that the sack originates, and thus the body becomes
attached to the internal ventral surface of the carapace and front of
head.
The thorax in two of the Orders bears no appendages, but in all common
Cirripedes it is furnished with six pairs of biramous, multiarticulated
cirri, which have a peculiar character, different from the limbs of
other Crustaceans, not being natatory, ambulatory, or branchial,
but "captorial" or fitted for sweeping the water, and thus catching
prey.[11] The cirri, at least the anterior pairs, can, besides other
movements, lengthen and shorten themselves; and this Milne Edwards[12]
states is the case with the Podophthalmia, and is considered by him
as an important character. The cirri of the first pair are attached
on each side close to the bases of the mandibles, and, as already
remarked, have some claim to be considered as maxillipeds or mouth
organs. The three or the four posterior pairs of cirri in the Balanidæ,
form a series somewhat distinct from the two or three anterior pairs,
thus recalling a characteristic feature in the Edriophthalmia.
[11] M. A. Hancock, in 'Annals and Magazine of Natural History,' 2d
series, 1849, p. 312, speaks of the cirri acting like a prehensile
net.
[12] 'Annales des Sciences Nat.,' tom. xviii, p. 121, 1852.
The mouth is prominent, and is formed by the partial confluence of the
labrum, palpi, and lower segments of the mandibles, and of two pairs of
maxillæ; it is capable of movement as a whole; in this respect we are
reminded of the Suctorial Entomostracans; but I believe the above type
of structure of the mouth is peculiar to Cirripedes.
The alimentary canal is simple, but can be distinctly divided
into--(1st) an œsophagus, singular from the bell-shaped expansion
of its lower end; (2d) the stomach, which is directed forwards and
then doubled back; and (3d) the rectum. There is no distinct liver.
The circulation is lacunal. In one family there are well-developed
branchiæ, which differ entirely in their homologies and position
from these organs in all other Crustaceans. In the nervous system,
the sub-œsophageal ganglions vary in concentration from that degree
observed in the lower Macroura, to that in the highest Brachyoura; but
the supra-œsophageal ganglions are always much less concentrated, and
are even embryonic in condition; presenting a difference not observed
in other Crustaceans. On the under side of the sub-œsophageal ganglion,
two nerves, apparently splanchnic, arise, and run almost parallel and
under the collar surrounding the œsophagus; they are very remarkable
from their great size, and from forming a plexus together with a
large branch, arising on each side from the collar close behind the
supra-œsophageal ganglion,--a structure unlike anything observed in
other Crustaceans. The eyes, as already remarked, are rudimentary, and
singular from being imbedded at a distance from the anterior end of the
animal. In the basal confluent segments of the outer maxillæ there are
two orifices, leading into little sacks, which I believe are olfactory
organs: again there are two other orifices on each side of the thorax,
beneath the first pair of cirri, leading into sacks, with a curious
elastic vesicle suspended within them; and these I can hardly doubt are
acoustic organs. Of these orifices and organs, there is no trace in the
same relative positions in any known Crustacean.
Cirripedes are ordinarily bisexual, in which they differ from all
Crustaceans: when the sexes are separate, the males are minute,
rudimentary in structure, and permanently epizoic on the females; to
these latter facts we have a partial analogy in some of the Suctorial
Entomostracans; but a far closer analogy in certain Rotifers, which are
considered by many naturalists as Crustaceans; but to the above subject
I shall almost immediately have to recur.
The male excretory organ is probosciformed and capable of the most
varied movements; it is single and medial; it is seated (in the one
instance in which this point can be safely judged of) at the extremity
of the abdomen, and therefore near the normal position of the anus;
in all these respects there is a very great difference from other
Crustaceans, in which the male organs are laterally double, and are not
seated at the extremity of the abdomen. In regard to the female organs,
the ovarian tubes and cæca inosculate together: there are no oviducts;
the ova, connected together by membrane, and so forming the "ovigerous
lamellæ," become exposed by the exuviation of the lining tunic of the
carapace or sack, and by the formation of a new tunic on the under side
of these lamellæ; a process, I believe, unknown in other Crustaceans.
