Having now given this short comparative sketch of the structure of a Cirripede, I may venture to express strongly my opinion, that the group is formed on a distinct type; as different from the other three or four main Crustacean groups, namely, the Podophthalmia, Edriophthalmia, Branchiopoda, and Entomostraca, as these differ from each other; the differences, moreover, being of the kind considered by the highest authorities on this subject, as the most important. It should be observed that there is no special blending at either end of the Cirripedial series, towards any one of the other main groups of Crustacea; it is hardly possible to take some one Cirripede, and say that it leads, more plainly than some other Cirripede, into ordinary Crustaceans. Moreover, a great range of structure, as we shall soon briefly show, is included within the group: I can adduce three or four undoubted Cirripedes, very considerably more different from each other, than any two members within the sub-class Podophthalmia, or within the Edriophthalmia, or the Branchiopoda, and quite as different as within the Entomostraca.
The opinion here expressed, that Cirripedes form a sub-class of equal value with the other main Crustacean groups, I am well pleased to find, accords with Mr. Dana’s[13] view, who remarks that this sub-class “has so many peculiarities of structure, that it should be regarded as a distinct type, rather than a subordinate division of the third (or Entomostracan) type.” M. Milne Edwards,[14] after dividing all Crustacea into two groups, divides one of them into four legions; and of one of these, the Entomostraca, he makes the Cirripedes a sub-group. I feel so entire a deference for any opinion on affinities or classification expressed by Milne Edwards, that I differ from him with the greatest hesitation. He does not give his reasons for assigning so subordinate a rank to Cirripedes, but I imagine it is from the nature of their metamorphoses: but if this be the case, I cannot understand why he should assign to his Branchiopods a rank equal to his Entomostracans. Moreover, I must repeat, that I do not believe that the larvæ do resemble the larvæ of Entomostracans and Branchiopods nearly so closely as at first appears to be the case. I may add, that Burmeister[15] has assigned to the Cirripedes a place amongst the Crustacea, almost equally subordinate to that given to them by Milne Edwards.
[13] ‘Crustacea: United States Exploring Expedition,’ p. 1407, 1852.
[14] ‘Annales des Sciences Nat.,’ tom. xviii, p. 120, 1852.
[15] ‘Beiträge zur Naturgesehichte der Rankenfüsser,’ 1834.
That Cirripedes have some special affinity to the Entomostraca, may be inferred from the fewness of the cephalic appendages, the biramous legs, the state of the abdomen, and the form of the carapace. Perhaps in the peculiar state of confluence of the lower segments of the gnathites, in the aborted antennæ, the rudimentary eyes, and in the minute parasitic males (when such exist), there is a more direct relation to the Suctorial division of the Entomostraca; but some of these resemblances are probably only analogical, resulting from the fixed condition of both groups. It should not be overlooked, that out of the three orders into which Cirripedes may be divided, in the two latter, the mature animal presents hardly any resemblance to an Entomostracan. From the distinct presence in either pupa or mature animal of the fourteen segments of the cephalo-thorax; from the apparent composition of the carapace, as will be subsequently explained; and from the concentrated condition of the nervous system, one is led to glance at the higher Crustacea; and here we shall find amongst the Podophthalmia, one aberrant group of low organisation, namely, that including Phyllosoma, Amphion, &c., in which more points of resemblance to Cirripedes may be detected, than, as I believe, in any other group whatever; for we here see that remarkable elongation of the head in front of the mouth, so eminently characteristic of Cirripedes; we have a carapace overlapping the thorax, which is sometimes free beneath; we have the abdomen sometimes almost obsolete; we have biramous legs: and especially we have the posterior cephalic and the first thoracic appendages more or less rudimentary and obsolete; and this, I infer from Mr. Dana, is a very rare phenomenon, though characteristic of all ordinary Cirripedes, in which the seventh and eighth segments with their appendages have disappeared. In the order including Phyllosoma, &c., namely, in the Macroura, the ganglions which give nerves to the five posterior thoracic limbs, are distinct from the great sub-œsophageal ganglion which supplies the several anterior appendages; this is the case with those Cirripedes in which all the infra-œsophageal ganglions are not concentrated into one. In the Macroura and Brachyoura, the first pair of legs almost always differs in structure from the others, so does the homologous or second cirrus in Cirripedes differ from the four succeeding pairs; in some few Macroura, the second leg is antenniformed, so in some few cases is the homologous (or third) cirrus; J. Vaughan Thompson was even struck by the resemblance in the curious, doubly pectinated spines on the anterior limbs of Mysis (allied to Phyllosoma[16]), and on those of many Cirripedes: these several latter resemblances may be small, but certainly I do not believe that they are accidental. Now the little group of Crustaceans, which includes Phyllosoma, &c., has lately been placed, by Milne Edwards, as a satellite amongst the Macrourous Podophthalmia; it leads into the Stomopoda, and likewise, as has been noticed by many authors, into the sub-class Branchiopoda, which latter sub-class is considered by Mr. Dana as only a part of the Entomostraca; this group, therefore, exhibits affinities radiating in several directions, and amongst these lines of relationship, one more must, I believe, be added, plainly directed towards the Cirripedia.
