Fig. 3, Carina, serving in the Chthamalinæ, also, as a rostrum.]

Now, the compartments in the shell of every sessile Cirripede, are

without exception constructed on the above three simple patterns. In

number, they are 8, 6 or 4, or all confluent together.

Considering this simplicity in growth and form of the separated

compartments, it seems at first surprising that the construction

and enlargement of the whole shell in Balanus, should long have been

viewed as a difficulty. But the radii, from growing against rectangular

indentations, or rather furrows, in the opposed compartments, come to

be set a little inwards; and their external surfaces assume a very

different appearance from the wall-portions of the compartments,

which grow against the surface of attachment. In different species,

the summits of the radii (and of the alæ) grow either very much more

obliquely than in the species figured, or more squarely--that is,

they extend from tip to tip of the adjoining compartments, parallel

to the basis. In this latter case, and when the surfaces of the radii

differ considerably in appearance from the walls, as in _Balanus

tintinnabulum_ (Plate 1), I am not at all surprised that the radii

should have been described as separate elements, and called "areæ

interjectæ," or "compartments of the second order:" for the shell of

this Balanus seems to be composed of six wedges with their points

upwards, namely, the parietal portions of the compartments, and of

six other narrower wedges, the radii, with their points downwards;

and the fact that these latter wedges consist simply of the sides

of the parietal portions, modified by growing against the adjoining

compartments, is completely masked. I should add, that sometimes the

radii are not developed, which simply means that the overlapping

lateral edges of the compartments have not been added to during growth.

The alæ are originally developed at the period of the metamorphosis,

as slight lateral protuberances in the upper part of the compartments;

from being overlapped, and therefore not exposed to external

influences, and from growing (as in the case of the radii) against

rectangular indentations or furrows in the adjoining compartments, they

generally assume an extremely different appearance from the parietes,

and might naturally be thought to have a very different nature. But

the alæ in all cases (as is obvious in Pachylasma) are nothing but

the protuberant lateral edges of the compartments, rendered thin and

modified during growth. In order that the margins of the alæ should be

received in an indentation, the upper internal surfaces of the walls

of the recipient compartments are thickened all round, excepting where

they receive the alæ. This thickened, upper, internal portion of the

walls or shell, together with the alæ themselves, form the part called

the sheath. The sheath sometimes blends insensibly into the lower parts

of the compartments, and then perhaps it would not be thought to be a

distinct element; but often it is abruptly separated by an overhanging

edge (see Pl. 9, fig. 5 _b_, 9 _b_; Pl. 20, fig. 1 _b_; Pl. 25, fig. 1,

K′) from the lower part, and then the sheath greatly complicates the

internal appearance, but not the essential structure of the shell. The

sheath acts beautifully, like an internal hoop, in strengthening the

shell round the orifice, where it is naturally weaker than at the lower

or basal end, where it adheres to the surface of attachment: in the

upper part of the shell, moreover, the sutures between the compartments

do not go straight through, but owing to the alæ projecting and being

overlapped, are extremely oblique; or the joints, in the language of

carpenters, may be said to be broken.

There is one other point of structure in the shells of the Balanidæ,

more especially of species like _Balanus tintinnabulum_, which adds to

their apparent complexity, namely, that the rim or orifice of the shell

formed by the upper ends of the compartments, projects considerably

above the opercular valves. In a young Balanus, immediately after the

metamorphosis, the operculum is attached by the opercular membrane all

round to the summits of the compartments, and there cannot be said to

be any orifice to the shell itself, but only an orifice or slit between

the opercular valves; but during growth, as the compartments are added

to at their basal edges, their upper ends are deserted, and cease to

enclose the sack, within which lies the animal's body. Hence the upper

ends come to project freely, either quite separately as in some species

of Pollicipes, where they cannot be said to form an orifice; or more or

less united into a ring so as to form an orifice, as in the different

species of Balanidæ. It follows, that to understand the real shape of

a Balanus, or rather of the cavity enclosing the animal's body, all

that part of the shell which projects above the opercular membrane,

may, in imagination, be removed as something extraneous, like so many

spines; not that I mean to say that these points of shell are dead; on

the contrary, they are often porose and penetrated by numerous threads

of corium. This upper part of the shell, thus produced so as to form

an orifice, no doubt serves to protect the less strong and moveable

operculum.

[Illustration: Fig. 4. Octomeris.

Fig. 5. Chthamalus.

Fig. 6. Chamæsipho.

Fig. 7. Balanus.

Fig. 8. Tetraclita.

_a_, Rostrum; _b_, Rostro-lateral, _c_, Lateral, _d_, Carino-lateral

compartment; _e_, Carina.

Horizontal sections through the Shells of the principal genera of

Balanidæ, showing the arrangement of the Compartments. Genera 4, 5, and

6 belong to the Chthamalinæ; 7 and 8 to the Balaninæ.]

_Number and Arrangement of the Compartments._--I have already stated

that the shell, in every one of the Balanidæ, consists of eight, six,

or four compartments, or of all fused together into a single piece;

and that the compartments themselves are all constructed on the three

simple patterns of which woodcuts (figs. 1, 2, 3) have been given.

They are arranged in a certain definite order. The type arrangement is

found amongst the Chthamalinæ, as might have been expected, inasmuch

as this sub-family is so closely related to the ancient genera

Pollicipes and Scalpellum, whence all the Thoracic Cirripedia may be

said to radiate. In Octomeris (fig. 4) the type-arrangement of the

compartments, eight in number, is well shown; the rostrum and carina

resemble each other, and have alæ on both sides, and therefore are

overlapped on both sides: the rostro-lateral compartments have radii

on both sides, and therefore overlap the adjoining compartments on both

sides; the lateral and carino-lateral compartments have radii on their

carinal, and alæ on their rostral sides; and therefore overlap on one

side, and are overlapped on the other side. Now the shell of every

other sessile Cirripede differs, I believe, from that of Octomeris,

only in the fusion together or abortion of some of the eight normal

compartments: in one genus, however, Catophragmus, outer whorls of

small compartments, arranged like the lower valves in the capitulum

of Pollicipes, are superadded. The genus Chthamalus (fig. 5) differs

from Octomeris only in the carino-lateral compartments being aborted,

(as will presently be discussed), and hence has six compartments.

Chamæsipho (fig. 6) differs from Chthamalus only in the rostro-lateral

and lateral compartments being fused together; and hence has only four

compartments. In Balanus (fig. 7) and the whole sub-family of the

Balaninæ, the rostrum is compounded of the true rostrum, as seen in the

type Octomeris, and of the two rostro-lateral compartments; hence this

compounded rostrum has radii instead of alæ on both sides, and there

are only six compartments. Tetraclita (fig. 8) and Elminius differ

from Balanus only in having the carino-lateral compartments absent, and

probably aborted; hence there are only four compartments. Lastly, in

Pyrgoma, all the compartments are blended together into a single piece.

In Pollicipes, the old type-form of the whole order, and in Scalpellum,

we have four valves, (answering to the operculum), surrounding the

aperture leading into the sack, and the valves below are arranged in

successive whorls, with a strong tendency to alternation. For, the

rostrum alternates with, that is faces the interval between, the two

scuta; the carina alternates with the two terga; and the upper lateral

valves alternate with the scutum and tergum on each side. These four

valves, namely, the carina and rostrum, which resemble each other

in structure, and the pair of upper latera, which are larger than

the other lateral valves, together form the uppermost whorl, or that

beneath the scuta and terga. In the next whorl we have the rostro-

and carino-lateral valves, alternating with those above them; and

beneath them there are generally other valves, which decrease in size

and display the same tendency to alternation. The valves here just

specified, namely, the rostrum, carina, and three pairs of lateral

valves, in the Lepadidæ, are so much larger, and are so much more

commonly present, than the other valves of the capitulum, that to

them alone I affixed special names. Now if amongst sessile Cirripedes

we look to that genus, viz., Catophragmus, which comes in its whole

structure the nearest to Pollicipes, one of the Lepadidæ, we find (as

in fig. 4), firstly, a rostrum and carina resembling each other, and

a pair of lateral compartments, larger than the other lateral pairs;

these four valves alternating with the opercular valves: and, secondly,

we find, but forming part of the same whorl, a pair of rostro-lateral

and a pair of carino-lateral compartments, which, just as in

Pollicipes, are larger than the exterior and lower valves. These lower

little valves, I may remark, decrease in size in the successive whorls,

and tend to alternate in position, just as in Pollicipes. Observing

these several striking points of correspondence in the valves, (and

indeed in the whole structure), of Catophragmus and Pollicipes, one

is strongly inclined to suspect that in Catophragmus, and therefore

in Octomeris and other sessile Cirripedes, although the rostro- and

carino-lateral compartments appear to lie in the same whorl with the

rostrum, carina, and large lateral compartments, yet that they really

belong, as in Pollicipes and Scalpellum, to a lower whorl. Now if a

very young shell of Balanus, immediately after the metamorphosis, be

examined, the carino-lateral compartments will be found not to have

been developed; they first appear after two or three zones of growth

have been added to the other compartments; bearing in mind that in

Pollicipes and in Catophragmus the lower whorls are added successively

during growth, we find in this fact strong confirmation of the view

that the carino-lateral compartments normally belong to a whorl beneath

that including the rostrum, carina, and lateral compartments. Whether

the rostro-lateral, like the carino-lateral compartments, are developed

subsequently to the others, I have had no opportunity of ascertaining,

and therefore cannot confirm the above analogical conclusion, namely,

that they, also, belong to a lower whorl.

In the sub-family Balaninæ, which includes Balanus (woodcut 7), and

Tetraclita (woodcut 8), the shell is characterised by not having

rostro-lateral compartments, and by the rostrum having radii: now in

_Pachylasma giganteum_, which undoubtedly belongs to the sub-family

Chthamalinæ, at a very early age the rostro-lateral compartments become

blended with the true rostrum, making a compound rostrum, exactly like

the rostrum in the Balaninæ; distinct evidence of a similar fusion

is retained throughout life (Pl. 15, fig. 1) in all three species of

Chelonobia, which is undoubtedly a member of the Balaninæ. Hence, I

think, we may conclude that in all the genera of the Balaninæ the

rostro-lateral compartments are probably not aborted, but are blended

with a normal rostrum (resembling that in woodcuts 4, 5, 6), making

together a compound rostrum furnished with radii: it must, however, be

observed that I could not detect any actual evidence of this fusion

in Balanus, even immediately after the metamorphosis. In Chamæsipho

(woodcut 6), either the rostro-lateral compartments attain a most

unusual breadth, or, as is more probable, they have become confluent

with the lateral compartments, which in the Lepadidæ seem to be the

most persistent of all the lateral valves. In such genera as Tetraclita

and Chthamalus, in which the carino-lateral compartments are absent,

they may be fused with the lateral compartments or with the carina; but

seeing that they are normally developed later than the other valves,

it appears to be the simplest theory to assume, until the contrary

be proved, that they are aborted. Finally, the somewhat unexpected

conclusion that the shell (not including the operculum) of sessile

Cirripedes normally consists of eight valves, four belonging to an

upper whorl, and four to a lower whorl, all forced into a single ring,

and often more or less fused together, though not strictly proved,

is rendered highly probable. I will only further add, that the Basis

perhaps represents several whorls of the small valves or scales on the

peduncle of Pollicipes, fused together; the comparison of the basis

with the calcareous cup, forming the lowest portion of the peduncle in

Lithotrya, which has been made by some authors, I do not think is very

accurate, as the cup in Lithotrya seems to have a special relation to

the boring habits of that genus.

[Illustration: Fig. 9.

Basal edge of wall of compartment in _Balanus tintinnabulum_; _a_, _a_,

outer lamina; _b_, _b_, inner lamina; _c_, _c_, longitudinal septa

uniting the inner and outer laminæ with their ends denticulated.]