The metamorphoses are highly complex. The larva in its first stage
bears a very close general resemblance, in having three pairs of
natatory appendages, the first being uniramous and the two others
biramous, and in having a single eye on its broad anterior front, to
the larvæ of most Entomostracans; but I cannot avoid the belief, that
this resemblance is only apparent, and not essential; and of false
resemblances, how many instances occur in the animal kingdom! In the
larva, when first freed from the egg, both pairs of antennæ are in
process of formation within envelopes: the mouth is probosciformed
and capable of movement, but is destitute of gnathites; it occupies a
position between the three pairs of natatory limbs; and these limbs I
must believe, for reasons hereafter to be assigned, answer (improbable
as I am well aware it must at first appear) to the second, third,
and fourth thoracic legs of the archetype Crustacean: the two hinder
pairs of limbs apparently soon become captorial, or fitted to secure
prey. Now, I cannot find in the published accounts of the larvæ of
Entomostracans, any that answer to this description.
The larva in the last stage might be included in the vast class of
Entomostracans: the attachment of the eyes to the singular apodemes
produced inwards from the basal segment of the great prehensile
antennæ, and the development of only the posterior six pairs of
thoracic limbs, are its chief peculiarities: but its rudimentary mouth,
owing to its transitional or pupal condition, renders the assignment of
its proper rank difficult.
* * * * *
Having now given this short comparative sketch of the structure of
a Cirripede, I may venture to express strongly my opinion, that the
group is formed on a distinct type; as different from the other
three or four main Crustacean groups, namely, the Podophthalmia,
Edriophthalmia, Branchiopoda, and Entomostraca, as these differ from
each other; the differences, moreover, being of the kind considered
by the highest authorities on this subject, as the most important. It
should be observed that there is no special blending at either end of
the Cirripedial series, towards any one of the other main groups of
Crustacea; it is hardly possible to take some one Cirripede, and say
that it leads, more plainly than some other Cirripede, into ordinary
Crustaceans. Moreover, a great range of structure, as we shall soon
briefly show, is included within the group: I can adduce three or four
undoubted Cirripedes, very considerably more different from each other,
than any two members within the sub-class Podophthalmia, or within the
Edriophthalmia, or the Branchiopoda, and quite as different as within
the Entomostraca.
The opinion here expressed, that Cirripedes form a sub-class of equal
value with the other main Crustacean groups, I am well pleased to find,
accords with Mr. Dana's[13] view, who remarks that this sub-class
"has so many peculiarities of structure, that it should be regarded
as a distinct type, rather than a subordinate division of the third
(or Entomostracan) type." M. Milne Edwards,[14] after dividing all
Crustacea into two groups, divides one of them into four legions;
and of one of these, the Entomostraca, he makes the Cirripedes a
sub-group. I feel so entire a deference for any opinion on affinities
or classification expressed by Milne Edwards, that I differ from
him with the greatest hesitation. He does not give his reasons for
assigning so subordinate a rank to Cirripedes, but I imagine it is from
the nature of their metamorphoses: but if this be the case, I cannot
understand why he should assign to his Branchiopods a rank equal to his
Entomostracans. Moreover, I must repeat, that I do not believe that
the larvæ do resemble the larvæ of Entomostracans and Branchiopods
nearly so closely as at first appears to be the case. I may add, that
Burmeister[15] has assigned to the Cirripedes a place amongst the
Crustacea, almost equally subordinate to that given to them by Milne
Edwards.
[13] 'Crustacea: United States Exploring Expedition,' p. 1407, 1852.
[14] 'Annales des Sciences Nat.,' tom. xviii, p. 120, 1852.
[15] 'Beiträge zur Naturgesehichte der Rankenfüsser,' 1834.
