Part 52

_Structure of Parietes._--The parietes are solid, and composed of successive layers of shell; the inner surface varies in condition in the same species, being either smooth or marked with branching impressed lines, or mamillated, or often irregularly punctured for the entrance of tubuli. Owing to these tubuli, the walls, when externally disintegrated, often become punctured. In certain depressed varieties of both _C. stellatus_ and _scabrosus_ the walls are supported by irregular depending columns, placed along either one side or both sides of the sutures. In _C. intertextus_ we have the remarkable character of the wall of the shell growing (I presume after a certain age) almost rectangularly inwards, thus forming a rather wide, flat, calcareous rim round the central basal membrane. In _C. Hembeli_ the internal basal edges of the parietes, in one moderately young specimen, were rugged with irregular points, but presented no other remarkable appearance; but in five old and very large specimens, the whole basis was calcareous, being absolutely continuous with the inner lamina of the parietes, showing that the latter had grown flatly inwards all round, and had then become confluent in the middle, so that there was no longer any basal membrane; excepting, no doubt, that which had existed in the younger stage, and which would be preserved in a functionless condition between the surface of attachment and the inflected parietes.

When an opercular valve or compartment is dissolved in acid, layers of tissue are left, and these are seen to be penetrated by tubuli, which enter at the punctures before mentioned on the inside of the shell: these tubuli often stand in groups of three or four together; they are about 1/10000th of an inch in diameter. Besides these irregularly scattered tubuli, there are in the opercular valves of _C. antennatus_, innumerable smaller parallel tubuli, running to the external investing membrane.

_Structure of the Radii and Alæ._--The radii, when developed, are always rather narrow. Their recipient furrows are generally nearly as broad as the radii themselves. Their edges are either quite smooth, as in _C. antennatus_; or very finely crenated; or, as in _C. dentatus_ and _Hembeli_ (Pl. 18, fig. 3 _a_, 5 _a_), so strongly crenated as to make the suture, both externally and internally, toothed: in these two species, the radii are ribbed in transverse lines parallel to the basis, each rib corresponding with one of the projecting and interlocking teeth on the sutures. In _C. intertextus_, and much less plainly in most specimens of _C. scabrosus_ (Pl. 19, fig. 1 _a_, 2 _a_), we have a structure in appearance very different, for the radii here consist of several very oblique plates, (_i. e._ nearly parallel to the parietes) on both sides of the sutures, which are interfolded or locked together: I believe that this structure is a mere modification of that in _C. dentatus_ and _Hembeli_, the transverse ridges on the radii of those species being here developed into oblique plates. We shall hereafter meet with a similar structure in the genus Verruca; to which genus, until meeting with these two species of Chthamalus, I had thought that the interfolding sutures had been confined. The alæ have their edges generally finely crenated: during diametric growth (when such takes place), they are rarely added to above the level of the opercular membrane, and hence their summits are oblique: in _C. intertextus_, however, the alæ are laterally added to above the opercular membrane, and their recipient furrows are likewise added to, of which fact I have seen no other distinct instance in any genus; hence on both sides of the sutures, in the sheath of this species, the lines of growth are upturned. In some much disintegrated specimens, both of _C. stellatus_ (var. _depressus_) and of _C. antennatus_, the radii have been corroded away, and the diametric growth is effected exclusively by the growth of the alæ, which are moreover much exposed, and rendered conspicuous. The sheath descends a moderate distance down the shell. When a shell is boiled in potash, the sutures (excepting when abnormally calcified together, as very often happens with some species) always fall apart, showing that the union is simply by animalised matter.