_Structure of the Individual Compartments._

If the basal margin of a compartment, for instance, of _Balanus

tintinnabulum_, be examined, it appears sufficiently complicated,

being composed of an outer and inner lamina, separated by longitudinal

septa, which are denticulated at their bases; and the tubes formed

by these longitudinal septa are crossed by transverse septa. On the

other hand, in some cases, as in the genera Chthamalus and Elminius,

each compartment consists of a simple shelly layer. These two extreme

states graduate into each other: we have, firstly, on the internal

surface, quite irregular points and ridges; these become regular,

causing the internal surface to be longitudinally ribbed; then these

ribs themselves become finely furrowed on their sides and at their

lower ends, producing sharp, minute ridges, the ends of which I have

called the denticuli; and, lastly, some of the denticuli on the

adjoining longitudinal septa become united into a solid layer, forming

the internal lamina of the wall. These denticuli do not generally

cover the whole surface of the longitudinal ribs, but leave a portion

near the outer lamina of the compartment smooth. The denticulated ends

of the longitudinal septa project beyond the basal edge of both the

outer and inner laminæ, and enter the mouths of the tubes (where such

occur) in the basis, and thus strengthen the shell. The whole of the

internal lamina generally is more or less striated longitudinally,

thus displaying its origin from the union of the inner edges of the

longitudinal septa. I need only further remark that on the internal

surface of the outer lamina, between the main longitudinal septa, there

are generally (as in the woodcut) smaller longitudinal ridges, which do

not reach the inner lamina, and on this account alone are not called

septa.

Tubes are formed by the longitudinal septa, between the outer and

inner laminæ. These tubes are almost square, and are occupied by

threads of corium, which enter at pores left open between the edge of

the compartment and that of the basis on which it rests. The tubes

extend high up the compartments; but in the uppermost part they are

generally cut off by thin, transverse, calcareous septa, deposited by

the ends of the threads of corium; a cancellated structure being thus

produced. Or the uppermost part of each tube becomes filled up solidly

with compact shelly layers, which are always first thrown down on the

side of the tube facing the outside, and thus greatly strengthen the

shell: in several instances, as in _Balanus perforatus_ and _Tetraclita

porosa_, in which the disintegration of the upper part of the shell is

a necessary element in its growth for the enlargement of the orifice,

these filled up tubes become exposed. In Coronula and Tubicinella, the

tubes in their upper parts are, I believe, crossed only by transverse

membranous septa.

_Anomalies and exceptions._--In Tetraclita (Pl. 10, fig. 1 _g_, 1 _h_)

from the branching of the longitudinal septa, several irregular rows

of tubes are formed. In certain varieties of _Balanus balanoides_

(Pl. 7, fig. 2 _b_), and in _B. cariosus_ (Pl. 7, fig. 3 _b_), slight

branching ridges on the internal surface of the walls, seem to

answer to the longitudinal septa, and produce, during the downward

growth of the shell, extremely irregular cells, and short tubes. In

_Balanus vinaceus_ (Pl. 2, fig. 7 _d_), the internal lamina, instead

of being solid, as in every other species, is left cancellated, and

thus betrays, much more plainly than usual, its origin in the united

denticuli of the adjoining longitudinal septa. In _Balanus porcatus_,

between the main longitudinal septa, there are (Pl. 6, fig. 4 _e_)

what may be called rudimentary and disconnected longitudinal septa. In

Coronula and its allies (Pl. 16, fig. 6, and Pl. 17, fig. 4 _c_) it

is the outer lamina of the compartment which is anomalous; for in the

two or three lower zones of growth, it forms only a ledge on each side

of the longitudinal septa; which ledges, higher up, become confluent,

and so form an ordinary outer lamina. In Coronula, also, the wall of

each compartment (see transverse section, Pl. 16, figs. 5, 7) is very

remarkable from being deeply folded, the folds being on their internal

faces firmly calcified together, and on their external faces closely

pressed together (often with a neatly serrated suture), so that the

whole nature of the shell might be, as has happened, easily quite

misunderstood; and the walls be considered as very thick, instead of

being, as is really the case, very thin. In Chelonobia (Pl. 15, fig.

1), however, the walls are truly of such great thickness, that the

nature of the relative parts might likewise be misunderstood; in this

genus the ovarian tubes enter the walls, extending up between the

longitudinal septa, or, as they may here be more naturally called,

the radiating septa. I will specify a few more peculiarities worthy

of remark:--in some species of the sub-genus Acasta, clefts are left,

covered only by membrane, on the lines of suture (Pl. 9, figs. 7 _a_,

8 _a_), between the compartments, just above the basis; and in other

species the basis is perforated by numerous membrane-covered, minute

orifices. In Platylepas, each compartment has one deep inward fold (Pl.

17, fig. 1), somewhat analogous to the three folds in Coronula; this

fold is produced into an internal midrib, supporting and rendering

convex the membranous basis; in this genus, also, the rostrum, owing

to its midrib, is generally thrust a little on one side, and the shell

thus rendered asymmetrical. In _Chamæsipho scutelliformis_ the shell

is symmetrically perforated (Pl. 19, fig. 4 _a_) by four apertures.

Lastly, in _Chthamalus Hembeli_ and _intertextus_, after a certain age,

the basal edges of the walls become inflected, and continue to grow

inwards till they entirely take the place of the true membranous basis.

_Structure of the Radii._

_Radius._--This term, as we have seen, is applied to that side of the

compartment, the growth of which is modified, by abutting against and

overlapping the adjoining compartment. Hence the structure of the

radius is essentially the same with that of the parietal portion of the

compartment. When best developed, as in _Balanus tintinnabulum_, the

radius consists of an outer and inner lamina, separated by denticulated

septa, extending in horizontal lines parallel to the basis, and is

consequently perforated by minute tubes or pores. The tubes become

filled up solidly much more commonly than do the parietal tubes; and

the inner lamina, in such cases, is hardly distinct from the outer

lamina. The denticuli often fail, or are present only on the lower

sides of the septa; and very frequently the edge of the radius can only

be said to be crenated. Notwithstanding these frequent anomalies, if a

series of species and genera be taken, it is certain that there is, as

might have been expected, a close relationship in internal structure,

between the radii and the parietes. The edge of the radius is received

in a slight furrow (generally marked like a seal, with the impression

of the denticulated septa) in the opposed compartment: sometimes the

outer edge or lid of the recipient furrow, is so broad as to give the

false appearance of a radius having been developed, at least in the

lower part of the shell, on both sides of the suture. A crest of corium

runs into each suture between the edge of the radius and the furrow in

the opposed compartment; and when the radius is permeated by pores (as

in woodcut 10), threads of corium branch off this crest, and enter the

pores. In the lower part of the shell, these crests of corium project

from the corium forming and surrounding the sack; but in the upper

part of the shell, above the opercular membrane, and therefore above

the sack, the corium is produced up each line of suture as a separate

ribbon. In proportion as these ribbons extend more or less near to the

summit of the shell, so do the edges of the radii continue to be added

to, to a greater or less height from the basis; and consequently their

summits become less or more oblique.

[Illustration: Fig. 10.

Edge of the radius of _Balanus tintinnabulum_. _a_, outer lamina; _b_,

inner lamina; _c_, denticulated septa, uniting the two laminæ.]

_Peculiarities in the Structure of the Radii._--In some of the species

of Tetraclita, in which genus the walls consist of several rows of

tubes, the radii are likewise perforated by several rows; and in some

of the other species (Pl. 10, fig. 1 _h_), the edge, or disarticulated

surface of the radius, is marked by irregularly branching ridges; and

these evidently correspond with the branching septa or ridges of the

wall. In Chelonobia, the outer lamina of the radius, as well as of

its recipient furrow, is of extraordinary thickness; and this lamina,

in _C. testudinaria_ (Pl. 14, fig. 1 _a_, 5, _b_, and Pl. 15, fig.

1, _f_), is modelled into sharp transverse ridges and valleys. In

the Chthamalinæ, the radii, like the parietes, are simply solid; and

apparently in consequence, for the sake of strengthening the sutures,

the edges of the radii, and of the recipient furrow in Octomeris

(Pl. 20, fig. 3 _a_) and in _Chthamalus dentatus_ and _Hembeli_ (Pl.

18, fig. 3 _b_, 5 _a_), are neatly dentated. In some other species

of Chthamalus (Pl. 19, fig. 1 _a_), the radii present a slight

modification of this structure, the sutures being formed by oblique

interfolding laminæ. In the radii of Coronula and Tubicinella, there

is a peculiarity, in apparent connection with the fact, that in these

genera the parietal tubes are not crossed by transverse calcareous

septa, namely, that the pores by which the radii are permeated keep

unclosed throughout their length, and open into a special longitudinal

tube (Pl. 16, fig. 7, _d′_), which runs along that margin of the wall,

whence the radius arises. In Coronula the wall is of extreme thinness,

and in conformity so is the true radius, but that the shell might not

thus be rendered very weak, complementary or pseudo-radii are developed

on their inner sides (Pl. 16, fig. 7, adjoining the true radii A _d_,

C _d_, and shaded by distant convex lines). Even in the allied genus

Xenobalanus, in which the whole shell tends to become rudimentary,

traces of these pseudo-radii (Pl. 17, fig. 4 _b_, _d_) can be detected.

In Coronula, though the radii (Pl. 16, fig. 7, A _d_, C _d_) are, by

the above special means, rendered thick, and though the alæ also are

thick (C _a′_, D _a′_), yet together they do not equal in thickness the

folded walls; and consequently, there is left between the radii and alæ

square chambers (_v_), occupied by the branching ovarian tubes.

_Structure of the Alæ._

These project, generally abruptly, from the sides of the upper part of

the compartments; they appear from the first growth of the shell; they

are overlapped by the radius and by part of the wall of the adjoining

compartment; they are thinner, and have, owing apparently to being

overlapped, a very different aspect from the parietal portion; but

they do not differ from it in essential nature. They are solid, that

is, they are never permeated by pores; but their edges are generally

crenated, and there is, in some cases, as in Chelonobia, sufficient

evidence that these crenations answer to the horizontal septa on

the edges of the radii (also often reduced to mere crenations), and

consequently, likewise, to the longitudinal septa of the parietes.

In Coronula the edge of each ala consists of a medial ridge, sending

off denticulated septa on _both_ sides, and is therefore anomalous as

compared with the alæ in other genera, but corresponds in structure

with the similarly anomalous radius of Coronula. In order to allow of

the growth of the edge of the ala, a fine thread of corium runs up the

narrow furrow in which the edge is lodged, proceeding from the corium

of the sack. In proportion as this thread runs up higher or lower,

so are the summits of the alæ rendered, during growth, less or more

oblique.

_Structure of the Sheath._

As the compartments overlap each other, the edges of the alæ would have

projected, and the inner surface of the orifice of the shell would not

have been smooth and rounded, had not that part of each wall, which

does not overlie an ala, been thickened so as to allow of the formation

of a shoulder or indentation, against which the edge of the ala fits

and abuts. The thickened portions, and the alæ themselves, together

form the sheath, of which the use seems to be to strengthen, like a

broad internal hoop, the upper part of the shell round the orifice,

where naturally it is weak. The sheath is composed of successive, fine,

shelly layers, which extend, as the shell is added to at the basal

margin, lower and lower down on the inner surface of the walls. The

lower edge of the sheath either simply projects a little inwards, or

more commonly is formed into a sharp depending ridge, as represented

in fig. 1, K′, Pl. 25. In some species of Pyrgoma (Pl. 13, fig. 2

_b_), the sheath reaches nearly to the bases of the compartments;

and in Chelonobia (Pl. 14, fig. 4 _e_ _c_ _e_), the inner layer of

shell surrounding the sack, which seems to correspond more nearly

to the sheath than to the inner lamina of the walls, actually rests

on the basal membrane. The opercular membrane is generally, but not

invariably, attached only a little way above the lower edge of the

sheath: at each exuviation, a new opercular membrane is formed, and is

attached to the next lower zone of the sheath; the old membrane being

cast off, but a circular slip of it is left, investing the last zone.

