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labium or movable lip. It is very conspicuous in young animals, and frequently in adult forms, particularly among the Amphipoda. It is represented by two small osseous disks in Palinurus, and a single small triangular plate in Cancer. Corresponding with this labium posteriorly is another that protects the opening between the mandibles in this direction. This is also supported frequently by osseous plates; but this organ is not constantly developed beyond a limited extent, except in a few instances. In Palinurus it consists of a central osseous plate, having a suture through the median line; from this base it projects in two long membranous sacs, supported on the outer or posterior surface by one or two osseous plates (Pl. III. fig. 22). It is this organ, it appears to me, that represents and is homologous with the lip-plate or metastoma in Eurypterus, Pterygotus, &c., that has been so fully described by Huxley, Woodward, and Salter.

The fifth or next succeeding pair of appendages is that which Prof. MilneEdwards has called the deutognathe. It is what has been known in popular carcinology as the first pair of foot-jaws, and first maxilla or siagnopoda in the History of the British Sessile-eyed Crustacea,' the latter name being suggested by Prof. Westwood "as the Greek equivalent for the Latin name of the five pairs of appendages succeeding the mandibles, which were collectively termed pattes-mâchoires by Cuvier, Savigny," &c.

The deutognathe in all known forms of Crustacea exists in the adult stage in an embryonic condition; it is small in size, feeble in power, and consists, in different genera and families, of a varying number of thin squamiform plates. Each joint of the typical limb, as far as present in the adult condition (Pl. III. fig. 23), offers no very exceptional distinction from the same in the embryonic stage (fig. 24).

The tritognathe, or sixth pair of appendages, supports the idea of the adult form bearing a close resemblance to that of the zoëa or embryonic condition still more decidedly (Pl. III. figs. 25 & 26).

The seventh pair of appendages, the tetartognathe of Milne-Edwards's nomenclature, is the first pair of mâchoires auxiliaires of Savigny, or the anterior mâchoires or foot-jaws of most authors.

These, in the adult Brachyura, are still more embryonic in appearance. In Maia and Cancer they are very reduced in size and apparent importance (Pl. III. fig. 27); but in some less highly developed types, such as the Amphipoda and Isopoda, where they are generally recognized under the name of maxillipeds (Pl. III. fig. 28), they assume a more important feature, and bear a not very distant resemblance to the typical form from which they are supposed to depart. In Nebalia they closely resemble the posteriorly succeeding pairs of limbs; but in this genus the whole of these gradually degenerate to the embryonic condition as they recede from this point.

In the larval or zoëa stage of Crustacea they are wanting in the higher forms.

These three pairs of limbs appear to me to offer an interesting and valuable example of the manner in which any great changes in the variation of the structure of an animal takes place. The crowding together so to speak of the three posterior somites of the cephalon, so as to bring, as much as possible, the several pairs of appendages within the limits of the oral region, so crushes them in their position, that their usefulness as separate organs must be much impeded. It would therefore appear that the crowding of appendages together interferes with and arrests the progress of their development, while they are best suited to exist under the altered conditions where they are the least inconvenient. That they are of little or no importance in the

economy of the animal can, I think, be demonstrated in the habits of their life-a circumstance which, I think, can be shown in the slight variation of their structure in the adult stage from that of the larval form, to depart in the anterior members towards the mandibular form, and posteriorly to put on conditions most consonant with the usefulness of the succeeding appendage; that is, while the anterior ones feebly approximate the mandibular form, the posterior have attached to them parts resembling immature branchial organs. These seven pairs of appendages are all that belong to the cephalon or head; and it appears to me that, however closely any of those that succeed may be associated with them in functional purposes, they are homologically distinct, and, as members of separate portions of the body, they should be named and distinguished in a scientific nomenclature more in accordance with their homological relationship than with their functional power.

The next pair of appendages is the first that belongs to the pereion or thorax in the Crustacean type of animals. It is the eighth pair in posterior rotation, but is generally named by authors according to its relation to the mouth. It is the pemptognathe of Milne-Edwards's more recent nomenclature, the second pair of mâchoires auxiliaires of Savigny, and the second pair of maxillipeds or foot-jaws of most carcinologists. It is the fourth siagnopodos according to Professor Westwood's suggestion, and the first pair of gnathopoda of the History of British Sessile-eyed Crustacea,' according to the nomenclature of the author of this report.

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This multiplication of names for a single appendage, signifying, as they severally do, various affinities, is by no means flattering to the students of Crustacea; but, to a large extent, it occurs from the circumstance that while one anatomist has contemplated the animal in the adult and higher concentrated forms, others have contemplated it in the more imperfect types. It is therefore the object in this report to bring together these several and various discrepancies, and demonstrate the relationship of parts through their various degrees of growth and change, and retain by one fixed name the same part however it may vary in structure or functional conditions through all stages of variation in Crustacean life.

In Crustacea the eighth pair of appendages in the structure of the animal is the first pair that belongs to the body. In the Brachyura it exists in the same type as is found in the zoëa or larva form (fig. 29), from which it varies only in the more robust character of some of the joints of which it is constructed (Pl. III. fig. 30). In this state it varies in form and degree only within a limited range, gradually becoming more pediform in character as we examine it through the Macrura (Pl. III. fig. 31) in the descending order until we reach Squilla (Pl. III. fig. 32), where we find it developed as a large and important organ that gives a decided and distinguishing feature to the animal. Through this genus we are led to the Eriophthalmia (Pl. III. fig. 33), among which we find that in the Amphipoda it is formed on the same type as in Squilla, but gradually approaching in its general characteristics and appearance those of the succeeding pairs of legs, until in the Isopoda it is in most families uniform with them.