That Cirripedes have some special affinity to the Entomostraca, may
be inferred from the fewness of the cephalic appendages, the biramous
legs, the state of the abdomen, and the form of the carapace. Perhaps
in the peculiar state of confluence of the lower segments of the
gnathites, in the aborted antennæ, the rudimentary eyes, and in the
minute parasitic males (when such exist), there is a more direct
relation to the Suctorial division of the Entomostraca; but some of
these resemblances are probably only analogical, resulting from the
fixed condition of both groups. It should not be overlooked, that
out of the three orders into which Cirripedes may be divided, in
the two latter, the mature animal presents hardly any resemblance
to an Entomostracan. From the distinct presence in either pupa or
mature animal of the fourteen segments of the cephalo-thorax; from
the apparent composition of the carapace, as will be subsequently
explained; and from the concentrated condition of the nervous system,
one is led to glance at the higher Crustacea; and here we shall find
amongst the Podophthalmia, one aberrant group of low organisation,
namely, that including Phyllosoma, Amphion, &c., in which more points
of resemblance to Cirripedes may be detected, than, as I believe, in
any other group whatever; for we here see that remarkable elongation
of the head in front of the mouth, so eminently characteristic of
Cirripedes; we have a carapace overlapping the thorax, which is
sometimes free beneath; we have the abdomen sometimes almost obsolete;
we have biramous legs: and especially we have the posterior cephalic
and the first thoracic appendages more or less rudimentary and
obsolete; and this, I infer from Mr. Dana, is a very rare phenomenon,
though characteristic of all _ordinary_ Cirripedes, in which the
seventh and eighth segments with their appendages have disappeared.
In the order including Phyllosoma, &c., namely, in the Macroura, the
ganglions which give nerves to the five posterior thoracic limbs, are
distinct from the great sub-œsophageal ganglion which supplies the
several anterior appendages; this is the case with those Cirripedes in
which all the infra-œsophageal ganglions are not concentrated into one.
In the Macroura and Brachyoura, the first pair of legs almost always
differs in structure from the others, so does the homologous or second
cirrus in Cirripedes differ from the four succeeding pairs; in some
few Macroura, the second leg is antenniformed, so in some few cases is
the homologous (or third) cirrus; J. Vaughan Thompson was even struck
by the resemblance in the curious, doubly pectinated spines on the
anterior limbs of Mysis (allied to Phyllosoma[16]), and on those of
many Cirripedes: these several latter resemblances may be small, but
certainly I do not believe that they are accidental. Now the little
group of Crustaceans, which includes Phyllosoma, &c., has lately
been placed, by Milne Edwards, as a satellite amongst the Macrourous
Podophthalmia; it leads into the Stomopoda, and likewise, as has been
noticed by many authors, into the sub-class Branchiopoda, which latter
sub-class is considered by Mr. Dana as only a part of the Entomostraca;
this group, therefore, exhibits affinities radiating in several
directions, and amongst these lines of relationship, one more must, I
believe, be added, plainly directed towards the Cirripedia.
[16] M. Martin St. Ange ('Mémoire sur l'Organ. des Cirripèdes,'
1835, extrait des 'Savans Etrangers,' tom. vi) has compared the
mouth of Lepas with that of Phyllosoma, and has given comparative
figures; but the resemblance is founded, I believe, on quite false
homologies.
One naturally wishes to ascertain how far Cirripedia are highly or
lowly organised and developed; but in all cases this, as it seems to
me, is a very obscure enquiry. Mr. Dana considers that, in Crustacea,
the greater or less centralisation of all the appendages round the
mouth is the main sign of high development; on this view, the anterior
part of a Cirripede, from being so much elongated, must be considered
as very low in the scale; the whole posterior part of the body, on the
other hand, is, in ordinary Cirripedes, brought close to the mouth; but
this is effected by the abortion of the seventh and eighth segments of
the cephalo-thorax and of the whole abdomen, and so, I presume, would
not, in Mr. Dana's estimation, raise the class much in the scale. Von
Baer[17] considers that the perfection of the type of any animal is in
relation to the amount of "morphological differentiation" which it has
undergone; on this view, Cirripedes ought to stand high in the scale,
for they differ much morphologically from the type of the class to
which they belong; as indeed is shown by the long time that elapsed
before their true position, namely amongst the Crustacea, was even
suspected; but something more must, I think, be added to Von Baer's
definition; for, to take as an example the eyes of a Cirripede,--as
seen in the first larval stage, there is only one eye, and that most
simple; in the pupa there are two, both compound, and furnished with
complicated muscles; lastly, in the mature animal there are still
two, but of very minute size, often almost confluent, and of the
simplest structure; hence, then, there has been much morphological
differentiation, but it is almost a contradiction in terms to speak, in
relation to such a case, of _perfection_ of type; and what has happened
to one organ, might happen to other organs, and so to the whole animal.