_Basis._--The basis is always membranous; but we have seen, in _C. intertextus_, that the walls form a flat ledge all round the base, and that in _old_ specimens of _C. Hembeli_, they grow so far inwards and become so completely confluent, that they might most easily be mistaken for a true calcareous basis. I may add, that in one elongated specimen of _C. stellatus_ from La Plata, the walls had likewise grown rectangularly inwards, forming a flat base, and had then turned upwards in the middle, forming a medial crest, with the edges not quite calcified together. The true basal membrane is very obscurely divided into concentric slips. I observed in several species, attached to the lower surface, an excessively fine network, quadrangular or hexagonal, of yellow vessels, which seemed insensibly to pass into the sheets, discs, and globules of cement, by which the membranous basis adheres to the supporting surface. I saw, in _C. antennatus_, numerous irregular, bifurcating, and inosculating cement-ducts, of unequal diameter, often crossing each other, and sending off branches ending in points: the older ducts, instead of being solidly filled up with cement, were only divided by septa. I did not succeed, in any species, in discovering the cement-glands.

_Mouth._--The _labrum_ is slightly bullate, with the middle portion depressed, but not forming a notch; in some species it is hairy, and in some pectinated with short spines. The _palpi_ are of moderate size. The mandibles have from three to five main teeth, the number sometimes varying even in the same species: the lower teeth are either plainly double laterally, or very obscurely double, or to all appearance quite single: a rather large lower portion of the mandible is finely pectinated. The maxillæ are always notched under the two or three large upper spines: the notch bears some fine spines: beneath the notch there are some large spines, and at the inferior angle some smaller ones.

_Cirri._--The first and second pairs are always very short compared with the four posterior pairs. The rami of the first pair are slightly unequal. The third pair, in length and arrangement of the spines, very closely resembles the three posterior pairs; in _C. intertextus_, however, the few basal segments, chiefly on the anterior ramus, are thickly clothed with bristles, like the segments of the second cirrus. In _C. antennatus_ (Pl. 29, fig. 2), the anterior ramus of this same third pair is usually (one single specimen being excepted) much elongated, having at least twice as many segments as the posterior ramus, but the number is variable; and these segments, either all, or only the upper ones, instead of having their spines regularly arranged in pairs, in a double row, are surmounted each by a circle of spines: I suspect that these elongated rami of the third cirrus act as antennæ. It can hardly be an accidental coincidence, that certain genera, as Lysmata and Pandalus, amongst the Macrourous Crustaceans, have the same leg (homologically the second thoracic limb) elongated and antenniformed. Certain varieties of _C. stellatus_ and _cirratus_, also, have the anterior ramus of this same third cirrus considerably elongated. We are thus reminded of the remarkable variability in the numbers of the segments, and in the arrangement of their spines, in the cirri of _Tetraclita porosa_; in that species, however, it was chiefly the terminal segments of the _posterior_ ramus of the third cirrus which were so highly variable. The three posterior pairs of cirri in Chthamalus support from three to five pairs of main spines on each segment, the number often varying in the same species, with some minute intermediate bristles. The dorsal surfaces of these segments, in some of the species, are serrated in an upward direction.

_Body, &c._--The body does not present any particular character: in _C. scabrosus_ there is a slight ridge running from the base of the first cirrus towards the adductor scutorum muscle: this ridge is clothed with a few hairs; there are also some hairs at the carinal end of the sack. In _C. dentatus_, also, there are hairs on the outer tunic of the prosoma. In two species which I opened, there were no cæca to the stomach. The ova vary in length from 13 to 14/2000th of an inch in length; they are packed in two lamellæ lying on each side of the animal's body. The larva just escaped out of the egg, in _C. stellatus_, _scabrosus_ and _dentatus_, had a large probosciformed mouth.

_Branchiæ._--These present a very singular amount of difference within the limits of the same genus. In _C. stellatus_ and _antennatus_ we have a simple fillet, tapering a little, barely plicated, and about half, or more than half, as long as the sack; in _C. scabrosus_ the branchiæ are entirely aborted, or are perhaps represented by the slight hairy ridge at the carinal end of the sack: in _C. dentatus_, on the other hand, each branchia consists of two large folds, barely plicated, almost covering the whole side of the sack, so that here the branchiæ are developed to an unusual degree, more than in Balanus, and as in Coronula and its allies: in the same manner as in these latter genera, the outer fold is considerably larger than the inner fold.