Hence the whole upper part of the sheath above the opercular membrane,

comes to be thus invested; and is marked by circular lines, one above

the other, caused by the successive exuviations. This investing

membrane often supports rows of minute bristles, directed upwards.

Generally, a film of shell is deposited, at the period of the formation

of each new opercular membrane, on that part of the sheath which lies

immediately beneath. This innermost film or thin layer of shell, on

the lines of suture between the compartments, breaks joint, at least

in some cases, with the underlying shelly layers,--that is, the suture

in this last-formed film does not lie exactly over the suture in the

subjacent layers of the sheath. In Tubicinella, the sheath extends down

close to the basis; and what is unique in this one genus, the opercular

membrane, gradually thinning out downwards, closely adheres to the

whole inner surface of the shell. In Tubicinella and in Xenobalanus

(Pl. 17, fig. 4 _b_), the sheath separates easily into separate

successive rims of shell; and this structure evidently is for the sake

of facilitating the breakage of the upper end of the shell, which, as

we shall presently see, is necessary to allow of the increase in size

of its orifice.

_Structure of the Basis._

This, in several genera and species, is composed of simple membrane,

and consists of successive, concentric, circular slips, added round the

outside, at each period of growth. In some species of Tetraclita and

Balanus the basis is calcareous, but diaphanous, very thin, smooth, or

somewhat granulated. In other cases it consists of a single calcareous

lamina, either smooth, or with ridges radiating from its centre; it

is formed of two laminæ, (as is most usual in Balanus,) separated by

radiating septa. These septa, as well as the radiating ridges in the

case of the single lamina, are homologous with the longitudinal septa

of the parietes. The denticulated ends of the latter enter the mouths

of the tubes formed by the radiating septa of the basis: threads of

corium pass between the denticuli of the parietal septa, and thus

enter the basal tubes. The ends of these threads of corium generally

deposit transverse calcareous septa, exactly as within the parietal

tubes. When the basis is thick the septa themselves (_ccc_) between

the proper basal tubes, become porose, (or rather cancellated,) and

they sometimes expand into a very thick, cancellated layer, separating

the outer lamina (_a_) of the basis from the proper basal tubes, which

always lie close under the inner lamina (_b_). This structure differs

only slightly from that seen in the parietes of Tetraclita, in which

the branching of the longitudinal parietal septa, produces thick walls,

formed of several rows of tubes or pores. With respect to peculiarities

in structure of the basis, _Balanus lævis_ offers the most remarkable

case; for here, in specimens which have grown crowded together, the

whole interior appears sometimes to have become too much elongated or

too deep for the animal's body, and consequently the lower part of

the deeply-concave basis has been filled up (Pl. 4, fig. 2 _a_) by

thin, irregular, calcareous diaphragms. In elongated specimens, also,

of _Balanus balanoides_, the shell sometimes appears to have grown

too long for the animal's body; but in this case the membranous basis

becomes extremely convex inwards; it still reaches the basal edges of

the parietes all round, but in the middle it is raised high above the

surface of attachment; yet sometimes threads of the cementing tissue

depend from the middle part to the surface of attachment. In _Balanus

terebratus_ (Pl. 8, fig. 2 _a_, 2 _b_), and in some species of Acasta,

the basis is riddled, as previously stated, by numerous, minute,

membrane-covered orifices. In _B. declivis_ the membranous basis is

always extremely oblique, owing to the rostral end of the shell being

twice as high as the carinal and opposite end.

[Illustration: Fig. 11.

Portion of edge of basis of _Balanus tintinnabulum_, _a_, _a_, outer

lamina; _b_, _b_, inner or upper lamina; _c_, _c_, _c_, porose or

cancellated radiating septa.]

Regarding the very remarkable means by which the basis of sessile

Cirripedes is cemented to the surface of attachment, it will be

convenient to defer for a little the description, on account of its

necessary length.

_Structure of the Opercular Valves (Scuta and Terga)._

These are situated on each side of the slit or orifice leading into the

sack; from their shape, their powers of movement, their separation by

flexible membrane from the shell, to which they serve as a lid, they

appear at first as if they constituted an element very distinct from

the shell itself, but this is not the case. They are, together with the

opercular membrane, as essentially as the whole of what is externally

visible, a part of the modified carapace, of which they occupy the

upper or posterior extremity: from tracing the metamorphoses, or

even by comparison of a Balanus with Pollicipes, there can be no

doubt of the truth of this conclusion. The opercular valves are four

in number,--a pair of scuta and a pair of terga; but the latter in

_Coronula diadema_ and _reginæ_, are either aborted or represented by

a mere rudiment; and in Xenobalanus both scuta and terga are quite

absent. In several cases, more especially in the genus Pyrgoma (Pl.

13, fig. 1 _b_), the scutum and tergum on each side are calcified

together, so that sometimes not even a trace of the line of junction

can be discovered. In most cases the scutum is firmly united, being

articulated in a manner presently to be described, to the tergum; but

in Coronula, Tubicinella (Pl. 17, fig. 3 _c_), and Platylepas, the ends

of these valves are simply approximated.

_Scuta._--These valves are important, inasmuch as the animal's body

is attached to them; in Pl. 25, fig. 1, the broken line, surrounding

_a_, _b_, shows where the body has been cut, in removing the scutum

on the near side, the other scutum, S, being left articulated to the

tergum, T. In shape the scuta are generally sub-triangular; but in some

species of Pyrgoma and in Chelonobia, &c. they are much elongated. The

lines of growth are usually prominent; and along the occludent margins

the alternate, or sometimes every third or fourth line, is developed

into a knob, which produces a serrated edge, serving to lock the two

opposed valves together; there is, however, no trace of this structure

in Coronula and Tubicinella. In some species of Pyrgoma, a ledge of

considerable breadth (Pl. 13, fig. 3 _e_, &c.) is developed along

the occludent margins of the two scuta, as well as of the two terga,

giving them an anomalous structure. The _Terga_ differ considerably

in outline in the different genera and species: their shape approaches

more nearly to a triangle than to any other regular form; but there is

generally a projection or spur on the basal margin, on the side towards

the scutum. In some species of Pyrgoma, the tergum is of so irregular a

shape as to defy description. In most cases, a longitudinal depression

or furrow runs down the valve, from the apex to the extremity of the

spur; and it not rarely happens that the sides of this furrow become

folded inwards and almost closed. The spur probably answers to the

basal point of the usually sub-rhomboidal tergum in Pollicipes and

Scalpellum.[22] The tips of the terga in some species of Balanus, &c.,

are specially modified into sharp points or beaks (Pl. 2, fig. 3 _b_,

3 _d_), bowed a little inwards, and projecting considerably above the

tips of the scuta; this is effected by the medial, uppermost part

of the valve being internally thickened and hardened, and then, by

the disintegration of the two margins and the external surface, the

internal modified portion becomes exposed. The whole valve, also, at

least in such cases as in _Balanus psittacus_, appears to be forced

slowly upwards in the articular furrow of the scutum. I am assured, by

a competent observer, that the beaks of the terga in _B. porcatus_ can

give an object placed within the orifice of the shell a sharp tap.

[22] In comparing the Tergum of one of the Balanidæ with that

of a typical member of the Lepadidæ, for instance, that of

Balanus with that of Pollicipes, apex corresponds with apex:

the extremity of the spur in Balanus corresponds with the basal

point of the whole valve in Pollicipes: the scutal margin, (which

in Balanus homologically extends down to the extremity of the

spur), corresponds with the scutal margin of Pollicipes: the

carinal margin in Balanus corresponds with the _upper_ carinal

margin in Pollicipes: the basal margin of Balanus on the carinal

side of the spur, corresponds with the _lower_ carinal margin in

Pollicipes: lastly, (and this is the chief difference), in Balanus

there is no appreciable occludent margin, the apex of the valve

being brought close to the upper angle of the scutal margin; in

Chthamalus, however, there is yet left some remnant of an occludent

margin,--which margin in Pollicipes is conspicuous.

The scutum and tergum, with the few exceptions above stated, are

articulated together at a large or open angle. The articulation (see

Pl. 11, fig. 5 _b_, _c_, _d_, and fig. 6 _b_, _c_) is effected by the

margin of the tergum being a little inflected, and lodged in a furrow

in the margin of the scutum. This furrow in the scutum has its further

border generally prominent and often reflexed or curved over; I have

called it the articular ridge; it, also, is lodged in a furrow in the

upper part of the tergum, which again is bordered by a ridge, viz., the

articular tergal ridge. So that in both scutum and tergum there is an

articular furrow, bordered in each case, on one side by the margin of

the valve, and on the other side by the so-called articular ridge. In

Chelonobia (Pl. 14, fig. 1 _b_) the articular ridge of the scutum is

horny. When, as often happens, the scuta and terga have been much worn,

the manner of their articulation (Pl. 18, fig. 1 _a_) is pretty well

shown even from the outside; in this case their external appearance

is very different from what it is in those individuals (fig. 1 _c_)

of the same species, which have not suffered disintegration. This

articulation of the scuta and terga is prefigured amongst the Lepadidæ,

in _Pollicipes mitella_, and in Lithotrya.

The scuta are brought together by a short, strong, straight, adductor

muscle (Pl. 25, fig. 1, _a_); its attachment leaves (with very few

exceptions, as in Tubicinella) a rounded impression, or even pit, on

the under side of the valve in its upper part. This pit is frequently

bounded, on its lower side, by a sharp ridge, which, though not in

actual connexion with the adductor muscle, I have, for convenience

sake, called the adductor ridge; it serves apparently to give support

to the animal's body; in some few cases (as in _B. psittacus_, Pl. 2,

fig. 3 _c_) it is confluent at its upper end with the articular ridge,

and converts the whole basi-tergal corner of the valve into a deep

cavity. In some of the species of Pyrgoma (Pl. 12, fig. 5 _c_, 7 _b_),

and in some varieties of Creusia, this adductor ridge is enormously

developed, so as to depend far beneath the true basal margin, or

that to which the opercular membrane is attached. At the basi-tergal

corner of the valve, there is generally a small pit or impression, and

sometimes distinct crests, for the attachment of the lateral depressor

muscle. At the rostral end there is, also, a small cavity formed by the

overfolding of the occludent margin (rarely furnished with crests) for

the attachment of the rostral depressor muscle. In the Terga, at the

basi-carinal corner, there are usually crests, though sometimes feebly

developed, for the attachment of the tergal depressor muscle. But in

Chelonobia, Coronula, Tubicinella, Platylepas, and in some other cases,

there are no crests. The crests, when well developed, are furnished

with rectangular sub-crests or denticuli on both sides; in fact they

resemble, and are probably homologous with, the denticulated ribs or

septa in the parietes, radii, and basis. Altogether the scuta and terga

are attached, as far as muscles are concerned, to the shell and sack,

by three longitudinal pairs.