Lastly, under a physiological point of view, and taking the Balanidæ as
the most perfect type of the class, the sub-œsophageal portion of the
nervous system is highly concentrated; the organs of sense, excepting
the eyes, seem more largely developed than in ordinary Crustaceans; the
circulating system is of the simplest kind, being only lacunal; special
Branchiæ, however, are developed by the metamorphosis of, as I believe,
a special organ, occurring only in the Lepadidæ; the digestive organs
are very simple, from not having any distinct liver; the generative
system is very low, for both sexes are generally united in the same
individual; and the testes and ovaria closely resemble each other.
On the other hand, the thoracic limbs are, to a considerable extent,
specialised in their structure and functions; only the three posterior
pairs strictly resembling each other. Lastly, the dermal and muscular
systems are complicated, and not, to use Professor Owen's term, by mere
vegetative repetition, as will be obvious to any one who will study the
beautifully constructed and modified carapace--that is the operculum,
shell and basis--of a Balanus. On the whole, I look at a Cirripede
as a being of a low type, which has undergone much morphological
differentiation, and which has, in some few lines of structure, arrived
at considerable perfection,--meaning, by the terms perfection and
lowness, some vague resemblance to animals universally considered of a
higher rank.
[17] English Translation, in 'Scientific Memoirs,' 1853, vol. i, p.
228.
* * * * *
It has been seen that I divide the Cirripedia into three orders,--the
Thoracica, Abdominalia, and Apoda; between which the fundamental
difference consists in the limbs or cirri being thoracic in the first,
abdominal in the second; and entirely absent in the third. For the sake
of showing the range of character in Cirripedes, to which allusion has
been made, I will briefly indicate the leading differences in each
order. In the Thoracica, three families are included,--the Balanidæ,
or sessile Cirripedes, the Verrucidæ, remarkable from their quite
asymmetrical shell, and the Lepadidæ, or pedunculated Cirripedes. The
great difference in external appearance between these three families is
known to all naturalists. Even within the one family of Lepadidæ there
are great differences in external appearance, as will be admitted on
comparison of Lepas, Pollicipes, Conchoderma, &c.; but we have also
important internal differences, as in the case of Anelasma, in which
the cirri are barely articulated, and are not capable of seizing prey,
whilst the mouth is almost probosciformed, with the outer maxillæ
and palpi rudimentary: still more important are the differences in
Alcippe, in which the cirri of the first pair act as brushes; the
second, third, and fourth pairs being quite aborted; and the fifth and
sixth pairs consist only of four segments, with one of the two normal
rami converted into a crenated, button-like projection, for the sake
apparently of triturating food; Alcippe, also, is very remarkable in
being destitute of a rectum and anus. In this same genus Alcippe, in
Ibla and Scalpellum, there are either separate males or Complemental
males, some of which are so utterly abnormal in their characters, that
by no definition which I could frame, could they be included even in
their proper Order, much less in their proper Family.