_Affinities._--_C. intertextus_ is the most distinct species of the genus, as shown by the peculiar radii and alæ, by the scuta and terga being calcified together, by the character of the third pair of cirri, and by the inflected parietes forming a ledge round the membranous basis; but in this latter respect _C. intertextus_ resembles _C. Hembeli_. _C. Hembeli_, in its serrated radii, is closely related to _C. dentatus_; and this latter species differs in the structure of its radii only in degree from certain varieties of _C. stellatus_. Lastly, _C. intertextus_, in its peculiar radii, closely resembles _C. scabrosus_, and this latter species does not differ much from the other species. Hence the genus Chthamalus has no claims to be subdivided into smaller genera.

_Range._--The species are found all round the world, from (as far as I have seen) 54°-55° north, to Cape Horn, in 55°-56° south. All the species, of the habits of which I know anything, are littoral; and in many parts of the world are excessively numerous, quite covering large spaces of the coast-rocks, and often coating the coast-shells. _Chthamalus dentatus_ is littoral, like the other species; but it often lives attached on _Balanus tintinnabulum_ and _amphitrite_, on the bottoms of ships arriving in British ports from the west coast of Africa. I do not know of any instance of more than two species occurring in the same region. Some of the species have large ranges: _C. scabrosus_ extends from the Falkland Islands and Tierra del Fuego to Peru; and _C. stellatus_ has an enormous extension over almost the whole world, excepting the west coast of South America and Australia. I do not believe any species of the genus, owing probably to their littoral habits, have hitherto been found fossil.

1. CHTHAMALUS STELLATUS. Pl. 18, fig. 1 _a_-1 _h_.

LEPAS STELLATA. _Poli._ Testacea Utriusque Siciliæ (1795), Tab. 5, fig. 18-20.

---- DEPRESSA (_var._) Ib., Tab. 5, fig. 12-16.

CHTHAMALUS STELLATUS. _Ranzani._ Memoire di Storia Naturale (1820), Tab. 3, fig. 21-24.

---- GLABER (_var._) _Ranzani._ Ib.

---- STELLATUS. _Philippi_ (!). Enumeratio Mollusc. Siciliæ.

LEPAS PUNCTATUS. _Montagu_ (!). Testacea Britannica (1803).

_Shell white or gray, generally much corroded and punctured: radii (when present) narrow, with their sutural edges most finely crenated; tergum with the crests for the depressor muscle depending barely beneath the basal margin._

_Var._ (_a_, communis) fig. 1 _a_, 1 _f_: _Shell conical depressed, upper part corroded, walls folded, sutures moderately plain or obliterated; radii not developed; orifice broadly oval_.

_Var. (b): Shell elongated, sub-cylindrical; sutures obliterated; surface much corroded; orifice almost circular._

_Var._ (_c_, communis) fig. 1 _c_, 1 _e_, 1 _h_: _Shell conical, folded, sometimes covered by membrane; radii developed, narrow; orifice sub-hexagonal, toothed._

_Var._ (_d_, fragilis) fig. 1 _d_: _Shell conical, smooth, thin; compartments easily separable, pale-coloured; radii developed, narrow; orifice large, toothed, sub-hexagonal._

_Var._ (_e_, depressus) fig. 1 _b_, 1 _g_, 1 _h_: _Shell much depressed, surface much corroded, smooth: alæ largely exposed, marked by lines of growth: radii not present; parietes on the under side often supported by pillars; orifice sub-hexagonal._

_Hab._--Southern shores of England, Ireland, Isle of Man, Mediterranean, Madeira, Cape de Verde Islands, Southern United States (Charlestown), West Indies, Brazil (Bahia), Rio Plata (Guritti Island), Red Sea, Philippine Archipelago, Coast of China, Gulf of Corea, Oregon or Northern California.

