CLASS—CRUSTACEA.
Sub-Class—CIRRIPEDIA.

Crustacea attached by the anterior end of the head, by cement proceeding from a modified portion of the ovaria; archetype composed of seventeen segments, with the three first of large size, and almost always developed into a carapace, not wholly exuviated, and capable of various movements; antennæ none; eyes rudimentary; mouth prominent, formed by the partial confluence of the labrum, palpi, mandibles, and two pairs of maxillæ; thorax attached to the internal sternal surface of the carapace, generally bearing six pairs of captorial, biramous, multiarticulated limbs; abdomen generally rudimentary; branchiæ, when present, attached to the under sides of the carapace; generally bisexual, when unisexual, males epizoic on the female; penis single, generally probosciformed, seated at the posterior end of the abdomen; oviducts none; metamorphoses complex.

Within the memory of many living naturalists, Cirripedes were universally looked on as belonging to the Molluscous kingdom; nor was this surprising, considering the fixed condition of their shells, and the degree of external resemblance between, on the one hand, Lepas and Teredo, and on the other hand, between Balanus and a Mollusc compounded of a patella and chiton. It is remarkable that this external false appearance overbore, even in the mind of Cuvier, his knowledge of their internal structure, namely, their lateral jaws, articulated appendages, and regular ganglionic nervous system, which now strike us as such conclusive evidence of their position in the great Articulate kingdom. Straus[5] was, I believe, the first who, in 1819, maintained that Cirripedes were most closely allied to Crustacea. But this view was disregarded, until J. Vaughan Thompson’s[6] capital discovery, in 1830, of their metamorphoses, since which time, Cirripedes have been almost universally admitted amongst the Crustaceans. It is well known, that it is hardly possible to give a definition of this great class, which shall include every member of it; nevertheless, even if the mature Cirripede alone be considered, the following characters, viz. the slight separation of the head and thorax, the latter generally bearing six pairs of appendages, and the being enclosed in a carapace—together with the periodical exuviation of the greater part of the external membranes, would, perhaps, suffice to show that it should be classed amongst Crustacea.

But it still remains undecided what rank in this class Cirripedes should hold. Before briefly discussing this point, it is indispensable to indicate their essential characters, which I will immediately attempt. For as long as it remained doubtful which was their anterior extremity, which the ventral or dorsal surface; as long as the peduncle was thought by one naturalist to be the legs, by another the abdomen, in a modified condition, it was hopeless to compare Cirripedes with ordinary Crustaceans, and assign to them their due rank.

In the larva in the first stage, an eye and two pairs of antennæ are in process of formation or are developed; here, then, according to the analogy of all Crustaceans, we have evidence of the existence of the first three cephalic segments. The mouth always consists of three pairs of gnathites, and hence again, from analogy, this part may be inferred to be formed of, and supported on, three other segments; making thus far six segments. In two Orders out of the three into which Cirripedes may be divided, namely, in the Abdominalia and Apoda, eight quite distinct segments succeed the mouth; of these the first differs slightly from the seven succeeding segments, and may, I think, be safely considered as forming the seventh (cephalic) segment. The next seven segments resemble each other in all essential respects, and are no doubt the normal, seven thoracic segments. These, in both the above orders, are succeeded by three smaller segments, which differ in structure from the thoracic segments, and must be abdominal. Hence we here have, altogether, seventeen segments. It should, however, be observed that in the two orders just referred to, each includes only a single species; but I know of no good reason why, on this account, their structure should be valued the less. In the third order, the Thoracica, which includes all common Cirripedes, two segments with their appendages are missing out of the eight that should succeed the mouth; from the open interval in the pupa, between the mouth and first pair of natatory legs, and from some other reasons, I believe that the two missing segments are the seventh and eighth, or last cephalic and first thoracic segments, and that they have coalesced close posteriorly to the mouth.[7] In the order Thoracica, the abdomen is quite rudimentary, though often still bearing caudal appendages; in the pupa, however, of this order, as in the mature animal of the two other orders, it is formed of three segments. Hence I conclude that, notwithstanding the absence of the above two segments with their appendages in the Thoracica, the archetype Cirripede may be safely said to be composed of seventeen segments.