In the second order of Abdominalia (Pl. 23 and 24) the seventh or last
cephalic segment is quite distinct, and bears rudimentary organs,
answering to the first pair of maxillipeds of ordinary Crustaceans, of
which organs, and of the segment supporting them, there is no trace in
the Thoracica: the seven succeeding thoracic segments are destitute
of any appendages; but the three segments of the abdomen bear three
pairs of cirri. The mouth is peculiar in the labrum being developed
into very large, moveable, lancet-formed organ; and the lower end of
the œsophagus is armed with beautiful discs of teeth, and brushes of
hairs,--a structure confined to this order. The male resembles the
male of Alcippe; and the latter genus seems to be the connecting link
between the Thoracica and Abdominalia. But the most important character
of this latter order, in which it differs from Alcippe, and all other
known Cirripedes, is in its metamorphoses; all the first changes are
merely indicated by changes in form in an egg-like larva, without the
development of distinct organs; and the last, or pupal condition, which
is attained within the sack of the parent, is very peculiar, by the
entire absence of natatory limbs.
The third order of Apoda is the most peculiar of all; it contains,
like the last, only one known species: the most acute naturalist, I
am convinced, if he had not made the class his special study, would
never even have suspected that this animal was a Cirripede. We see
much magnified in Pl. 25, fig. 7 a naked, plainly-articulated animal,
resembling the larva or maggot of a fly, attached by two threads;
and these threads, on analysis, can be clearly shown to be the last
rudiment of the carapace, specially modified. The last cephalic, the
seven thoracic, and the three abdominal segments, are all equally
destitute of appendages. The mouth is suctorial, and constructed on a
plan unlike, I believe, anything known in the articulate kingdom; for
the mandibles and maxillæ have rotated on their axes, and stand back
to back; they can act only by tearing open a slit, and this action is
performed in a hood, formed by the confluence of the broad palpi and
labrum. Although the œsophagus is distinct, there is no stomach or
anus. Lastly, owing to there being no carapace, the ova are developed,
differently from in all other Cirripedes, within the thorax.
I will close this preliminary discussion on the confines and type of
the sub-class, by recalling attention, now that a sketch has been given
of the three Orders, to the remark before made, that a wide range of
structure is included within it, and by reurging that the Cirripedia
should be ranked, not as one of the subordinate groups, but as one of
the main divisions of the Crustacea.
_On the Sexual Relation of Cirripedes._
Cirripedes are commonly bisexual or hermaphrodite, but in Ibla,
Scalpellum, and Alcippe, members of the Lepadidæ in the order
Thoracica, and in Cryptophialus in the order Abdominalia, the sexes
are separate. As two of these genera were described in my former
volume, and two others (Alcippe and Cryptophialus) are described in
this volume, I may as well here give a brief summary of the facts as
yet known on this very curious subject. The Males, in the above four
genera, present a wonderful range of structure; they are attached in
the usual way by cement proceeding from the not-moulted antennæ of the
pupa, to different parts, in the different species, of the female.
These males are minute, often exceedingly minute, and consequently
generally more than one is attached to a single female; and I have
seen as many as fourteen adhering on one female! In several species
the males are short-lived, for they cannot feed, being destitute of a
mouth and stomach. As the females are longer lived, successive crops
of males, at each period of propagation, become attached to her.
It is the females in the above genera which retain the characters
of the genus, family, and order to which they belong; the males
often departing widely from the normal type. Some of the males are
rudimentary to a degree, which I believe can hardly be equalled in the
whole animal kingdom; they may, in fact, be said to exist as mere bags
of spermatozoa. So widely do some of them depart in every character
from their class, that twice it has happened to me to examine specimens
with a little care, and not even to _suspect_, until a long period
afterwards, that these males were Cirripedes.[18]
[18] In my volume on the Lepadidæ (p. 200) in searching for
analogies for the permanently epizoic and rudimentary condition
of the male Cirripedes, I quoted two cases, which I believe are
now known not to be analogous; namely, the _Syngamus trachealis_
of Von Siebold, and the worm-like Hectocotyle, which latter was
quite lately supposed to be a male Cephalopod, but has now been
ascertained to be only one of the arms of the male wonderfully
adapted and organised as a sperm-receptacle. The Asplanchna, the
mouthless male of a Rotifer, (p. 292) alone remains for me.