_Growth of the Whole Shell, and Its Microscopical Structure._

The opercular valves are added to along their basal margins alone;[23]

the animal's body, together with the several muscles, becoming attached

at each period of growth lower and lower down to the valves; this

no doubt is effected by the absorption of the upper surfaces of the

muscles, and the formation of new fasciæ on their lower surfaces. The

opercular membrane, which, though thin and flexible, forms part of

the general outer surface of the animal as much as does any portion

of the rigid shell, with which indeed it is strictly homologous, is

periodically moulted, together with the integuments of the whole

included animal. The new opercular membrane is of course each time

formed a little larger than the old one. In Coronula and Tubicinella,

however, several successive opercular membranes are preserved one

over the other, and the outside membrane gradually disintegrates; in

these cases the undermost opercular membrane is formed wrinkled and

considerably too large, so as to allow of being stretched, before it

is finally cast off. In Tubicinella, the opercular membrane runs down,

adhering to the inner surface of the shell, to nearly the basis, and

hence during the diametric growth of the shell, it is longitudinally

split, and is repaired by slips of new membrane, which resemble the

radii in form and in direction of the lines of growth.

[23] In some species of Pyrgoma, the ledge (_limbus occludens_)

which is added along the occludent margin of both scuta and terga,

and in some species of Balanus a narrow rim, or slight protuberance

which is added along the carinal margin of the terga, offer

unimportant exceptions to the rule, that the opercular valves grow

only at their basal margins.

The basis is added to only round the circumference, and hence

increases in diameter, and, when concave, in depth. The compartments

grow at their basal margins, where they are in contact with the basis;

hence the shell is added to in height, and, owing to the outward

inclination of the compartments, also, in basal diameter; but the

compartments likewise, in most cases, grow along both lateral margins,

that is, on the edges of the radii and alæ; and hence the upper part

of the shell, also, increases in diameter. The orifice of the shell,

moreover, thus becomes enlarged. In some cases the shell is destitute

of radii, only sutures being present, that is, the compartments do not

grow laterally; and sometimes, as in the whole genus Pyrgoma, there

are not even sutures, the compartments having been fused together:

in both these cases, the shell can increase in diameter only at the

base; and the orifice, it might have been thought, would necessarily

have remained, to the destruction of the animal, of the same minute

size, as when first formed after the metamorphosis: this certainly

would have been the case had not the upper ends of the compartments,

surrounding and forming the orifice, been nicely adapted always to

yield, in a certain limited degree, to the disintegrating influences

to which every shell is exposed, but which most Cirripedes can resist;

and the disintegration of the narrow end of a conical tube, of course

increases the diameter of its orifice. In Tubicinella, in which the

shell is furnished with narrow radii, and does increase in diameter

from top to bottom, the increase is not sufficient in proportion to

the continued elongation of the shell; to compensate for this, the

orifice is enlarged at short intervals by the breakage of the upper

end of the shell, for which purpose (as explained under the genus) it

is evidently constructed. Hence we see that, in certain Cirripedes,

decay or disintegration, and breakage, are necessary elements in their

growth! It is a remarkable fact, which I cannot explain, that in

some species in which the orifice of the shell is usually increased

by disintegration, if individuals are so situated that they are not

exposed to sufficiently energetic disintegrating influences, as may be

inferred from the well-preserved condition of the whole surface of the

shell, then the radii become developed, and the orifice is increased in

size by the diametric growth of the upper part of the shell: I have

seen instances of this in _Tetraclita porosa_, and _purpurascens_, and

in _Balanus perforatus_: it appeared, but of course erroneously, as if

the lateral growth of the compartments had been subjected to the will

of the animal.

Considering the strength of the shell of sessile Cirripedes, the

separation of their compartments one from another and from the basis,

during growth, has justly been thought a surprising circumstance. In

most Chthamalinæ and in some species of Balanus, however, if the shell

be boiled in caustic potash, the compartments fall apart with a touch;

this shows that their union is due to animal and probably to organised

matter, and the growth of such matter between the opposed edges of

the compartments, and their consequent gradual separation, offers

no particular difficulty. But in many Balani, boiling in potash for

hours does not seem even to weaken, in the least degree, the sutures,

which are wonderfully strong--the shell often breaking rather than

yield on these lines; if, however, the shell be dissolved in acid,

the animalised tissue which is left easily separates on the lines of

suture, and if this tissue be boiled in potash, the remnants of the

compartments fall quite separate. These facts seem to me to show, that

the compartments in such cases are joined along the lines of suture by

tissue, which must be in a calcified state, but which, nevertheless,

continues to grow by intersusception; in other words, I believe that

the tips of the complicated ridges and points interlocking on the lines

of suture, are not separated from each other by films of corium or

simple animal matter, but are actually united by corium in a calcified,

yet still growing condition.

In ordinary Crustaceans, the growth is periodical and sudden; a new

and larger carapace, for instance, is formed under the old one, and

after the exuviation of the latter, the new one soon hardens, and does

not subsequently increase in size; so it is in the case of Cirripedes,

with the membranes of the body, and even with certain parts, as

the opercular membrane, of the external covering. But a Cirripede

cannot, like a crab, crawl into some crevice and remain protected

till its shell becomes hardened; hence, probably, it is that the

shell is never[24] wholly moulted. Even if the margins of the opposed

compartments and of the basis were to grow rapidly, the shell would

necessarily be much weakened on the lines of suture, and unable to

withstand the heavy breakers, to which so many species of sessile

Cirripedes are exposed. On the other hand, although the margins are

thus compelled to grow slowly, they do not grow continuously, as may

be seen in the zones of increment on all the valves, corresponding, I

believe, with the periods of exuviation of the membranes of the body. A

layer of shell, often very thin, seems to be generally deposited over

the whole internal surface of the several valves, at the same time that

the marginal zones are added; so that the only essential difference

in the growth of the external covering, in Cirripedes as compared

with ordinary Crustaceans, is that the old shell is not cast off, but

adheres to the outside of the new shell, and that the margins are

added to (in certain definite directions) slowly yet not continuously,

instead of the whole being formed at a single period.

[24] In the genus Alcippe, and in Cryptophialus, the whole of

the external membranes are moulted, excepting the surface of

attachment; but then these Cirripedes live in cavities which

they form for themselves, and are thus protected. In Lithotrya

the membrane of the peduncle, with its little valves or scales,

is moulted, but here, again, this very part is protected by the

tubular cavity, which the animal forms and inhabits. Neither of

these three genera belong to the Balanidæ, or sessile Cirripedes,

which we are now more especially describing.

If, now, a section of one of the shelly zones of growth be carefully

examined, it can in some cases be distinctly seen to be formed of

successive, excessively fine laminæ; but the animalised tissue (which

differs much in amount in different Cirripedes) left after the shell

has been dissolved in acid, exhibits, in most cases, neither laminæ nor

any other structure whatever. The shell seems to be the actual pulpy

corium, or true skin, in a calcified condition, but generally with its

cellular structure modified and much reduced: I have taken a bit of

recently-formed shell of Tetraclita and of Coronula, with the corium

still adherent to its under surface, and after dissolution in acid, I

could not distinguish the part, which had just before existed as shell,

from the corium itself. In the case of Coronula, immediately prior

to the period of moulting and growth, I found the unaltered corium so

charged, as to effervesce, with carbonate of lime, either in a state of

dissolution, or in granules too minute to be visible under the highest

powers.

The sutures between the several compartments and the basis are covered

by thin membrane, which is continually splitting during the growth

of the opposed edges of the underlying shell; but previously to each

splitting, a new slip of membrane is, I believe, already formed under

the old one; so that the corium is not even momentarily exposed. Owing

to this manner of growth, the slips of membrane consist of successive

rims united together; in most cases, these soon become abraded from the

older parts of the shell, but are sometimes preserved. The last-formed

slip of membrane over a suture is homologous with the opercular

membrane; and both are strictly analogous with the ring of flexible

membrane, forming the joint of the leg of a crab. In the latter case,

the flexible membrane and hardened crust are both moulted together:

in the opercular membrane, there is a double line of splitting, one

close round the opercular valves, and the other at the basal edge of

the sheath, and the intermediate portion is moulted, but with a zone of

membrane left adherent to the non-moulted valves and sheath: lastly,

in the slips of membrane covering the sutures, there is only a single

line of splitting, and no portion, I believe, is moulted; the rims of

membrane on each side remaining adherent on the compartments and basis,

until worn away.

The opercular membrane, when closely examined, exhibits no structure,

except that it can sometimes be plainly seen to be composed of

successive, numerous, excessively thin laminæ. Occasionally, however,

it presents the false appearance of being permeated by parallel and

anastomosing vessels: this appearance is clue to one or more of the

component laminæ having been wrinkled before a succeeding lamina was

thrown down and attached to its under side. If a small piece of an

opercular valve of Tubicinella, with the opercular membrane adhering

to it, and with the corium under both, be dissolved in acid, it may

be clearly made out that the corium under the valve has gone on

being converted into shell, whereas under the opercular membrane it

has been converted and condensed into fine constituent laminæ of

chitine. Inasmuch as the successive layers of shell, during each period

of growth, go on encroaching on those of the membrane, the line of

junction between the shell and chitine becomes oblique or bevelled. The

membrane on this bevelled line of junction assumes a slightly different

aspect to what it has elsewhere; it becomes yellowish or brown, thicker

and very much tougher. In many genera it is also furnished with a row

of small bristles. At the period of exuviation the opercular membrane

separates just outside this modified portion, leaving the latter

adherent, as a rim or slip, on the valves. If, however, the opercular

membrane be rudely torn off before its proper period of exuviation,

it carries with it the as yet continuous, but already modified, slip.

A slightly indented line may sometimes be traced before the period of

exuviation, showing where the separation will take place: what produces

this line I know not. The coloured, thickened, and modified slips of

opercular membrane, which are thus retained adhering to the valves,

and which together form an investing membrane, have been considered

by most authors as the epidermis; but they have no more right to be

thus called than has any other part of the opercular membrane. Exactly

similar slips of membrane are left investing the sheath. So, again, the

membrane which, when well preserved, invests the walls of the shell,

is made up, as already stated, of successively adherent slips, which

originally covered the lines of suture.[25]

[25] In the case of Coronula there is a peculiarity, described in

the last section of this Introduction, (under the head of Cementing

Apparatus), namely, that the two or three last-formed, exterior

zones of the Basal membrane continue for a period to increase in

width; being, as I believe, dragged one from over the other, with

fresh laminæ of membrane continually thrown down. In this same

genus, and in Tubicinella, the walls of the shell are invested

by membrane, which is doubled inwards under their basal edges;

and as the latter grow, the investing parietal membrane splits

and separates from the basal membrane, and is pulled outwards and

downwards. This inflected, often broad border of membrane, seems to

me more strictly comparable with the opercular membrane, than with

those narrow, thickened rims of yellowish membrane which in other

Cirripedes cover the suture between the basal edges of the walls

and the basis.

The little bristles above alluded to, which arise from the slips of

membrane left adherent on the opercular valves, sheath, and walls,

stand in rows; a row corresponding to each period of exuviation of

the opercular membrane. The bristles are generally largest on the

opercular valves and sheath; in _Balanus tintinnabulum_, they are

from 1 to 2/1000ths of an inch in length, but they are longer in some

other species. I may here mention, as showing the connexion of these

bristles with the opercular membrane, that similar bristles occur in

_B. perforatus_, scattered over the surface of that membrane, and

are necessarily moulted with it. In the imbedded genera Coronula and

Tubicinella, none of these bristles exist. When a portion of valve or

shell, furnished with bristles, is dissolved in acid, tough, sinuous,

and apparently hollow, threads are seen to run from their bulb-like

bases, into and up the corresponding layer, which, before dissolution,

existed as shell; and they terminate internally in very fine points,

which I believe are united to the underlying corium. These threads, or

_tubuli_,[26] as I have called them in my volume on the Lepadidæ, are,

in _Tetraclita porosa_, about 1/5000ths of an inch in diameter, but

only half that size in _B. tintinnabulum_. On parts of the shell where

there are no bristles, similar tubuli penetrate the shelly layers, and

come to the surface. The tubuli running to the lowest and last-formed

row of bristles, just after a period of exuviation, are so delicate as

hardly, or not at all, to be distinguished; in the row above, they are

plain and longer, and for the next two or three upper rows they are, in

some cases, as in _Tetraclita porosa_, longer and longer, having been

added to during each successive thickening of the valve. These tubuli

consist of chitine, and no doubt first existed as threads of corium;

they are so tough that they must serve to strengthen the successive

layers of shell, but I imagine their chief function is to keep up the

vitality of the newly-formed layers of shell. May we not, also, venture

to suppose that by their means, some degree of sensibility is given

to the bristles? I need only further remark, that in some species of

Balanus and of Chthamalus, the under side of the shell is penetrated

by irregular pores, large enough to be visible to the naked eye, into

which threads of corium penetrate; but these can hardly be said to

appertain to the microscopical structure; and are more nearly related

to those pores and furrows, formed by the greater or less development

of the longitudinal septa, and in which the threads of corium deposit,

or rather become changed into, transverse septa, or solid shelly

matter, as previously described.

