Fig. 7. TERGUM (internal view).
Sessile Cirripedes, partly from being attached to surfaces having very different characters, partly from undergoing a varying amount of disintegration, and partly from unknown innate causes, are extremely variable. Under the head of Variation, [in the Family Balanidæ] and [under the Genus Balanus], I have enlarged on this subject, and have shown that there is scarcely a single external character which is not highly variable in most of the species. As whole groups of specimens often vary in exactly the same manner, it is not easy to exaggerate the difficulty of discriminating species and varieties. It is absolutely necessary in most cases, for mere identification, that the valves of at least one specimen in a group should be disarticulated and well cleaned (for which purpose caustic potash is most useful), so that the internal characters may be examined. Whoever attempts to make out from external characters alone, without disarticulating the valves, the species, (even those inhabiting one very confined region, for instance the shores of Great Britain,) will almost certainly fall into many errors: hence it is, and can thus only be accounted for, that I have not seen one collection of British specimens with all the species, though so few in number, rightly discriminated; and in the large majority of cases, either two or three species, certainly distinct, were confounded together, or two or three varieties, as certainly not distinct, were separated from each other.
On the Names Given to the Different Parts of Cirripedes.
In my former volume I have stated that I found it indispensable, in part owing to the extreme confusion of the nomenclature previously used, to attach new names to several of the external parts of Cirripedes. Almost all these names are applicable to the [Balanidæ], or sessile Cirripedes, and to the [Verrucidæ]; but a few additional names are requisite, which, together with the old names, will, I hope, be rendered clear by the accompanying woodcuts. In sessile Cirripedes, the whole of that which is externally visible, may for convenience sake be divided into the operculum or opercular valves (valvæ operculares), and the shell (testa), though these parts homologically present no real difference. The operculum is seated generally some little way down within the orifice of the shell; but in very young specimens and in [Verruca], the operculum is attached to the summit of the shell, and the shell, without the operculum being removed, can hardly be said to have any orifice; though, of course, the opercular valves themselves have an aperture for the protrusion of the cirri.
The shell consists of the basis (called the support by some authors), which is membranous or shelly, and flat or cup-formed, and of the compartments (testæ valvæ), which vary from eight to four in number, and occasionally are all calcified together.
The compartment, at that end of the shell where the cirri are exserted through the aperture or lips of the operculum, is called the carina (fig. [1]); the compartment opposite to it is the rostrum,—these two lying at the ends of the longitudinal axis of the shell. Those on the sides are the lateral compartments; that nearest the carina, being the carino-lateral (testæ valva carino-lateralis), that nearest the rostrum, the rostro-lateral, and the middle one, simply the lateral compartment; but these three compartments are rarely present together. The rostro-lateral compartment, which always resembles fig. [2], and may be always known by having radii on both sides, is often absent; and not rarely the lateral and carino-lateral compartments are confounded together, or one is absent; in such cases the compartment that is left is simply called the lateral one. The compartments are separated from each other by sutures, which are often so fine and close as to be distinguished with difficulty. The edge of a compartment, which can only be seen when disarticulated from its neighbour, I have called the sutural edge (acies suturalis).
Each separate compartment consists of a wall (paries), or parietal portion (pp in woodcuts), which always grows downwards, and forms the basal margin; and is furnished on the two sides either with alæ (fig. [4]), or with radii (fig. [2]), or with an ala on one side and a radius (fig. [3]) on the other.