In _Scalpellum Peronii_, and _villosum_, the males are but little
abnormal, for if classified independently of their sexual relations,
they would be considered as immature specimens of a new genus, standing
next to Scalpellum; in _Scalpellum rostratum_, the male would form
another and rather more distinct genus. The males, in the latter, are
attached to the other sex, between the basal edge of the labrum and
the adductor scutorum muscle; but in _S. Peronii_ and _villosum_ they
are attached lower down, in the furrow between the two scuta, and are
thus protected: in these three species, the internal parts of the male
present nothing particular. In Ibla, the males are attached low down
within the sack of the female; they may be said to consist of a mouth
surmounted on a long peduncle, for there is no capitulum or general
covering, and the whole thorax is in a rudimentary condition, the cirri
being reduced to two distorted pairs. As these males certainly moult
several times and grow a little, they must feed; and as they have no
cirri fit for action, they must seize their food by the contortions of
their peduncle, which we know homologically consists of the three first
segments of the head. The movements of the peduncle must, also, supply
those of the probosciformed penis, almost invariably present with other
Cirripedes, but here absent. If compelled to class these males without
regard to the female, great difficulty would be experienced; we could
hardly place in the family of the Lepadidæ, a Cirripede without a
capitulum, and without cirri, those very organs which give their name
to the class, and with a thorax reduced to the dimensions of a lower
lip; yet, if the presence of a peduncle did determine the classifier to
place these males amongst the Lepadidæ, then undoubtedly the character
of the mouth, &c. would fix their position next to Ibla.
The males of _Scalpellum vulgare_, _ornatum_, and _rutilum_, resemble
each other in all essential points, and differ wonderfully in
appearance and structure from all ordinary Cirripedes. They consist of
a minute flattened bag with a small orifice at the summit, and at the
lower end attached by the cemented pupal antennæ. On each side of the
orifice, there is a pair of calcareous beads, representing the two
scuta and two terga of ordinary Cirripedes; and between the scuta a
minute black eye is generally conspicuous. In _S. ornatum_ the beads, I
may remark, on the two sides are not equal; those either on the right
or on the left side, being larger than those on the opposite side, so
that the animal externally is asymmetrical. Inside, within a tubular
sack, the thorax is lodged, supporting four (instead of six) pairs
of limbs; and these, instead of forming biramous, multiarticulated,
captorial cirri, are reduced to almost a rudiment, supporting a few
long sharp spines, which apparently act only as defensive organs. At
the end of the thorax there is seated a large abdominal lobe, which
does not occur in the other sex. Hence the thorax, though rudimental,
has been specially modified. Of the mouth and stomach there is not a
vestige. Constructed as these males are, assuredly they have no claim
to be ranked amongst the Lepadidæ or pedunculated Cirripedes; nor is it
possible to class them in any group whatever of ordinary Cirripedes.
In _S. vulgare_ the males are attached, often several together, to the
extreme edges of the two scuta, and therefore immediately over the
orifice leading into the sack; in _S. rutilum_ and _ornatum_, they are
attached in concavities on the under side of both scuta, just above the
depression for the adductor scutorum muscle. In the former of these
species, the pit for the reception of the male is formed by shelly
matter not having been deposited over a certain space on the under side
of the valve; and the pit is converted by a covering of membrane into
a pouch. As there are two scuta so there are two pouches, in each of
which a male is lodged; hence, according to the Linnean nomenclature,
_Scalpellum ornatum_ may be said to belong to Diandria monogynia. As
these males, from being mouthless, soon die, they are succeeded by
successive pairs; the pupa being led by a wonderful instinct to crawl
into the pouch, and there undergo its metamorphosis.