[26] I regret that I have used this term "tubuli"; for the threads

thus designated, I believe, are not the same with the _tubuli_ of

Dr. Carpenter, which are not left after dissolution in acid. I have

seen tubuli, as called by me, in the shell from the leg of a crab,

after having been placed in acid.

_Muscles of Sack._

In the pupa, the thorax, as we shall hereafter more fully see, is

continuous with, and opens into the large anterior end or front part of

the head; but during the metamorphosis (Pl. 30, fig. 2), the thorax of

the Cirripede becomes, owing to the almost transverse position occupied

by the young animal within the pupa, to a great extent internally

separated from the anterior end,--which anterior end forms, as we know,

either the peduncle or the basis. Hence it comes to pass that the

body or Thorax (Pl. 25, fig. 1) is lodged within a sack (_f_) within

the shell. The chitine membrane lining this sack is excessively thin

and transparent, but less so in Xenobalanus and Tubicinella; it is

obviously continuous with that investing the body of the animal; it is

also essentially continuous with the opercular valves and membrane,

and consequently with the whole shell. It is periodically moulted. It

is lined by corium, as is likewise the surrounding shell; hence the

corium is double round the sack, as indeed might have been expected

from the shell and opercular valves (at least their upper parts) being

formed by the prolongation, as is obvious in the pupa, of the posterior

edges of the carapace. Between the two folds of corium, which are

united together by transverse ligamentous fibres, branching out at

both extremities, like the roots and branches of a tree, we have the

longitudinal muscles, which go to the opercular valves; and likewise a

layer-like mass of branching ovarian tubes (Pl. 25, fig. 1, _g_): the

ovarian tubes, however, are often confined to the base of the sack.

In Xenobalanus, the two folds of corium are united by longitudinal

membranous septa, making a series of quite peculiar, square tubes.

The above-mentioned muscles are attached at their upper ends to the

opercular valves, and at their lower ends to the basis. There are, in

fact, three pairs, but the pair attached to the basi-carinal angles of

the two terga (Pl. 25, fig. 1, _i_), are almost invariably confluent,

forming one great bundle; the second pair is attached to the lateral

or basi-tergal corners of the two scuta, and are hidden in the figure;

the third pair (_h_) is attached also to the scuta, to their rostral

angles. These muscles can only act as depressores; they are often

extremely powerful; they belong to the voluntary class, for they

are transversely striped. By their action, the opercular valves are

capable of varied slight movements, within the limit allowed by the

width of the flexible opercular membrane. By the action of the lateral

scutal depressores, the orifice leading into the sack is opened, the

movement being generally aided by the protrusion of the cirri. By the

sudden contraction of the rostral scutal depressores, the blows which

are sometimes given by the beaked terga at the opposite end of the

operculum, are probably effected. By the contraction of all three pairs

of muscles, the opercular valves are held down with quite surprising

force. The valves can be raised only by the action of the animal's body

against the basis.

In Coronula these muscles are more spread out, and do not extend

down to the basis; their lower portions, as is likewise the case in

Tubicinella, do not exhibit transverse striæ, and hence tend to pass

either into the involuntary class, or into ligament. This condition of

the muscles, in the above two genera, accords with the little-developed

state of their opercular valves. In Xenobalanus, there is no longer

any evidence of the muscles being collected into five or six bundles,

for they are thinly and almost uniformly spread out, and show in no

part transverse striæ. I may add that in much elongated specimens

of _Balanus balanoides_, these muscles become in their lower part

ligamentous, and destitute of striæ.

_Branchiæ._

In the Balaninæ, a pair of Branchiæ is always present: they lie on

each side, in a somewhat curved position, in the angle between the

sides of the shell and the basis. In Pl. 25, fig. 1, they are exactly

covered, on the further side, by the body of the animal. They are

attached near each other at the carinal end of the sack in a vertical

line, and likewise on each side in a transverse line, extending from

close beneath the spur of the tergum towards the point of attachment

of the body to the scutum. In Balanus, as in the figure (Pl. 25, fig.

3) of _B. tintinnabulum_, each branchia consists of a medial fold of

skin, a little curved conformably with the sack, and slightly tapering

towards its rostral and free extremity; but this fold is almost hidden

by the vertical sub-folds or membranous ridges, themselves plicated and

sub-plicated, which project on both sides: these vertical folds are

free at their tips: at their lower attached ends, they are thickest.

On the side nearest the wall of the shell, the whole branchia has a

bilobed appearance, owing to a very deep indentation caused by the

projection of the scutal lateral depressor muscle; the sub-folds on

this side are also more plicated. The branchia essentially is an inward

plicated fold of the membranes of the sack; for its outer, very thin

tunic is continuous with and moulted with that lining the sack; and

within it we have two layers of delicate, pulpy, transparent corium,

united together (as is best seen in Coronula) by ligamentous fibres,

branched at their two ends, all exactly as in the corium surrounding

the sack. There are here no distinct vessels, any more than in

other parts of the body, but a fluid could easily circulate in the

interspaces of the corium. From the large size of this organ, and its

simplicity of internal structure, being adapted exclusively to expose

a great surface of skin to the water, I do not doubt that it has been

correctly considered as a respiratory organ. By the voluntary movements

of the opercular valves (_i. e._ part of the carapace) the water is

constantly being pumped in and out of the sack; the movement, indeed,

may be almost compared to the heaving of a man's chest. Moreover,

the branchiæ on each side are attached so closely to the spur of the

tergum, that each time the latter is moved, the whole branchia must, I

think, be agitated, and the folds opened, as by the action of a lever.

In our two commonest, tidal, sessile Cirripedes, viz. _Balanus

balanoides_ and _Chthamalus stellatus_, I have observed that, when left

uncovered by water, they kept the orifice of their operculums a little

open, with a bubble of air within their sacks, so that the orifice

was in fact closed by a thin septum of water, with air beneath; when

disturbed, they closed their operculums with force, and expelled the

bubble of air with a clicking noise, which has been noticed by Dr.

Coldstream,[27] and has been thought to be made by the movement of the

operculum itself. _Bal. crenatus_, a deep-water species, when out of

water, keeps its operculum closed.

[27] 'Encyclopædia of Anatomy and Physiology;' article Cirrhopoda.

In Coronula, Platylepas, Tubicinella, and Xenobalanus, each

branchia[28] consists of two unequal folds, both plicated on both

sides: in the two latter genera, they extend far down the deep and

elongated sacks, and hence the area of surface altogether gained is

extremely great. In most of the species of Chthamalus, the branchia

consist of a small fillet barely plicated: in the allied _Chamæsipho

columna_, they are rudimentary, forming a smooth little pouch only

1/100th of an inch in length: in _Chthamalus scabrosus_ they are quite

aborted, being perhaps represented by a slight hairy ridge; but in

_Chthamalus dentatus_, and therefore within the limits of the same

genus, the branchiæ (and this seems to me a singular fact) are large,

each being composed of two plicated folds, as in Coronula. Tapering

filaments situated near the bases of the cirri, such as those occurring

in several species of the Lepadidæ, are not found in any sessile

Cirripede; but I have observed nearly similar filaments, projecting

upwards and inwards at the base of the sack, in several species of

Balanus and in Coronula; those which I examined were simply occupied

by delicate corium, and no doubt must aid in exposing a greater surface

of corium to the circumambient water.

[28] Burmeister has given a good figure (Tab. 2, fig. 10) of the

branchiæ of Coronula, (but the two folds are shaded too unequally),

in his 'Beiträge zur Naturgeschichte der Rankenfüsser.'

In my former volume on the Lepadidæ, I have described the _ovigerous

fræna_ occurring on the two sides of the sack, to which the

_ovigerous lamellæ_ are attached by a peculiar glandular secretion:

in the Balanidæ there are no ovigerous fræna, but the branchiæ just

described are identical with the fræna in essential structure and

in position; differing only in being placed a little nearer to the

carinal end of the sack, and in being generally (but not always)

larger and more plicated: seeing this, and that in _Alcippe lampas_,

and in some species of Pollicipes,--the genus which comes nearest to

the Balanidæ,--the ovigerous fræna are large and are destitute of

glands, and have therefore lost their normal function of supporting

the ovigerous lamellæ, I can hardly doubt that the _branchiæ_ in the

Balanidæ are the _ovigerous fræna_ of the Lepadidæ in a modified

condition; a transformation of function not greater than that of the

swimming bladder of a fish into the lungs of the higher Vertebrata.[29]

[29] There is, I conceive, no foundation for the belief of some

authors that the branchiæ of the Balanidæ are in any way connected

with the ovaria.

_Thorax and Body._

_Parts of the body included within the shell or carapace._--These

parts (Pl. 25, fig. 1) consist of the prominent mouth, and of the

thorax (_c′_), with its largely developed portion, called the prosoma

(_c_), and with its appendages. The abdomen is quite rudimentary,

being represented merely by a small portion of membrane surrounding

the anus, and sometimes inserted like a wedge between the inwardly

inflected posterior thoracic segments; in only two genera (Catophragmus

and Pachylasma), its nature is rendered somewhat plainer by supporting

caudal appendages. The probosciformed penis lies folded under the

thorax; and I believe (from what is seen in the anomalous genus

Proteolepas), that it normally arises from the ventral surface of

the terminal point of the rudimentary abdomen.[30] The thorax is

laterally compressed, the ventral surface being very narrow, with

the bases of the cirri placed closely together. It consists, in

appearance, of two very different portions; one a soft, more or less

rounded bag, which I have called the prosoma; and the other, which

supports the five posterior pairs of cirri, is narrower, invested

with stiffer membrane, and is more or less distinctly composed of

five segments. These segments (Pl. 26, fig. 8) on their dorsal and

dorso-lateral surfaces, are generally driven like wedges one into the

other, with their points directed anteriorly: on the ventral surface

the articulations are transverse. The prolongation (_e_) of the thin

membrane (_a_) surrounding the anus (_b_), that is, the rudiment of

the abdomen, which sometimes carries caudal appendages, almost divides

(in appearance, whether really I know not) the hindermost thoracic

segment along the medio-dorsal line, into two parts. I have given the

above drawing of these segments, but with the dorsal surface much

flattened, in _Coronula diadema_; in most species of Balanus, however,

the wedges formed by one segment being driven into another, are much

sharper; on the other hand, in Xenobalanus they are nearly straight and

transverse. The three posterior segments are always the most distinct;

the two next segments are also distinct laterally, but along the dorsal

surface they become, excepting in Xenobalanus and some few other

cases, completely confluent. The greater distinctness of the posterior

segments is conformable to what takes place in the higher Crustacea.