[16] M. Martin St. Ange (‘Mémoire sur l’Organ. des Cirripèdes,’ 1835, extrait des ‘Savans Etrangers,’ tom. vi) has compared the mouth of Lepas with that of Phyllosoma, and has given comparative figures; but the resemblance is founded, I believe, on quite false homologies.
One naturally wishes to ascertain how far Cirripedia are highly or lowly organised and developed; but in all cases this, as it seems to me, is a very obscure enquiry. Mr. Dana considers that, in Crustacea, the greater or less centralisation of all the appendages round the mouth is the main sign of high development; on this view, the anterior part of a Cirripede, from being so much elongated, must be considered as very low in the scale; the whole posterior part of the body, on the other hand, is, in ordinary Cirripedes, brought close to the mouth; but this is effected by the abortion of the seventh and eighth segments of the cephalo-thorax and of the whole abdomen, and so, I presume, would not, in Mr. Dana’s estimation, raise the class much in the scale. Von Baer[17] considers that the perfection of the type of any animal is in relation to the amount of “morphological differentiation” which it has undergone; on this view, Cirripedes ought to stand high in the scale, for they differ much morphologically from the type of the class to which they belong; as indeed is shown by the long time that elapsed before their true position, namely amongst the Crustacea, was even suspected; but something more must, I think, be added to Von Baer’s definition; for, to take as an example the eyes of a Cirripede,—as seen in the first larval stage, there is only one eye, and that most simple; in the pupa there are two, both compound, and furnished with complicated muscles; lastly, in the mature animal there are still two, but of very minute size, often almost confluent, and of the simplest structure; hence, then, there has been much morphological differentiation, but it is almost a contradiction in terms to speak, in relation to such a case, of perfection of type; and what has happened to one organ, might happen to other organs, and so to the whole animal. Lastly, under a physiological point of view, and taking the [Balanidæ] as the most perfect type of the class, the sub-œsophageal portion of the nervous system is highly concentrated; the organs of sense, excepting the eyes, seem more largely developed than in ordinary Crustaceans; the circulating system is of the simplest kind, being only lacunal; special Branchiæ, however, are developed by the metamorphosis of, as I believe, a special organ, occurring only in the Lepadidæ; the digestive organs are very simple, from not having any distinct liver; the generative system is very low, for both sexes are generally united in the same individual; and the testes and ovaria closely resemble each other. On the other hand, the thoracic limbs are, to a considerable extent, specialised in their structure and functions; only the three posterior pairs strictly resembling each other. Lastly, the dermal and muscular systems are complicated, and not, to use Professor Owen’s term, by mere vegetative repetition, as will be obvious to any one who will study the beautifully constructed and modified carapace—that is the operculum, shell and basis—of a [Balanus]. On the whole, I look at a Cirripede as a being of a low type, which has undergone much morphological differentiation, and which has, in some few lines of structure, arrived at considerable perfection,—meaning, by the terms perfection and lowness, some vague resemblance to animals universally considered of a higher rank.
[17] English Translation, in ‘Scientific Memoirs,’ 1853, vol. i, p. 228.