This species is very widely distributed and extremely common. On the coast-rocks of the southern shores of England it is, in parts, even more numerous than the _Balanus balanoides_, with which it often grows mingled. As already stated, it is often confounded in British collections with this species of Balanus, under the name of _Balanus punctatus_; that Montagu had this Chthamalus in view, when describing his _Lepas punctatus_, is certain, from his original specimen in the British Museum, but whether this was the case with his predecessor, Pulteney, in the Dorset Catalogue, I do not feel so sure. _Chthamalus stellatus_ varies, as we shall immediately see, extremely in appearance. Some of the varieties, as _var. depressus_, which in external aspect are especially distinct, I have no doubt are really varieties, but whether this is the case with some of the forms from the more distant localities, is a little more doubtful; but I beg that it may be observed, that I have, in the case of every one of the varieties, and of all the specimens from distant localities, cleaned with potash and most carefully examined the disarticulated valves, and likewise dissected the included animal's body.

_General Appearance._--British specimens are usually conical, and have their walls folded, corroded in their upper parts, with the radii not developed, and the sutures more or less obliterated; in this state the orifice of the shell is entire, and very broadly oval: I have seen specimens in this condition from Madeira, Brazil, and the Gulf of Corea. The shells when crowded, are rendered cylindrical, and more or less elongated, with the sutures as viewed externally quite obliterated; the surface rugged and much disintegrated; and the orifice nearly circular: in this extreme condition I have seen specimens from England, from near Genoa, and from the mouth of the Plata: the specimens from these last two localities were of remarkable size, being half an inch in diameter, and rather more than half an inch in height. Again, other British specimens (fig. 1 _c_), though not nearly so common, are rather steeply conical, and have not only their sutures distinct, but narrow radii are plainly developed on apparently both sides of the sutures; in this case the orifice is slightly toothed, and is rather elongated: I have seen specimens in this condition, but with their walls rather more deeply folded, from the Cape de Verde and Philippine Archipelago. Other specimens, from some unknown tropical sea, differed only in the walls being but slightly folded, and being so perfectly preserved as to be externally covered with membrane: this latter circumstance gave the specimens a peculiar appearance. In this condition were some specimens (fig. 1 _d_) attached to oysters sent to me by Professor Agassiz, from Charlestown; and which differed from all the others that I have seen, in the thinness and smoothness of their compartments, and in the facility with which the compartments separated from each other and from the surface of attachment: hence I have called this form, which, until finding more important differences, I must rank as a variety, _fragilis_: it has stronger claims than the other varieties to be specifically separated; but I suspect that it has been exposed to some peculiar modifying conditions.

Lastly, we have the variety (fig. 1 _b_) excellently figured and described by Poli, under the name of _L. depressa_: this is much depressed, with the walls thin, not folded, with the surface much corroded, with the sutures very distinct, with the radii not at all or barely developed, but with the alæ largely developed, and marked with lines of growth, resulting from the diametric growth of the shell: the orifice is hexagonal, but broadest towards the carinal end: most of the specimens, _but not all_, have on the under sides of their compartments rugged pillars depending from beneath the alæ, for the purpose, apparently, of supporting the much depressed shell. This much depressed variety attains a larger basal diameter (but not a greater bulk or internal capacity) than any other variety, namely, sometimes three quarters of an inch. The great peculiarities of this variety result, apparently, from its much depressed form, deeply corroded not folded walls, and considerable diametric growth; from the latter cause the alæ are largely developed; as I can find neither internally nor externally any fixed diagnostic character I have not hesitated to rank this form as a variety. Poli found his specimens mingled with the ordinary _C. stellatus_, on the shores of Sicily; and I collected at St. Jago, in the Cape de Verde Archipelago, some specimens nearly as well characterised, also associated with the common variety.

