POLYTREMA. _De Ferussac._ Dict. Classique d'Histoire Naturelle, 1822.
LEPAS. _Gmelin._ Systema Naturæ, 1789.
BALANUS. _Bruguière._ Encyclop. Method., 1789.
---- _Lamarck._ Animaux sans Vertèbres, 1818.
[105] From a note by the Editor, it appears that Schumacher's essay
appeared before the number of the Journal containing Leach's paper,
so that Schumacher's name must be adopted.
[106] I have not seen a _complete_ copy of this work, and give the
title from a catalogue; the running heading of the part containing
the Cirripedia, is "Opuscoli Scientifici."
_Compartments four, sometimes externally calcified together: parietes
permeated by pores, generally forming several rows. Basis flat,
irregular, calcareous, or membranous._
_Hab._--Throughout the tropical and warmer temperate seas.
_General Appearance._--The shell is conical, more or less depressed,
and very rarely, even when growing in crowded groups, becomes
cylindrical or elongated. The orifice is seldom large, generally
diamond-shaped or oval. The colour is either nearly white or purple,
occasionally even inky black, or very dark green, and sometimes of a
pale pink peach-blossom. The surface is sometimes smooth, but more
commonly longitudinally ribbed; the outer lamina of shell is very often
wholly corroded away, excepting close to the basis, leaving the solidly
upfilled parietal tubes exposed: these give the shell a striated
appearance, or they appear like flattened tapering points adpressed to
its surface (Pl. 10, fig. 1 _b_): Lamarck attempted to express this
appearance, by using the specific name of _stalactiferus_. The colour
of the shell depends, to a considerable extent, on the colour of the
shelly matter in these exposed parietal tubes. We shall presently see
that the corrosion and disintegration, to which some of the species
are so liable, plays an important part during their growth. The radii
are either well developed, as in most of the species; or they are
entirely absent, as in the great majority of specimens of _T. porosa_
and _serrata_. In many individuals of _T. porosa_ and _purpurascens_
not only are the radii absent, but the four compartments are calcified
together without any trace, on the external surface, of the sutures.
The largest specimen which I have seen of _T. porosa_, which is the
largest species, was two inches in basal diameter, and nearly one inch
and a half in height.
_Scuta._--These valves are sub-triangular, and either longitudinally or
transversely elongated. Externally, the growth-ridges are moderately
prominent, and in _T. costata_ and _cœrulescens_ they are crossed by
longitudinal striæ. Along the occludent margin, the inflected extremity
of each alternate growth-ridge is generally much thickened,--a set of
teeth being thus formed, by which the two valves are locked together.
In _T. porosa_, this character is variable, for sometimes every
alternate ridge, and sometimes only two or three ridges, separated
from each other by several growth-ridges, are thus developed into
teeth. The _articular_ ridge is either moderately prominent, or is
extremely prominent, as in _T. cœrulescens_; but the lower edge in no
case depends as a free, hinge-like style, as sometimes in Balanus.
The _adductor_ ridge is generally well developed and distinct from
the articular ridge: in _T. purpurascens_ it is very blunt: in _T.
serrata_ it is united to the articular ridge half way up it, thus
forming a deep tubular cavity running up to the apex of the valve: in
_T. cœrulescens_, the adductor ridge is very short, and is united to,
or almost continuous with, the lower end of the articular ridge, a
small sub-cylindrical tubular cavity being thus formed. Small crests
exist for the attachment of the rostral and lateral depressor muscles,
in most of the species, excepting _T. purpurascens_ and _costata_, in
which, however, more especially in the former, there are, instead
of crests, minute pits for the attachment. These crests vary much in
prominence in the same species.
