In the sketch of the three Orders given in the [Introduction], it will have been seen that the differences in their structure are so great, that it would have been hardly possible to have given a single blended account of the whole Class. But as all common Cirripedes are included in the present Order, here would have been the natural place for a full description of their external and internal structure. Having, however, been necessarily, yet perhaps unfortunately, led to give, in my former volume, a description of this kind of the Lepadidæ; and as it is necessary to give a similar account of the other great family of the Order, namely, the [Balanidæ], I have found it more convenient to make this latter account comparative and supplemental to the former one on the Lepadidæ, and so serve for the Order, rather than attempt to give a separate description in full of it. For this latter plan would have involved much useless repetition, as, on account of the many exceptions and limitations necessary to almost every statement, there is little choice between a description of great length and a mere diagnostic character of the Order, such as I have given above.

The [Thoracica] may be divided into three very natural Families, of nearly equal value; firstly, the [Balanidæ], or sessile Cirripedes, which may be subdivided into two sub-families, also very natural, the [Balaninæ] and [Chthamalinæ]; secondly, the [Verrucidæ], containing only one genus; and thirdly, the Lepadidæ, or pedunculated Cirripedes. These three families differ from each other, besides in mere external appearance, almost exclusively in the relation of the different portions of their external covering or carapace, and of the muscles moving such portions. In the [Balanidæ], the four opercular valves surrounding the orifice leading into the sack, are capable of other movements, besides being opened and shut; whereas all the other valves are immoveably united together. In the Lepadidæ, the valves answering to the opercular valves, are furnished with a muscle only for shutting them; whereas the peduncle answering to the basis in the [Balanidæ] is capable of various movements. In the [Verrucidæ] the shell is singularly asymmetrical; only half of the operculum (either the right or the left side, this varying even in the same species) being moveable; the other half being immoveably united to the remaining valves; and the whole shell has only one muscle serving to shut the moveable half of the operculum. All the internal parts and organs are very similar in the above three Families. If, however, the internal structure of one of the two sub-families, into which the [Balanidæ] may be divided, namely, of the [Balaninæ], be compared with that of the Lepadidæ, several important differences may be detected;—on the one hand, in the [Balaninæ], the presence of branchiæ, the extremely complicated cementing apparatus, the difference in structure between the third and succeeding pairs of cirri, the large palpi, the notched labrum, and the laterally double teeth of the mandibles;—and on the other hand, in the Lepadidæ, the presence of ovigerous fræna, caudal appendages, bullate labrum, and often prominent olfactory orifices. But if the Lepadidæ be compared in these several respects with the other sub-family, or [Chthamalinæ], which cannot possibly be removed out of the family of [Balanidæ], many of these differences break down and disappear, in some or all of the species.

The Lepadidæ include, as has previously been noticed, a much greater range of difference than the [Balanidæ]; and this is what might have been expected, for it is the most ancient family, and extinction has done its work, separating genera, which, in accordance to analogy, we may suppose were once more nearly connected by intermediate forms. The Lepadidæ, in one sense, may be taken as the type of their order; for they have undergone less “morphological differentiation;” that is, they differ the least from the last larval stage, and seem to give the most general idea of a Thoracic Cirripede. On the other hand, if we mean, as some authors do, by the word type, that form which, in the group in question, has been most modified, and illustrates every peculiarity of its class in the strongest manner, then we must look to the [Balaninæ], and to its typical genus, [Balanus], for the most Cirripedial form. In this genus the different portions of the carapace differ most, and subserve to a certain extent different ends, and in minute structure are most complicated; here the cementing apparatus, which offers the main characteristic of the whole sub-class, is most complex; here the several pairs of cirri differ most from each other in structure and action; here the peculiar branchiæ (organs apparently derived from the modification of another organ, itself confined to Cirripedes, viz., the ovigerous fræna) are best developed; here the nervous system is most highly concentrated; and, lastly, here we meet with the largest and most massive species of the whole group.

1. Family BALANIDÆ, (or Sessile Cirripedes).

Cirripedia without a peduncle; scuta and terga furnished with depressor muscles; other valves united immoveably together.

TABLE OF CONTENTS.

Page
Structure of shell[34]
”of the individual compartments[43]
”of the radii[45]
”of the alæ[47]
”of the sheath[48]
”of the basis[49]
”of the opercular valves (scuta and terga)[51]
Growth of whole shell and microscopical structure[54]
Muscles of sack[61]
Branchiæ[63]
Thorax and body[65]
Muscular system[68]
Movements and muscles of the cirri[71]
Mouth[74]
Cirri[81]
Caudal appendages[85]
Alimentary canal[85]
Circulatory system[87]
Nervous system[88]
Eyes and vision[93]
Acoustic organs[95]
Olfactory sacks[97]
Male organs of generation[97]
Female organs of generation[100]
Metamorphoses and homologies[102]
Larva, first stage[103]
Larva, second stage[109]
Larva, last or pupal stage[110]
Act of metamorphosis[126]
On the homologies of the carapace[131]
Cementing apparatus[133]
Affinities, classification, variation[152]
Rate of growth, exuviation, &c.[156]
Geographical range and habitats[159]
Geological history[172]

Almost every one who has walked over a rocky shore knows that a barnacle or acorn-shell is an irregular cone, formed generally of six compartments, with an orifice at the top, closed by a neatly-fitted, moveable lid, or operculum.[19] Within this shell the animal’s body is lodged; and through a slit in the lid, it has the power of protruding six pairs of articulated cirri or legs, and of securing by their means any prey brought by the waters within their reach. The basis is firmly cemented to the surface of attachment. The whole shell, basis, and operculum consists, as we have already seen, of the first three segments of the head, modified into a singularly constructed carapace, which encloses the mouth and rest of the body. The anterior extremity of the shell is situated in the centre of the basis, where indeed, by due care, the antennæ of the pupa may be always detected; and the posterior extremity is directed vertically upwards.