The shell itself is dirty-white or gray, or brown: in some varieties, however, the white is nearly pure: internally the parietes are generally tinted dull purple. The corium of the sack is dark greenish-black, with a white edge to the lips lining the aperture between the opercular valves.

_Structure of Shell and Radii, &c._--The under side of the parietes is either quite smooth, or marked with slight, branching, depressed lines; or mamillated; or irregularly studded with large pores. I have already alluded to the pillars, depending from the under sides of the alæ in most specimens of _var. depressus_: these pillars tend to form ridges, parallel to the sides of the compartment, like those we shall presently see in certain specimens of _C. scabrosus_. The radii are very narrow, when best developed; their edges, when disarticulated, can be seen, when examined by a strong lens, to be finely crenated. The edges of the alæ are likewise very finely crenated.

_Scuta._--The outline of these valves varies considerably in specimens taken out of the same group: we have either a nearly equilateral triangle (fig. 1 _f_) or the tergal margin (1 _e_) is much shorter than the other margins. There is always a deep depression for the adductor muscle, and a small pit of very variable depth for the lateral depressor. But the tergal margin offers the greatest variability; here we see a very prominent articular ridge or fold, having either a straight edge or a single or double prominence (fig. 1 _e_-1 _h_). In specimens in the same group we find considerable variation in these points; but the amount of difference is sometimes so great, that I long hesitated whether to rank some of the varieties as species. The _Terga_, likewise, vary greatly in shape and width: in some of the commonest varieties (and in _var. depressus_) the valve is very narrow, with the under surface channelled or concave: in other varieties the valve is much broader and flatter. The spur is but slightly developed. The crests for the depressor muscles barely descend beneath the basal margin of the valve. The articular ridge, in some varieties (as in specimens from Madeira and the Cape de Verde Islands), is extremely prominent and straight (1 _f_); in others, it is little prominent and deeply sinuous (1 _h_). This great variability in the articular margins of the scutum and tergum seems to be mainly due to the corrosion to which these valves have been subjected, and their consequently modified growth: in some specimens the articular ridge of the scutum, and in others that of the tergum, has been largely developed, in either case their mutual outlines being greatly affected.

_Branchiæ._--These are narrow, hardly at all plicated, elongated, being about half as long as the sack. In a specimen from La Plata, this organ ended rather more abruptly in a point than it did in other specimens.

_Mouth._--The crest of the labrum is usually hairy, but in a specimen from Bahia (Brazil) there were some very fine teeth. The palpi vary somewhat in shape, and sometimes have a row of bristles along their basal exterior margins. The mandibles usually have four main teeth, the lowest one being confluent with the inferior pectinated angle: in _var. depressus_, in the same individual, there were only three teeth on one side of the mouth and five on the other; the lower main teeth are laterally double, but generally one tooth of each pair is so small and obscure as to be perceived with difficulty. In the maxillæ, there are some large spines above the notch, and in the notch some fine ones: in _var. depressus_, and in a cylindrical _var._ from La Plata, there was quite a tuft of small spines above the notch.

_Cirri._--The outer surface of the pedicel of the second cirrus bears a tuft of long, fine, plumose hairs: the terminal segments of the rami of this cirrus sometimes (as in the La Plata specimens) support a clump of coarsely pectinated spines. In specimens having six segments in the shorter ramus of the second cirrus, the shorter ramus of the third cirrus had fifteen segments. The two rami of the third cirrus are usually equal in length and in the number of their segments; but in the Brazilian specimen there were fifteen segments in the posterior, and twenty-six in the anterior ramus; in another specimen, fixed on a tropical Perna, there were in the two rami of this third cirrus eighteen and twenty-four segments. In the three posterior pairs of cirri each segment carries either four or five pairs of main spines: the segments vary a little in the degree to which they are elongated, being most elongated in the var. from La Plata, with an elongated shell.