In the classification of Crustacea, the relation and number of the segments of the different parts of the body, are viewed both by Prof. Milne Edwards[8] and Mr. Dana,[9] as of the highest importance. I may premise that both these authors divide the Crustacea into Podophthalmia, Edriophthalmia, and Entomostraca; Milne Edwards making a fourth legion, the Branchiopoda, and another division, including Limulus, of equal value to the above four legions altogether; whereas Dana sinks Limulus and the Branchiopoda under his Entomostraca. As far as concerns our present discussion on Cirripedes, the first three divisions, as valued by Dana, will best serve as standards of comparison; but it is not unimportant to our present purpose, as showing how difficult it is to weigh the value of the higher divisions of a Class, to observe the wide difference in opinion of two naturalists, so eminent for their knowledge of the class in question and for their high abilities.

In the order Thoracica, including all common Cirripedes, the cephalic and thoracic segments are as much confounded together (but with coalescence and abortion of two middle segments) as in most Podophthalmia; but in the two other orders, the cephalic and thoracic segments are as distinct as in the Edriophthalmia. The number of the segments, however, which strictly appertain to the anterior part of the head and mouth, being only six, is an Entomostracan character; on the other hand, the first pair of cirri in the Thoracica, has some claim from their position, apparent functions, and separation from the succeeding pairs, to be said to belong to the mouth; on which view, the first nine segments would, in function, be cephalic, as in the highest Crustaceans. The fewness of the segments of the abdomen, and their not bearing in two of the orders appendages, is an Entomostracan character.

Cirripedes are permanently attached, even before their final metamorphosis, by a tissue or cement, first debouching through the second pair of antennæ, and, subsequently, in most cases, through special orifices, penetrating the anterior part of the head; this cement proceeds from glands, which certainly are modified portions of the ovarian system. This fact I consider of the highest classificatory importance, for it is absolutely the one single character common to all Cirripedes, besides such as show only that these animals belong to the articulated kingdom, and are Crustaceans. No structure of this kind has hitherto been observed in any other member of the class or kingdom. Even in the Suctorial Entomostracans, which become immoveably attached to the fish on which they prey, the males are free; and the means of attachment, as far as known, are quite different.

Both the first and second pairs of antennæ are absent in the mature animal; for the three terminal segments of the antennæ of the pupa, which may always be found cemented under the centre of the surface of attachment, are functionless, after maturity. The eyes are rudimentary, and are singular from being seated far from the anterior extremity of the head. In their rudimentary state, and in the absence of antennæ, we have characters common with certain Suctorial Entomostracans; and this similarity apparently arises from the fixed condition of the animals of both groups.

The carapace, which covers the dorsal surface of the larva in the first stage, in the last larval or pupal stage is developed so as to enclose, like a bivalve shell, the whole body. In the mature Cirripede, the whole external covering, whether shell and operculum, or capitulum and peduncle, can be conclusively shown to be the carapace of the pupa, modified. In thus enclosing the mouth and whole body, the modified carapace resembles that of several Entomostracans; but in being apparently formed (as I hope hereafter to show) by the development of the third segment of the head, and in consisting generally of distinct sclerodermic plates, arranged in an imbricated order, there is, I think, a closer resemblance to the same part in some of the Podophthalmia. The carapace, or portions of the carapace, being capable of other movements, besides merely opening and shutting, differs, I believe, from that of all other Crustaceans; as it likewise does[10] in the greater part not being periodically moulted.

Moreover, in all Cirripedes there is another striking peculiarity connected with these parts, namely, the exclusive attachment of the whole thorax or included body to the internal ventral or sternal surface of the carapace and head. In the pupa, the thorax, as in all Crustaceans, opens into, and is continuously united with, the large anterior part of the head; but from the singular fact that the thorax of the young Cirripede is developed not within the thorax, but within the head of the pupa (Pl. 30, fig. 2), with its longitudinal axis placed at right angles to that which it held in the pupal condition (the mouth and the whole exterior being developed conformably with that of the pupa), it comes to pass after the metamorphosis, that the Cirripede is, as it were, internally cut in twain (compare Pl. 25, fig. 1, and Pl. 30, figs. 2 and 3). Thus it is, as will hereafter be more fully explained, that the sack originates, and thus the body becomes attached to the internal ventral surface of the carapace and front of head.