_Terga._--These valves present no essential differences from those of
Balanus; they are sometimes beaked, and the beak is hollow and occupied
by a thread of corium, as in that genus. The external surface of the
valve is often depressed in the line of the spur, but there is never a
longitudinal furrow with the edges folded in, as in Balanus. The spur
is very short in _T. purpurascens_. In _T. radiata_, the articular
ridge is remarkably prominent. The crests for the depressor muscles are
well developed in all the species. The shape of the terga is variable
in nearly all the species, and greatly so in _T. porosa_.
_Compartments._--Owing to there being only four compartments, the
lateral pair are large; the size of the carina relatively to the
rostrum varies, according as its alæ have been added to during
diametric growth. The walls are very thick, and are composed of
numerous tubes, in some species as many as fourteen or fifteen rows
being exposed on the basal margin (Pl. 10, fig. 1 _g_). The tubes
are generally angular, and slightly elongated in the ray of the
circular shell; sometimes they are nearly circular and small. New
tubes are formed only at the basal edge of the outer lamina, by the
bifurcation of the septa which form the tubes. In very young specimens
there is only a single row of tubes; and in _T. rosea_ this holds
good throughout life: in this species (fig. 3 _d_) the tubes, in the
single row, are large and quadrangular, and the outer lamina of shell
is strengthened by numerous, small, internal, longitudinal plates.
I believe the branching septa, which separate and form the parietal
tubes, correspond with the longitudinal septa in the more simple
walls of Balanus. The tubes become solidly filled up, in their upper
parts, with hard, and generally coloured shelly matter. The degree
to which they are filled up differs in the different species; the
external side of each tube is always first thus coated. The thin outer
lamina of shell, in several of the species, commonly disintegrates and
disappears; the upfilled parietal tubes being thus exposed. The inner
lamina of the walls is generally smooth, but in _T. radiata_ it is
longitudinally ribbed, as in most species of Balanus. The sheath is
generally dark-coloured; its lower edge does not project or overhang
the inner lamina, as is usual in Balanus, excepting in _T. serrata_,
and in some few varieties of _T. porosa_.
The _Radii_, when developed, are either narrow or broad, with their
summits either oblique or extending in a straight line from the top
of one compartment to that of another. In _T. serrata_, I have not
seen a single specimen with the radii developed; in _T. porosa_, they
are very seldom developed, and then, apparently, only in quite young
specimens, in which they are narrow; in _T. purpurascens_, they seem to
be about as often developed as not, and when present they are broad;
in _T. costata_, _cœrulescens_, and _radiata_, they are always largely
developed. In some specimens of the species, in which the radii are
not developed, even the sutures do not reach the external surface;
the outer lamina of the parietes being continuous all round, so that
the shell seems formed of a single piece. Even in such specimens, the
four compartments, viewed from within, can be seen to be distinct; and
the sutures can generally be traced across the whole thickness of the
parietal tubes; in this latter case, when the sutures are broken open,
the radii are seen to be represented (Pl. 10, 1 _h_) by a few small
sinuous ridges. Owing to the disintegration of the upper and outer part
of the shell, and the consequent exposure of parts of the sheath and
alæ, the radii sometimes appear as if developed, when such is not the
case. With respect to the internal structure of the radii, they are
formed, in _T. purpurascens_ and _costata_, of tubes, like those of the
parietes, and therefore according to the normal plan; whilst in the
other species they are formed by longitudinal sinuous ridges, sending
out on each side irregular denticuli; and the interspaces between
the ridges are filled up solidly during the growth of the radii, in
all the species, except in _T. radiata_, in which they are left to a
considerable extent open. These sinuous ridges, with their denticuli,
homologically represent the branching septa which form the parietal
tubes. The edges of the alæ are crenated in all the species, except in
_T. costata_.