Lastly, the males of Alcippe and Cryptophialus (Pl. 23, fig. 19, and
Pl. 24, fig. 19) are remarkable for their similarity to each other,
considering the essential dissimilarity of the two females. The females
live in cavities which they excavate in the shells of Molluscs, and
within which they are attached by a horny disc; this disc is the only
part of the outer integument which is not frequently moulted, and,
apparently in consequence, the males are attached to its edges. It
results from this position, that the males are protected by being
enclosed within the cavity excavated by the female; and it further
results, that the males are attached at a considerable distance from
the orifice of the sack of the female, into which the spermatozoa
have to be conveyed; and to effect this, the probosciformed penis is
wonderfully developed, so that in Cryptophialus, when fully extended,
it must equal between eight and nine times the entire length of the
animal! These males, like those last mentioned of Scalpellum, consist
of a mere bag, lined by a few muscles, enclosing an eye, and attached
at the lower end by the pupal antennæ; it has an orifice at its upper
end, and within it there lies coiled up, like a great worm, the
probosciformed penis, and beneath it a single testis, with a single
vesicula seminalis. These organs complete the whole organisation of the
male; for there is no mouth, no stomach, no thorax, no abdomen, and
no appendages or limbs of any kind. Yet all these parts are present
in the female. I know of no other instance in the animal kingdom
of such an amount of abortion. The whole exterior of these males
evidently is composed, as in all ordinary Cirripedes, of the three
first cephalic segments; of the fourteen succeeding segments of the
archetype Cirripede we have not a vestige, excepting the probosciformed
penis, which, from analogy, should arise from the ventral apex of
the seventeenth segment, the first three segments of the head being
counted in the seventeen. Here, then, fourteen out of seventeen
segments have aborted, the tip of the seventeenth having coalesced with
the third cephalic segment! I am tempted just to notice the case of
Proteolepas, in the order Apoda, as showing, within the limits of the
same sub-class, a wonderful amount and diversity in abortion; for in
Proteolepas, the three anterior cephalic segments are reduced to the
merest rudiment, encasing the cement-ducts, the fourteen succeeding
segments being unusually well developed; whereas in the above described
males, we have just seen the three anterior segments fully developed,
whilst the fourteen succeeding segments are lost or have coalesced
with the others; so that within the same sub-class all seventeen
segments of the archetype have almost disappeared.
It may be asked how I know that the several above described rudimentary
epizoons are really the males of the Cirripedes to which they are
attached. Even if the whole course of the metamorphoses had not been
known in three of the cases, the mere fact of these epizoons being
cemented by the three terminal segments of their peculiar, pupal
antennæ, would have been sufficient to have shown that they belonged
to the class of Cirripedes. In nearly every case, I was able to
demonstrate, and not in one or two but in many specimens, that these
epizoons were males; and as in several instances the spermatozoa
were developed, and as, notwithstanding, in no instance was there a
vestige of ova or ovaria, it may safely be concluded that they were not
hermaphrodites, and therefore required females of some kind. If these
epizoic Cirripedes had been independent animals, as they all belong
to the same sub-class, and all have such peculiar habits, it might
have been expected that they would have shown some special affinity
towards each other; but this is not the case; the epizoon of Ibla is
more nearly related to Ibla, and the epizoon of Scalpellum more nearly
related to Scalpellum, than are these epizoons to each other. If the
several epizoons were classed by themselves, they would be grouped in
divisions, corresponding with those of the Cirripedes on which they are
attached, which is just what might have been expected if these latter
were their females. There are, also, many special relations between
the male epizoons and the Cirripedes to which they are attached; thus,
the mouth of the epizoon of Ibla, is so like the mouth of Ibla, which
is peculiar in several respects, that I should easily have recognised
it as belonging to a member of that genus. _Scalpellum villosum_ is
remarkable as one out of only two or three members of the whole Family,
which is destitute of caudal appendages, so is its male epizoon; again,
_S. villosum_ is unusually spinose, so is its male epizoon; on the
other hand, _Scalpellum ornatum_ is remarkably smooth, so is its male
epizoon; and I could give other similar instances. Will it be believed
that these coincidences are accidental, and that the epizoons have no
special or sexual relation to the Cirripedes to which they are attached?
One other instance of coincident structure is so important, that it
must, even in this sketch, be noticed; the prehensile antennæ of
the pupa are most important and complicated organs, and differ in
the different genera of the same family; they are preserved in a
functionless condition throughout life, and in two instances I was able
accurately to compare these organs in the epizoon and in the Cirripede
to which it was attached, and they were identical in every particular.