The thorax in two of the Orders bears no appendages, but in all common Cirripedes it is furnished with six pairs of biramous, multiarticulated cirri, which have a peculiar character, different from the limbs of other Crustaceans, not being natatory, ambulatory, or branchial, but “captorial” or fitted for sweeping the water, and thus catching prey.[11] The cirri, at least the anterior pairs, can, besides other movements, lengthen and shorten themselves; and this Milne Edwards[12] states is the case with the Podophthalmia, and is considered by him as an important character. The cirri of the first pair are attached on each side close to the bases of the mandibles, and, as already remarked, have some claim to be considered as maxillipeds or mouth organs. The three or the four posterior pairs of cirri in the Balanidæ, form a series somewhat distinct from the two or three anterior pairs, thus recalling a characteristic feature in the Edriophthalmia.

The mouth is prominent, and is formed by the partial confluence of the labrum, palpi, and lower segments of the mandibles, and of two pairs of maxillæ; it is capable of movement as a whole; in this respect we are reminded of the Suctorial Entomostracans; but I believe the above type of structure of the mouth is peculiar to Cirripedes.

The alimentary canal is simple, but can be distinctly divided into—(1st) an œsophagus, singular from the bell-shaped expansion of its lower end; (2d) the stomach, which is directed forwards and then doubled back; and (3d) the rectum. There is no distinct liver. The circulation is lacunal. In one family there are well-developed branchiæ, which differ entirely in their homologies and position from these organs in all other Crustaceans. In the nervous system, the sub-œsophageal ganglions vary in concentration from that degree observed in the lower Macroura, to that in the highest Brachyoura; but the supra-œsophageal ganglions are always much less concentrated, and are even embryonic in condition; presenting a difference not observed in other Crustaceans. On the under side of the sub-œsophageal ganglion, two nerves, apparently splanchnic, arise, and run almost parallel and under the collar surrounding the œsophagus; they are very remarkable from their great size, and from forming a plexus together with a large branch, arising on each side from the collar close behind the supra-œsophageal ganglion,—a structure unlike anything observed in other Crustaceans. The eyes, as already remarked, are rudimentary, and singular from being imbedded at a distance from the anterior end of the animal. In the basal confluent segments of the outer maxillæ there are two orifices, leading into little sacks, which I believe are olfactory organs: again there are two other orifices on each side of the thorax, beneath the first pair of cirri, leading into sacks, with a curious elastic vesicle suspended within them; and these I can hardly doubt are acoustic organs. Of these orifices and organs, there is no trace in the same relative positions in any known Crustacean.

Cirripedes are ordinarily bisexual, in which they differ from all Crustaceans: when the sexes are separate, the males are minute, rudimentary in structure, and permanently epizoic on the females; to these latter facts we have a partial analogy in some of the Suctorial Entomostracans; but a far closer analogy in certain Rotifers, which are considered by many naturalists as Crustaceans; but to the above subject I shall almost immediately have to recur.

The male excretory organ is probosciformed and capable of the most varied movements; it is single and medial; it is seated (in the one instance in which this point can be safely judged of) at the extremity of the abdomen, and therefore near the normal position of the anus; in all these respects there is a very great difference from other Crustaceans, in which the male organs are laterally double, and are not seated at the extremity of the abdomen. In regard to the female organs, the ovarian tubes and cæca inosculate together: there are no oviducts; the ova, connected together by membrane, and so forming the “ovigerous lamellæ,” become exposed by the exuviation of the lining tunic of the carapace or sack, and by the formation of a new tunic on the under side of these lamellæ; a process, I believe, unknown in other Crustaceans.

The metamorphoses are highly complex. The larva in its first stage bears a very close general resemblance, in having three pairs of natatory appendages, the first being uniramous and the two others biramous, and in having a single eye on its broad anterior front, to the larvæ of most Entomostracans; but I cannot avoid the belief, that this resemblance is only apparent, and not essential; and of false resemblances, how many instances occur in the animal kingdom! In the larva, when first freed from the egg, both pairs of antennæ are in process of formation within envelopes: the mouth is probosciformed and capable of movement, but is destitute of gnathites; it occupies a position between the three pairs of natatory limbs; and these limbs I must believe, for reasons hereafter to be assigned, answer (improbable as I am well aware it must at first appear) to the second, third, and fourth thoracic legs of the archetype Crustacean: the two hinder pairs of limbs apparently soon become captorial, or fitted to secure prey. Now, I cannot find in the published accounts of the larvæ of Entomostracans, any that answer to this description.