_Diametric growth._--When first examining groups of _T. porosa_, in
none of which the radii had been developed, and in which, consequently,
the shell could not have grown in diameter, but only at its basal
margin, I was at first unable to comprehend, how the upper part of the
shell and the orifice could have acquired their proper proportional
width. The young shell, at its first formation, starts with an orifice
so small that a pin could hardly be inserted in it; and this in many
individuals is never increased in diameter by the diametric growth
of the shell; but in place of this, as the conical shell is added to
at its base, the whole upper part disintegrates and wears away, the
orifice becoming thus enlarged. We thus see that the corrosion and
wearing away of the upper part of the shell is a necessary element in
its growth. The development of the radii, which in some of the species,
as in _T. purpurascens_, at first seems to be quite capricious, really
depends upon the fact, whether the specimens have been exposed to
disintegration; for I have almost always found that when the outer
lamina of shell has been well preserved, the radii have been developed,
and the orifice has been enlarged by their growth, instead of by the
wearing down of the upper part of the conical shell.
_Basis._--This consists of a very thin, flat, though irregular,
translucent, calcareous plate, which towards the edges is sometimes
membranous. In _T. purpurascens_, the basis is entirely membranous.
When a portion of the calcareous base is dissolved in acid, a tissue
is left, composed of several laminæ, to which numerous bifurcating
cement-ducts are attached: even before dissolution, these delicate
bifurcating ducts can just be perceived by the aid of a simple lens.
_Mouth._--The several organs present no particular characters. There
are generally three teeth on each side of the notch in the labrum.
The palpi usually have parallel sides, but are club-shaped in _T.
purpurascens_ and _costata_. The mandibles have generally four teeth,
but there are five in _T. vitiata_, and only three in _T. costata_. The
maxillæ are notched. The outer maxillæ are bilobed in front.
_Cirri._--The segments of the three posterior pairs usually support
only three pairs of main spines, but there are four pairs in _T.
vitiata_ and _costata_: between each pair, there is either a tuft of
fine spines, or a single fine spine. The rami of the first cirrus
are unequal in length. In the third cirrus, the posterior ramus is
sometimes much elongated, but sometimes both rami are short and blunt.
Some of the segments in the third cirrus often support very coarsely
and doubly pectinated spines. Under the head of _T. porosa_, it will be
seen to what a remarkable degree the relative numbers of the segments
in the several cirri vary, even in specimens taken out of the same
cluster.
The _Branchiæ_ are well developed (at least in _T. porosa_ and
_costata_), as a large, plicated, tapering fold, with a small second
fold on the inner side at the base. In _T. porosa_ the _stomach_ is
destitute of cæca. The _vesiculæ seminales_ in this same species are
large, with their broad, blunt ends reflexed. The _ovarian_ tubes
surround the sack, and cover the basal plate.
_Distribution and Habitats._--This genus is confined to the tropics,
and to the warmer parts of the temperate seas: in the southern
hemisphere, it ranges south, to the Cape of Good Hope and to Van
Diemen's Land: in the northern hemisphere it does not appear to range
so far; I do not know of any authentic case of a species having been
found in the Mediterranean, or on the shores of the United States,
north of the West Indies. _Tetraclita porosa_ is found round the whole
world; _T. radiata_, also, has a very wide range, inhabiting the West
Indies, the East Indian Archipelago, and New South Wales. This latter
species, as well as _T. cœrulescens_, is often attached to the bottoms
of ships, adhering to _Balanus tintinnabulum_. The several species live
attached to tidal rocks, to littoral shells, or to massive corals. I
have met with two instances, in the West Indies and the Philippine
Archipelago, of _T. porosa_ adhering to wood. _Tetraclita porosa_
seems to feed chiefly on crustaceans: the number and the size of the
amphipods, isopods, and entomostracans, together with an annelid,
in the stomachs of some specimens from South America, was quite
surprising. As many as five species occur in the same region, in the
eastern half of the world; thus on the shores of New South Wales, we
have _T. porosa_, _vitiata_, _radiata_, _purpurascens_, and _rosea_; in
the Philippine Archipelago, we have _T. porosa_, _vitiata_, _costata_,
and _cœrulescens_.