The articulations between the segments are folded inwards, and are

formed of thin membrane, which in some cases, as in _Coronula diadema_,

forms a marked contrast with the much thicker, stiffer, and yellowish

membrane of the segments themselves; in _Balanus tintinnabulum_,

however, the whole membrane of the five thoracic segments is very

thin, excepting small wedge-shaped portions along the medio-dorsal

line. The infolded articulations between the segments supporting the

three anterior pairs of cirri (at least in the Balaninæ), are much

wider than those between the three posterior segments; the former

segments, with their cirri, being consequently capable of being moved

further apart from each other. Could there have been any doubt as to

the distinctness and reality of the five thoracic segments, it would

have been set aside by the arrangement of the muscles attached to

them, as will presently be described. I need only add, that in many

genera there are shield-like swellings at the exterior bases of the

pedicels of the posterior cirri, which I for some time thought were

the epimeral elements of the thoracic segments; but I now believe them

to be parts of the pedicels of the cirri. The basi-exterior margin,

moreover, of the pedicel of the third pair of cirri, in many species of

the Balaninæ (Pl. 25, fig. 1), is produced as a plate, thickly fringed

with fine hairs, half across the dorsal surface of the thorax; serving,

apparently, as a brush to clean the sack, or to prevent the ingress of

any intruding substance.

[30] Von Siebold and Stannius, in their 'Anatomie Comparée,' tom.

i, p. 473, and p. 440, (foot-note), consider the articulated

probosciformed penis as an elongated abdomen; a view which, at the

commencement of my examination, I was tempted to admit; but the

position of the caudal appendages on the dorsal basis of the penis,

suffice, I think, to show that this view is not correct; for these

caudal appendages evidently correspond with those borne on the

very extremity of the abdomen in the pupa. Nor, indeed, does the

position of the anus accord well with such a view.

The soft, rounded, bag-like portion of the body, which I have called

the prosoma, is usually separated by a notch from the five posterior

thoracic segments; at its upper end it may be said to carry the mouth

and first pair of cirri. The prosoma includes the main part of the

stomach and the broad ends of the vesiculæ seminales. It is always

clothed by very thin membrane, which in _Chthamalus dentatus_, is

hairy. In Tubicinella and Xenobalanus, the prosoma is much elongated,

being produced far down the deep sack. That the prosoma is mainly

formed by a great development of that segment (homologically the second

thoracic segment) which carries the first pair of cirri, is certain,

and I should not have hesitated to have said that it was exclusively

so formed, had not the first thoracic segment in the anomalous genus

Cryptophialus been developed as a distinct and free segment, not

attached to the carapace; showing that possibly in other Cirripedes,

the dorsal half of this first thoracic segment may be concerned in the

formation of the free prosoma.

_Muscular System._

_Attachment of the Body to the Shell._--The prosoma which carries the

posterior thoracic segments, and in appearance the mouth, is the only

part of the body which is attached to the general covering (Pl. 25,

fig. 1), namely, to the opercular valves. Except through the continuity

of the lining membranes of the sack, the body lies free within the

walls of the shell. The area of attachment (shown by a sinuous broken

line round _a_ and _b_) extends from about the middle of the two scuta

down to their basal margins. As these valves lie obliquely across the

orifice of the shell, the animal's body comes to be suspended almost

in the middle of the sack. The two scuta, as we have seen, have the

power of opening and shutting a little; and are brought together by

the adductor scutorum muscle (_a_), which is generally very powerful.

The body is attached to these valves, round and beneath the adductor,

so as to hide it until one of the valves be removed. The attachment

is chiefly effected by three pairs of widely expanded, superficial

muscles, two pairs of which are spread over the flanks of the prosoma,

and the third pair over its rounded (properly dorsal) surface, which

lies close to the rostral compartment (A, fig. 1) of the shell. I

should have stated, that my chief examination of the attachment of

the body to the scutal valves, has been made on _Coronula balænaris_,

and less closely on _Balanus tintinnabulum_. Within these three pairs

of superficial muscles, there are (besides the adductor) no less than

five other pairs; of these one long pair is attached at one end to

the basal margin of the labrum (_e_), and at the other end, to the

under side, near to the basal margin of the scuta: two other, shorter,

parallel pairs of muscles are attached at one end to the interspace of

membrane between the basal edge of the labrum and the adductor scutorum

muscle, and at the other end, to the under side of the scuta, above the

attachment of the first pair: the fourth and shortest pair curls close

under the adductor, and is there attached at both ends beneath it. The

action of these four pairs of muscles must be to draw back, from the

orifice of the shell, the mouth, and that interspace of body between

the basal margin of the labrum and the adductor muscle. This movement I

saw in living specimens. The last and fifth pair of muscles is small,

but of considerable length; it is a diverging pair, attached at the

converging end, above and exteriorly to the adductor muscle; and at

the diverging end, low down on the under side of the scuta; I am very

doubtful regarding the function of this pair. Altogether we have seen

that round and within the fleshy pedicel, by which the body is attached

to the scuta, there are no less than eight pairs of muscles. The

central space between these muscles is hollow, and here many lacunal

channels seem to converge. These muscles receive nerves from the

supra-œsophageal ganglions. The interspace above alluded to, between

the basal edge of the labrum and the adductor scutorum muscle, occupies

a very different position according as the animal's body is protruded

as far as it can be, or is retracted. It is homologically part of

the third cephalic segment; and consequently the mouth ought to have

stood posteriorly (_i. e._ above, in the position figured in Pl. 25,

fig. 1) to this interspace; yet, in fact, when the animal is retracted

within its shell, the mouth usually lies almost directly beneath this

interspace and the adductor scutorum muscle.

Besides these muscles of attachment, the prosoma is furnished with

several other muscles. There are superficial muscles running up

towards the basal margin of the sides of the mouth; and other deeper

muscles, to which, I presume, the movements of the mouth, as a whole,

are due. The muscles moving the gnathites do not, as far as I could

make out, extend beneath the basal edge of the mouth. There are, also,

powerful muscles giving movement to the basal segments of the pedicel

of the first pair of cirri. Again, there are superficial muscles

running to the next succeeding thoracic segment; the anterior ends of

which are separated by a clear interspace from the lower ends of the

above-described superficial muscles, by which the prosoma is attached

to the scuta. On each flank, moreover, but more deeply imbedded, are

the long flexor and extensor muscles, presently to be described,

running to the five posterior thoracic segments. The last muscle which

I need here mention, is a deep-seated diverging pair, attached near

the upper end of the stomach, on its ventral surface, and diverging

from this point to the sides of the prosoma high up beneath the mouth.

The probable action of this pair, as well as of the three superficial

pairs of muscles by which the body is attached to the scuta, is to

draw up the whole prosoma towards or from the orifice; and likewise to

contract it firmly, so as to serve as a fulcrum for the movements of

the five posterior thoracic segments, together with the cirri, which

they carry.

The muscles of these five thoracic segments are numerous and powerful;

they are also complicated, chiefly owing to the segments on their

dorsal and dorso-lateral surfaces being driven, like wedges, one into

the other. As far as I could make out, there are on each side three,

superficial, dorso-lateral and lateral muscles (generally, if not

always, destitute of striæ), which do not cross the articulations,

but extend merely from articulation to articulation; and of which

the function can be only to contract each separate segment, and

consequently to open out the intermediate infolded articulations;

the effect of this would be to separate slightly the cirri from each

other,--more especially those borne on the two or three anterior

segments, between which the infolded articulations are deeper or

broader. There are other more deeply imbedded, powerful, long,

dorso-lateral extensor, and ventri-lateral flexor muscles, attached

at one end within the flanks of the prosoma, and at the other end to

the successive segments of the thorax. The action of the former is to

straighten and stretch out the thorax; of the latter, or ventri-lateral

muscles, to retract it. In tracing these muscles, a fascia could be

seen to become attached to a segment, and then this same fascia would

run on to the next succeeding segment: the effect of this must be, that

each segment can be retracted and protracted either from the prosoma

as a fulcrum, or from the antecedent segment as a fulcrum: we have,

also, seen that each segment can, by the agency of the superficial,

non-striated muscles, contract itself. Hence these thoracic segments

are capable of diverse movements, as was very evident when the shell

of a living specimen was opened. By one movement in common, the

whole five posterior segments could be drawn back, so as to become

even partly imbedded in the prosoma: lateral, twisting or wriggling

movements were also quite distinct: the three posterior segments

seemed to be capable of less independent movements than the anterior

segments; and I observed that the more powerful flexor and extensor

muscles did not run into these three posterior segments. The cirri, of

course, partake of the movements of the thorax; and in watching, in an

uninjured specimen, the alternate, protruding, gracefully sweeping and

retracting movements of the posterior pairs of cirri, it was evident

that the thorax was the chief agent in their movement. Besides the

muscles now mentioned, there are some immediately to be noticed, which

extend from within the thoracic segments to within the pedicels of the

cirri.

_Movements and Muscles of the Cirri._

Although the cirri have not been described, it will be most convenient

here to treat shortly of their muscles. Each cirrus consists of a

pedicel, having a long basal and a short upper segment, supporting two

multiarticulate rami. The lower segment of the pedicel can be drawn

forward by an adductor muscle, attached low down within this segment,

and crossing at right angles (at least in the case of the anterior

cirri) the corresponding muscle of the opposed cirrus, on the central,

ventral surface of the thorax. This segment can also be drawn back

by a muscle springing from the dorso-lateral surface of the thorax,

and running only a little way within the segment: I am far from sure

that the lower segment does not possess other muscles. The short upper

segment of the pedicel can be moved backwards and forwards, as I saw

in living specimens, independently of the lower segment; this movement

being best seen in the anterior cirri, which are much more often moved

independently of each other than are the posterior cirri. The rami

are capable, I believe, of being moved backwards and forwards _as a

whole_, by the movement of the few lower segments, which are generally

more or less confluent. They can, also, be curled up and uncurled by

the combined movement of each separate segment. The uncurling seems

to separate the two rami a little laterally. Each ramus, at least in

the two or three anterior pairs, can be moved to a certain extent,

independently of the other ramus of the same cirrus; and the few

terminal segments, either of both rami or of one ramus, are often a

little moved and curled (and this is especially the case with the long

anterior ramus of the first pair), without the lower segments or the

pedicel being moved.

The flexor and extensor muscles, which, as I believe, move the upper

segment of the pedicel (_a_ and _b_, Pl. 29, fig. 1), are attached at

their upper ends to its basal margin, and are thus enabled to draw it

a little way down within the lower segment, and so move it. The short

flexor muscle (_c_), which is attached at its lower end within the

upper segment of the pedicel, and the longer extensor (_d_), also,

attached within this same lower segment, serve, I believe, to move the

lower, partially confluent segments of each ramus as a whole. In the

case of these muscles, and of those last mentioned, I am surprised that

the extensors (_b_) and (_d_) are not attached nearer to the exterior

and dorsal surface. Other muscles (_e_, _f_) attached at their lower

ends within the upper segment of the pedicel, run up each of the two

rami to their tips, with some of the fasciæ terminating within each

segment: of these muscles, the outer one (_f_, _f_) appears to be

the extensor, and the inner one (_e_, _e_) the flexor. But besides

these, there are other short flexor muscles (_g_, _g_) which run on

the anterior face,[31] from segment to segment, serving to pull the

front edge of one segment within the edge of the next lower segment.

These muscles differ much in plainness in the several genera: they are

very distinct in Coronula. In some specimens of this genus, a few of

the articulations between the basal segments of the rami having been

obliterated, the short muscles (_g_, _g_) running from articulation to

articulation were absent, and their presence and nature in the upper

segments thus rendered the plainer. The muscular system in the several

pairs of cirri seems to be the same, with the exception of the first

pair, in which the muscle answering, as I suppose, to (_a_), namely,

the flexor of the upper segment of the pedicel, is much spread out at

its lower end, and is there attached to the exterior surface of the

lower segment.