The full force of the excessive improbability of this resemblance,
and of the above coincidences in structure, on the supposition of the
epizoon and its support not being sexually related, will hardly be
perceived without referring to the facts given in detail in my former
volume.
Lastly, in the case of Cryptophialus (and indirectly in that of
Alcippe) the nature of the male epizoon is, I think, actually
demonstrated; for I traced both it and the female or ordinary form of
Cryptophialus, through the same several larval stages, from the egg,
enclosed within the sack of the female, to the pupa and mature animal.
Moreover, if the male nature and sexual relation to the supporting
Cirripede, be admitted in any one of these epizoons, then so close
is the agreement in habits, and to a certain extent in structure,
in all the foregoing epizoons, that probably no one admitting one
instance would dispute the others, and further evidence would even be
superfluous. Indeed, had it not been for the following facts, I should
not have brought forward, either here in abstract, or in other places
in detail, so many arguments and so much evidence.
I have as yet not entered in detail on the sex of the supporting
Cirripede: in Cryptophialus, Alcippe, and in one species of Ibla, I
was able to demonstrate in many specimens, that all the male organs,
internal and external, were entirely absent; and consequently that
these Cirripedes were _exclusively_ female. In _Scalpellum ornatum_,
also, there is no trace of external male organs (the state of the four
dried specimens not allowing the internal organs to be examined), and
there cannot be any reasonable doubt that this species likewise is
exclusively female. It should be borne in mind that the male organs,
external and internal, are most easily discovered, and that in the
above cases I had an abundant supply of excellent specimens. On the
other hand, in _Ibla Cumingii_, and in four species of Scalpellum, I
was able to demonstrate in the supporting Cirripede the presence of
all the male organs, as well as of the female; and in the vesiculæ
seminales of several specimens, both in the Ibla and in _Scalpellum
vulgare_, spermatozoa were contained; the male organs, however, not
being very amply developed. These species, consequently, are not
exclusively female, but are hermaphrodite, though having male epizoons
attached to them. This statement, I am well aware, is enough, at first,
to cast a doubt on all that I have said; but let any one reflect on
the evidence, of which I have here given a summary, and which has
been elsewhere given in full, and I think he must admit that at least
those epizoons which are exclusively male, and which are attached
to Cirripedes exclusively female, are sexually related and form one
species; but if he admit this, he cannot possibly escape from the
conclusion that some of the other epizoons, for instance that of _Ibla
quadrivalvis_, are the males of the hermaphrodites to which they are
attached,--these epizoons not exclusively impregnating the ova of a
female, but aiding the self-impregnation of an hermaphrodite. Hence I
have called these males _Complemental Males_, to show that they do not
pair with a female, but with a bisexual individual. Nothing strictly
analogous is known in the animal kingdom, but amongst plants, in the
Linnean class, Polygamia, closely similar instances abound.
In the series of facts now given, we have one curious illustration
more to the many already known, how gradually nature changes from one
condition to the other,--in this case from bisexuality to unisexuality.
Finally, in the four genera so often named, we meet the following
several cases, some of them even the most diverse, occurring in closely
allied species. (1st), a female, with a single male (rarely with two)
permanently attached to her, protected by her, and capable of seizing,
by the movements of its pedunculated body, any minute animals or
substances found within her sack; (2d), a female with successive pairs
of short-lived, mouthless males, inhabiting pouches on each side under
her scutal valves; (3d), a female with many, in one instance fourteen,
short-lived males, destitute of mouth, thorax, and appendages, but
furnished with a stupendously long male organ, attached to a thickened
portion of her outer integuments, but lying within the cavity which
she has excavated; (4th), an hermaphrodite with a male attached within
the sack, capable of feeding itself, as in the first case; (5th), an
hermaphrodite with from one to three males, organised like ordinary
Cirripedes, and apparently capable of seizing prey in the common way;
and attached between the scuta, and thus protected; (6th and lastly),
an hermaphrodite with from one or two up to five or six, short-lived,
mouthless males, like those in the second case, attached in one
particular spot, on each side of the orifice leading into the sack.