The larva in the last stage might be included in the vast class of Entomostracans: the attachment of the eyes to the singular apodemes produced inwards from the basal segment of the great prehensile antennæ, and the development of only the posterior six pairs of thoracic limbs, are its chief peculiarities: but its rudimentary mouth, owing to its transitional or pupal condition, renders the assignment of its proper rank difficult.

---

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.

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.

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.

---

It has been seen that I divide the Cirripedia into three orders,—the Thoracica, Abdominalia, and Apoda; between which the fundamental difference consists in the limbs or cirri being thoracic in the first, abdominal in the second; and entirely absent in the third. For the sake of showing the range of character in Cirripedes, to which allusion has been made, I will briefly indicate the leading differences in each order. In the Thoracica, three families are included,—the Balanidæ, or sessile Cirripedes, the Verrucidæ, remarkable from their quite asymmetrical shell, and the Lepadidæ, or pedunculated Cirripedes. The great difference in external appearance between these three families is known to all naturalists. Even within the one family of Lepadidæ there are great differences in external appearance, as will be admitted on comparison of Lepas, Pollicipes, Conchoderma, &c.; but we have also important internal differences, as in the case of Anelasma, in which the cirri are barely articulated, and are not capable of seizing prey, whilst the mouth is almost probosciformed, with the outer maxillæ and palpi rudimentary: still more important are the differences in Alcippe, in which the cirri of the first pair act as brushes; the second, third, and fourth pairs being quite aborted; and the fifth and sixth pairs consist only of four segments, with one of the two normal rami converted into a crenated, button-like projection, for the sake apparently of triturating food; Alcippe, also, is very remarkable in being destitute of a rectum and anus. In this same genus Alcippe, in Ibla and Scalpellum, there are either separate males or Complemental males, some of which are so utterly abnormal in their characters, that by no definition which I could frame, could they be included even in their proper Order, much less in their proper Family.

In the second order of Abdominalia (Pl. 23 and 24) the seventh or last cephalic segment is quite distinct, and bears rudimentary organs, answering to the first pair of maxillipeds of ordinary Crustaceans, of which organs, and of the segment supporting them, there is no trace in the Thoracica: the seven succeeding thoracic segments are destitute of any appendages; but the three segments of the abdomen bear three pairs of cirri. The mouth is peculiar in the labrum being developed into very large, moveable, lancet-formed organ; and the lower end of the œsophagus is armed with beautiful discs of teeth, and brushes of hairs,—a structure confined to this order. The male resembles the male of Alcippe; and the latter genus seems to be the connecting link between the Thoracica and Abdominalia. But the most important character of this latter order, in which it differs from Alcippe, and all other known Cirripedes, is in its metamorphoses; all the first changes are merely indicated by changes in form in an egg-like larva, without the development of distinct organs; and the last, or pupal condition, which is attained within the sack of the parent, is very peculiar, by the entire absence of natatory limbs.

The third order of Apoda is the most peculiar of all; it contains, like the last, only one known species: the most acute naturalist, I am convinced, if he had not made the class his special study, would never even have suspected that this animal was a Cirripede. We see much magnified in Pl. 25, fig. 7 a naked, plainly-articulated animal, resembling the larva or maggot of a fly, attached by two threads; and these threads, on analysis, can be clearly shown to be the last rudiment of the carapace, specially modified. The last cephalic, the seven thoracic, and the three abdominal segments, are all equally destitute of appendages. The mouth is suctorial, and constructed on a plan unlike, I believe, anything known in the articulate kingdom; for the mandibles and maxillæ have rotated on their axes, and stand back to back; they can act only by tearing open a slit, and this action is performed in a hood, formed by the confluence of the broad palpi and labrum. Although the œsophagus is distinct, there is no stomach or anus. Lastly, owing to there being no carapace, the ova are developed, differently from in all other Cirripedes, within the thorax.

I will close this preliminary discussion on the confines and type of the sub-class, by recalling attention, now that a sketch has been given of the three Orders, to the remark before made, that a wide range of structure is included within it, and by reurging that the Cirripedia should be ranked, not as one of the subordinate groups, but as one of the main divisions of the Crustacea.