[31] For a considerable time I thought that there were muscles

going to the spines, especially to those which arise from the upper

dorsal edge of each segment; but I have since ascertained that

these are the cases within which new spines, with their lower ends

doubled like the fingers of a glove hastily pulled off, are in

process of formation.

The backward and forward movements of the segments, both in the rami

and in the pedicels of the cirri, are apparently effected, as already

noticed, by the outer or inner (as the movement may be) basal edge of

one segment being drawn a little way down within the next succeeding

lower segment. If, at the same time, both the inner and outer margins

of all the segments were drawn one within the other, the whole limb

would necessarily be shortened; and I distinctly saw a shortening

action, with very slight movement in any other direction, in the

first and second pairs of cirri; and I think it almost certain that

this movement might be performed by the other cirri. If I correctly

understand a statement of Milne Edwards,[32] this is an important fact,

as he asserts that only the higher Crustaceans possess the power of

shortening their limbs.

[32] 'Annales des Sciences Naturelles,' tom. xviii, 1852, p. 121.

When a Cirripede is alive, the action of the cirri is really beautiful:

from the position of the thoracic segments, the posterior cirri (three

pairs in the Balaninæ and four pairs in the Chthamalinæ) form a sort of

semicircle facing the mouth: the anterior cirri stand further apart,

and are opposed in pairs to each other, with the first pair pointing

beyond the mouth. Together the cirri form a hollow cone, not circular

but elongated, with the mouth situated at the lower anterior end. The

posterior cirri are protruded, by the movement of the whole thorax,

curled up, close along the carinal end of the orifice; as they are

protruded, they diverge, both by the movement of their pedicels, and,

as I believe, by the separation of the thoracic segments. As the two

rami of each separate cirrus are uncurled, they also diverge a little;

as do the double rows of spines on the segments in each ramus, by their

elasticity. By the movement of the thorax, the cirri are then swept

towards the rostrum; and, lastly, they are brought perpendicularly down

towards the mouth with a rapid movement, which would be beautifully

adapted to catch any object floating or swimming in the water; hence

I have called the action of the cirri, captorial. When the shell of

a Balanus is broken open, the second and third pairs of cirri are

repeatedly clasped over the mouth with a convulsive movement, in a

manner indicating, I think, that their chief function is to seize

and carry to the mouth any object entangled by the sweeping movement

of the three posterior pairs. The first pair is also well adapted

to aid in this seizing action; but I suspect that the long anterior

ramus likewise acts as an organ of touch, warning the animal of

danger. The mouth being itself moveable as a whole,--the outer maxillæ

being capable of a backward and forward sweeping action, and being

furnished with orifices apparently olfactory,--the inner maxillæ having

more diversified movements,--the toothed mandibles overhanging the

œsophagus,--and the œsophagus itself possessing a powerful swallowing

movement, are all admirably adapted to secure any prey, when once

entangled by the cirri.

_Mouth._

The mouth, in the sub-family Chthamalinæ, cannot be distinguished

from that of the Lepadidæ, which has been pretty fully described

in my former volume. In the Balaninæ, however, the labrum differs

considerably in not being swollen; that is, in its outer and inner

fold of membrane being close together, and in having a central notch:

the palpi are also larger, and the lower teeth on the mandibles, are

laterally (Pl. 26, fig. 5) double, as will be more particularly stated

under these two sub-families. I have given a drawing (Pl. 26, fig.

1) of the mouth, seen from above, of _Balanus perforatus_, with the

right-hand palpus (_d′_) and outer maxilla (_a′_) cut off, in order

that the labrum (_e_), mandibles (_c_), and inner maxilla (_b_) might

be better shown; the cut-off bases (_x_, _x_) of the first cirrus on

each side are also shown. In fig. 2 we have the deep supra-œsophageal

cavity in _Bal. improvisus_ torn open and laid flat, with the inner

surfaces of the labrum (_c_) and outer maxillæ (_a_) exhibited, the

palpi, mandibles, and inner maxillæ having been removed. Figs. 3 and

4 will presently be referred to; they are parts of the mouth, with

the muscles, &c. removed, of Coronula. The mouth differs extremely

little in the different genera and species of the Balanidæ, much less

than amongst the Lepadidæ. In the Balaninæ, the crest of the labrum

is sometimes hairy, instead of having, as is usual, from two to six

teeth on each side of the central notch: in _Balanus improvisus_ (Pl.

26, fig. 2) and _eburneus_, and in _Chelonobia_, the crest on each

side of the central notch (_e′_) is furnished with a row of finely

graduated teeth. A sub-triangular portion of the inner fold of membrane

of the labrum, which overhangs the œsophagus, is always thickened and

yellowish; it is also often punctured in patterns (Pl. 26, fig. 2,

_f_), which, I believe, give attachment to little muscles that serve

to open the upper end of the œsophagus. Opposite to this thickened,

sub-triangular portion of membrane, the thin membrane forming the

supra-œsophageal cavity (or the cavity surrounded by the gnathites)

is strengthened by a pair of curved ribs (_h_, fig. 2) of thickened

yellowish membrane, running down from the inner bases (_a′′_) of the

bilobed outer maxillæ to the opening of the œsophagus (_g_): a broad

branch from each of these ribs supports the sides of the orifice of

the œsophagus; and this branch almost joins on to a slightly thickened

rim or bar (_f′_), which branches off from the upper part of the

sub-triangular (_f_) inner fold of the labrum. This structure, in _Bal.

improvisus_, is represented in Pl. 26, fig. 2, as well as it could be,

considering that the deep supra-œsophageal cavity has to be torn open;

and then laid flat.

The _Palpi_ (Pl. 26, fig. 6) differ little, except in size, in the

different genera, being squarish, more or less elongated, or even

approaching to club-shaped: in most of the Balaninæ they are larger

even than the mandibles, of which they normally form a part. Their

upper margins, especially towards their free extremities, are always

thickly clothed with spines; and there is generally a single row,

either short (_r_) or long, of spines of greater length, which arise

from a little above, and stand almost in a parallel line to, the basal

margin. On the internal surface, there is sometimes a row (_t_) of

very short little spines, which overhang the crest of the labrum. The

_Mandibles_ (Pl. 26, fig. 5) have from three to five teeth; the lower

point or angle is generally pectinated. In Coronula and its close

allies, there are some small teeth intermediate between the four or

five main teeth; and in these genera, though members of the sub-family

Balaninæ, the lower teeth exhibit only rudiments of being laterally

double.[33] The _Maxillæ_ sometimes have a notch under the upper large

pair of spines, and in _Octomeris brunnea_ there is a double notch: in

many species of Balanus, the inferior corner stands up like a step (Pl.

26, fig. 7, _a_): in many other genera and species, the whole edge is

straight. In all, or almost all cases, the row of spines on the middle

portion is double. The _Outer Maxillæ_ are always bilobed on their

inner faces, and are clothed with bristles. On all the gnathites, the

bristles are often doubly serrated.

[33] M. Martin St. Ange describes, in his 'Mémoire sur

l'Organisation des Cirripèdes,' pp. 15 and 32, "_une petite

langue_" in the mouth of Lepas; but I may venture to assert that

such does not exist; it is merely the point of union between the

outer maxillæ. M. St. Ange, in his comparison of the mouth of

Lepas with that of Phyllosoma, compares the mandible of the latter

with the palpus of Lepas; the first maxilla of Phyllosoma with the

mandible of Lepas; and so on with the other gnathites.

_Muscles and functions of the Gnathites, and their confluence._--The

outer maxillæ appear at first like a deeply-lobed lower lip, for they

reach over almost to the labrum (Pl. 26, fig. 1), and thus partially

cover the other organs; they are separately capable of a strong and

rapid, to and fro movement, by which no doubt they sweep any prey,

entangled by the cirri, towards the other gnathites. Each outer maxilla

is furnished with a pair of muscles, apparently a flexor and extensor;

there is also a little muscle between the two maxillæ, I presume for

the purpose of bringing them together. The outer and inner maxillæ

generally stand close together, and in several genera a little way

apart, from the mandibles; but there is no trace of any labrum or true

lower lip, bounding the mandibles and orifice of the œsophagus. The

outer and inner maxillæ and mandibles are not opposed in pairs to each

other, but against the thickened inner fold of the labrum; almost in

the same manner as the posterior pairs of cirri are not opposed one to

the other, but to the mouth.

I have described pretty accurately the muscles of the mandibles in

my former volume, and there given a drawing (Pl. 10, fig. 1) of them.

There are four muscles: first, the depressor muscle, which is the

largest, and is attached, at its upper end, to ligamentous apodemes

under the free toothed portion of the jaw; and at its lower end,

spreading considerably out, is attached to a concavity close above the

basal margin of the labrum; to understand the action of these muscles,

it should be borne in mind that the mandible almost faces the labrum.

In some genera, as in Coronula,[34] the swelling near the basal margin

of the labrum (Pl. 26, fig. 3, _k_), caused by the internal concavity

for the above muscle, is conspicuous. The depressor muscle is opposed

by a small elevator, attached to the mandible close by the depressor;

thence it runs upwards, and is united at its upper end to the base of

the palpus, at the point where the latter adheres to the labrum: I have

ventured to call this muscle the elevator, from being apparently so

well fitted for this purpose; but I feel some little doubt, from having

observed an apparent slight movement in the palpi of living Balani;

and this is the only muscle entering those organs. The free part of

the mandible is articulated on a square, thickened piece of membrane,

forming part of the side of the mouth (Pl. 26, figs. 3, 4, _c^1_;

and Pl. 10, fig. 1, _a_, _b_, in my volume on the Lepadidæ); to this

square piece of membrane, two short muscles are attached, one above the

other, and which ought, in the Plate in my former volume, to have been

represented crossing the depressor muscle at nearly right angles; at

their further ends they are attached to about the middle of the labrum,

where, at least in Coronula (Pl. 26, fig. 3, _i_), a slight concavity

can be detected. The action of these two muscles must be to draw the

whole mandible against the labrum; and the depressor muscle might, at

the same time, draw the toothed edge downwards, and thus force any prey

into the œsophagus.

[34] This is figured by Burmeister in his 'Beiträge zur

Naturgeschichte der Rankenfüsser,' Tab. 2, fig. 6.

The inner maxillæ are likewise furnished with four muscles, very

nearly as figured in my former volume (Pl. 10, fig. 10); namely, two

muscles, one inside and the other outside the curious apodeme, which

in the Balanidæ (Pl. 26, fig. 7, _b′_) is as invariably present as in

the Lepadidæ: these two muscles are attached at their lower ends to

the outer membrane of the mouth, close to its basal articulation: the

outer one of these two muscles would, I presume, act as an elevator,

and the inner one as a depressor; the free part of the organ working on

the top of the apodeme, like an axe, on a hinge, on the top of a pole.

But there is also a larger depressor muscle, in an analogous position

with that (_i. e._ the first-mentioned muscle) of the mandibles; and a

fourth muscle, crossing the latter depressor at nearly right angles,

and attached (as far as I could make out) on the side of the orifice

of the œsophagus, close under the mandibles: the action of this latter

muscle would be to draw the whole organ towards the labrum.

I must not conclude my description of the mouth, without drawing

particular attention to its peculiar compounded nature. It is

prominent, and is capable, as a whole, of movement; it is separated

from the body by a fold or articulation, which can be traced all

round. It is, as we have seen, composed of a broad labrum and three

pairs of gnathites, which latter have only their terminal segments

free; and these surround a conical hollow, at the bottom of which

lies the opening of the œsophagus. The prominence of the whole mouth

appears to result from the lateral fusion of the two basal segments of

the three pairs of gnathites. I have examined the mouth of ordinary

Crustaceans, and can see no trace of a structure like this. That there

has been some union of the parts is indisputable; for the palpi, which

in ordinary Crustaceans are quite free, are here firmly united to the

upper and outer corners of the labrum; and indeed, at first appear to

be more intimately connected with the labrum than with the mandibles.

The palpus on its upper and exterior surface, is in direct continuity

with the square thickened piece of membrane, on which the mandible is

articulated, and likewise with that side of the upper or free portion

of the mandible which faces the labrum. This face of the mandible,

beneath the toothed edge, is hollowed out or arched (Pl. 26, fig. 5,

_p_), owing to the above-mentioned continuity of its membrane with

that of the palpus. On the lower surface, the palpus is firmly united

to the lateral corners of the labrum; or indeed the corners of the

labrum may be almost said to be formed by the soft, swollen bases of

the palpi: the point of union, when viewed from the outside, is seen

to form a knob on the shoulder of the labrum, beneath the level of

its crest, and at this knob (Pl. 26, fig. 3, close to _d′_) several

thickened bands in the surrounding membrane unite. The free portion of

the palpus stands out transversely behind (_i. e._ anteriorly to, in a

homological sense) the labrum. I suspect that the palpus possibly may

consist of two segments, of which the terminal one is free, and the

lower one confluent with the labrum.

Before proceeding any further, I should observe that figs. 3 and 4, in

Pl. 26, represent the membranes of the mouth of _Coronula diadema_,

perfectly cleaned. In fig. 3, all the front part of the mouth has been

removed, the mandible on one side, the labrum with the two palpi, and

the œsophagus being alone left, and these are viewed from the inner

side; the front part, however, of the supra-œsophageal cavity has been

cut away. In fig. 4, the labrum, with the œsophagus, has been removed,

whilst the two outer maxillæ, the right-hand inner maxilla and mandible

(with the exterior and basal portions, _d_, _d′′_, of one palpus) are

seen from the outside; but in order that these parts should all be

shown, the whole of the right-hand side of the mouth has been spread

out, for the teeth of the mandible should have stood in a vertical

line between the two outer maxillæ. In the mandibles, the free upper

part is separated, by a distinct articulation, from the square piece

of thickened membrane (fig. 3, _c^1_) on which it is supported; and

this latter is separated by a second articulation from a portion of

thickened membrane (_c^2_), the basal edge of which forms the third and

lowest articulation, separating the mouth from the body. This basal,

thickened portion of membrane curls round and inwards, towards the

outer maxillæ or front of the mouth, and its terminal points sometimes

even penetrate a little way within the muscles, like apodemes: it is

not distinctly separated by any line or suture from the membrane, which

forms the whole broad labrum; so that I at first concluded that the

labrum dipped under the mandibles, and thus afforded a support on which

they were articulated; but this appears so opposed to all analogy,

that it is more probable that the above basal thickened portion of

membrane is truly the basal segment of the mandibles, completely

confluent with the labrum; and it is, I think, not very improbable

that even a large portion of what in appearance belongs to the labrum,

namely, those concavities to which the muscles of the mandibles are

attached, may, also, be part of the basal segment of the mandibles.

Whether or no there really are two segments beneath the upper free

portion of the mandibles, which have become laterally confluent with

other parts, I must think that the square thickened piece of membrane

(_c^1_) represents at least one segment. I may here observe, that Prof.

Milne Edwards seems to consider the mandible of the higher Crustaceans

as answering homologically to the haunch of the leg; but, according to

M. Brullé,[35] there ought to be two basal segments (sous-maxillaire

and maxillaire) bearing the proper mandible, and giving rise, on the

outer side, to the palpus,--a structure which perfectly corresponds

with my view of the mandible and palpus in Cirripedes.

[35] 'Annales des Scienc. Nat.,' 3d series, Zoolog., tom. ii, p.

271.

_Maxillæ_: the point whence the long apodeme (_b′_, Pl. 26, fig.

4 and fig. 7) arises, according to Audouin's views, must mark an

articulation, and this would separate the upper free segment from the

lower segments, which I believe to be laterally confluent with the

organs on each side. The thickened membrane, of which the upper free

part is formed, extends a little distance beyond the insertion of the

apodeme; and this small portion beneath the point of insertion may

possibly answer to the square, thickened piece of membrane, or second

segment, supporting the mandibles. Beneath it, a rather wide expanse of

thin, flexible membrane reaches down to the basal fold surrounding the

mouth, and may thus form the third segment.

_Outer Maxillæ_: the upper free segment has a spinose lobe (_a′′_,

Pl. 26, figs. 2 and 4), on its inner face, which may indicate a lower

and second, almost free segment. Passing over this, we have, on the

outside of the mouth, beneath the free, upper segments, an expanse of

membrane, which, on the side, close to the inner maxillæ, is perforated

(Pl. 26, fig. 4, _n_) by orifices which I believe are olfactory. In

some species, as in _Bal. eburneus_ and _improvisus_, there is a

longitudinal medial suture in this expanse of membrane, which I suppose

indicates the lateral confluence of the middle segments of the two

outer maxillæ. A short, transverse articulation or fold separates this

middle segment (fig. 4, _a^1_) of each maxilla from the third or basal

segment; and this latter (_a^2_) is separated from the body by a very

distinct fold, which (at least amongst the Lepadidæ) sends inwards a

short, medial, tongue-formed apodeme. Here, then, we apparently have,

as in the mandibles, two segments under the upper free segment of

each outer maxilla, laterally confluent with the adjoining organs.

But I must state that, in old specimens, and only in old specimens of

_Coronula diadema_, I have found under the outer maxilla an additional

transverse ridge and fold, which plainly shows how easily a mere

thickening of the membrane might be mistaken for an articulation. I

can, however, hardly persuade myself that the articulated membrane,

under the free part of the mandibles, which has now been figured and

described, has no homological signification; and the fusion of the

palpus and labrum seems too plain to be mistaken. Hence I must conclude

that the mouth, in the Cirripedia, does truly exhibit a compounded

structure of a very peculiar nature.

_Cirri._

There are always six pairs; each biramous and multiarticulated,

supported on a pedicel formed of two segments. A shield-like swelling

at the exterior bases of these pedicels often appears like another

segment; but such, I believe, is not its nature. The five posterior

pairs answer to the five pairs of ambulatory legs in the higher

Crustaceans; and as in the case of the latter, the three, or the four

hindermost pairs almost invariably resemble each other. The first

pair, which is homologous with the outer maxillipeds of ordinary

Crustaceans, is separated by an interval from the second pair;--though

this is not the case with the legs of the pupa, from which the cirri

are metamorphosed. These anterior cirri are attached to the lateral

edges of the mouth, namely to the thickened rim of membrane, forming

the supposed basal segment of the mandibles. They are capable of more

diversified movements than the other cirri: the anterior ramus is

always elongated, with the terminal segments more or less tapering,

and is directed beyond (or anteriorly to) the mouth: the shorter

ramus closely resembles in structure the rami of the second pair.

In the Chthamalinæ the second pair, and in the Balaninæ the second

and third pairs (as will be more particularly described under these

sub-families) differ in structure from the posterior pairs, from which

they are separated by a slight interval. The number of segments on

the posterior cirri is often great, amounting in Chelonobia even to

fifty. Each segment normally is furnished on its inner face, which

is usually somewhat protuberant, with from two to rarely eight or

ten pairs of long spines or bristles, placed in a double row; the

two spines in the lower pairs stand nearer to each other, and are

shorter than the spines in the upper pairs. Between each pair of

spines there is either a single, very thin bristle, or often a tuft of

such. The pairs are directed somewhat upwards, and they diverge when

the cirri are uncurled; their function is obviously to entangle the

prey. On the dorsal or exterior surface of each segment, close to its

upper margin, there is a tuft of spines, often composed of thicker

and thinner spines; these, I believe, serve to prevent any creature

intruding within the sack. On both sides of the upper margin of each

segment, there is generally a row of short, blunt, excessively minute

spines, which only deserve notice, inasmuch as it is by their increase

in number and size, and by the spreading out of the dorsal tufts,

and, lastly, by the increase of the little tuft intermediate between

the pairs of spines situated in front, that the segments on the two

or three anterior pairs of cirri become covered, like brushes, with

bristles. The bristles or spines on the second and third cirri are

often, especially in Tetraclita, doubly and coarsely pectinated. The

bristles on the pedicels follow the same arrangement as on the rami;

namely, being in regular pairs on the posterior cirri, and crowded

thickly, like a brush, on the anterior cirri. The segments in the

shorter ramus of the first cirrus, and in both rami of the second,

and often of the third cirrus, are broader than the segments of the

posterior cirri; they are, also, especially in the genus Balanus,

frequently produced in their upper, ante-lateral corners, into

remarkable prolongations (see Pl. 29, fig. 4, of the third cirrus of

_Bal. perforatus_), clothed on their inner surfaces, and at their

extremities, by numerous bristles. The number of the segments in each

cirrus is in some degree variable, and increases with age; this is

likewise the case, to a certain extent, with the number of the spines

borne on each segment.

As compared with ordinary Crustaceans, I presume the two rami answer

to the "_tige_" and "_palpe_" of Milne Edwards; and the pedicel (as I

have called it) to the two basal segments of the leg.[36] The "_fouet_"

or flabellum does not appear to be developed in any Cirripede; for

though the filamentary appendages in certain genera of Lepadidæ, might

at first be thought to be of this nature, yet their usual position

_beneath_ the basal articulation of the first pair of cirri, and the

occasional presence of more than one, proves, I think, that such is not

the case.

[36] According to this author's new nomenclature, the pedicel would

consist of the coxopodite and basipodite; the tige would be the

ischiopodite and following segments; and the palpe would be the

exopodite; the epipodite or flabellum being absent. ('Annales des

Sciences Naturelles,' tom. xviii, 1852.)

Though the structure of the cirri is very uniform, yet we meet with

some peculiarities. In Chelonobia, the segments of the posterior cirri

bear only two pairs of main spines; whereas in some varieties of

_Balanus balanoides_, they carry as many as ten pairs in a longitudinal

row; but in this latter species, the number of these spines varies,

in a singular manner, from six to ten pairs. In Tubicinella, the

pairs of spines on the segments of the posterior cirri are arranged

so closely one under the other, that they appear almost like a single

transverse row. Considering the whole family, the third pair of cirri

differs most in structure in the different genera. Thus, in _Chthamalus

antennatus_, the anterior (or outer) ramus (Pl. 29, fig. 3) is thicker

and much longer than the posterior (or inner) ramus; the number of the

segments in one instance being, in the two rami, 53 and 18; in the

longer ramus, the spines are arranged abnormally, tending to form a

little circle round each segment; and the whole ramus may be said to be

antenniformed, and I believe acts as an organ of touch: the relative

number of the segments, I may add, in the two rami and the arrangement

of their spines varies greatly in this species. In two other species of

the same genus Chthamalus, we have _occasionally_ the anterior ramus in

some degree antenniformed, so that this whole structure is variable.

In the allied _Chamæsipho columna_, it is the posterior or inner

ramus which is antenniformed, but this peculiar development is more

plainly marked in the case of the second pair of cirri than in that

of the third pair. In _Tetraclita porosa_ it is, also, the posterior

ramus of the third pair which is antenniformed; in this third pair,

and indeed in the other cirri, the relative numbers of the segments

vary extremely. A similar structure in the third pair, but in a lesser

and variable degree, may be observed in some of the other species of

Tetraclita. In _Balanus vestitus_, also, we have, in the third pair,

an analogous structure. It is scarcely possible to believe that the

circumstance of the second pair of legs, which answer to the third pair

of cirri, being antenniformed in certain decapod Crustaceans, is an

accidental coincidence; it must be owing to some special affinity in

the two groups.