Thus we see that not only in their relation to the body of the animal, but also in their most general appearance and affinities they are part of the same system of appendages as those posterior to them, and that their relation to those anterior arises from that crowding together of parts in the higher types of Crustacea that forces an abnormal form as the result.

This pair of appendages, as being the first attached to the "pereion" or body of the animal, may with consistency be called, as it really is, the first pair of perciopoda. But throughout the higher Crustacean forms the

first two pairs of appendages are functionally utilized as attendants upon the mouth; and where this is not the case they are formed as organs of prehension, more especially among the male animals. This is exemplified even in those species, as among the terrestrial Isopods, where the outward form is less striking, but the whole appendage is strengthened for grasping purposes. The next or ninth pair of appendages is almost if not universally formed upon the same type as the preceding. There is a departure in degree to be found, more pronounced in the Brachyura, in consequence of the appendages crowding so much on one another. Thus, while those that experience most the pressure of those that overlap them are precluded from attaining their fully developed forms, the external ones, or they that overlap the preceding, have, in order the more perfectly to fulfil their duties, extended their own surfaces, so as more effectually to protect the oral cavity, as an operculum covering the mouth.

These two pairs are variated so constantly from the other appendages of the pereion that I think it will be found convenient in most cases to designate them by distinguishing names. The Reporter has, in the Report on the Amphipoda in 1865 and elsewhere, called them gnathopoda, as fect or appendages connected with the mouth; and I see every reason why this. name should be adopted throughout the whole subkingdom, as one better adapted, both functionally and homologically, than those proposed either by Milne-Edwards's latest nomenclature, or the still less correct ones in popular use of previous authors.

In the larval form the second gnathopod is less advanced than the first, but in the adult stage it is larger and more efficient. An exception to this exists in Nebalia, where all the appendages of the percion are developed upon an immature or embryonic type. These gradually decrease in power and form the more they recede posteriorly. All these appendages exhibit the seven joints that are present in the formation of a single limb; and in those instances where there is a decrease in that number, the joint that is wanting is lost at the extremity. This appears to be very general through all the Brachyural and Macrural divisions.

In the higher forms both pairs of gnathopoda carry a secondary branch as well as another that has generally been known as the "flabelliform appenage." For these Milne-Edwards has proposed the name of endognathe for the primary or internal ramus, exognathe for the external or second ramus, and epignathe for that which is generally known as the "flabelliform appendage," and mesognathe for the fourth. But as the representatives or homotypes of these same appendages occur in different grades of Crustacean form, and whenever they do occur they bear the same relation to the limb from which they spring, it would be better that they should consistently be known by their homotypical character, rather than vary their name with every succeeding appendage. Thus the flabelliform appendage invariably springs from the coxa or first joint, and is homotypical of the branchial organs in other pairs of limbs; another is invariably connected with the basos or second joint, and the third has its origin in the ischium or third joint. One or all may be suppressed; but whenever either the one or the other is present it has its origin in its own peculiar joint, and as such should be identified in any scientific nomenclature. I therefore suggest the names of coxecphysis, basecphysis, and ischiecphysis for the several parts*, as branches springing from those joints, in whatever appendage they may be found. Thus the secondary branch that exists attached to the legs in Phyllosoma or the young of Palinurus is an ischiecphysis; in * The name of the joint being compounded with the word expvois, sprout or branch.

Mysis a very similar appendage is the basecphysis, while the branchiæ are, in all cases when present, the homologues of the coxccphysis.

The next five succeeding pairs of appendages are the true legs as they exist in the typical forms of Crustacea, and it is from the general appearance of them that the higher forms are known as Decapoda, or Ten-footed Crustacea. In a scientific point of view the name is incorrect and misleading; for in many of the Macrura and the Edriophthalmia they are twelve or fourteen in number, while in the Anomura the departure of the last two pairs of pereiopoda from the typical form is as great as the two first in many other forms; consequently the name of Decapoda, as well as Dana's name of Tetradecapoda, is both incorrect and homologically untrue. These five pairs constitute the tenth to the fourteenth pairs of appendages; but as they are limbs attached to the pereion, I have elsewhere suggested that they should be known as pereiopoda. Milne-Edwards, in his nomenclature, has not identified them with any distinguishing name; he merely calls the anterior pair, which is cheliform in many genera, by the name of bras (arms), and the rest pattes (feet), and it is remarkable that he should identify each one of the seven joints that is present in its construction by a distinguishing term; but the entire member he defines by an unscientific but popular phrase that is inconvenient, as it is found that the prehensile power is not confined to a single pair, but, as in Astacus and Homarus, is the property of other limbs, while in some, as in Scyllarus, it does not exist in any. Carrying this observation into other forms, we find that in certain Amphipoda the great chelate or arm-like organ exists in the fifth pair of pereiopoda, as in Phronima. Thus we see that the power of being developed into a grasping forceps or hand exists in each or all the pereiopoda in succession; therefore the term of arm, or bras, is inadmissible in a scientific nomenclature. I therefore propose to call these five pairs of appendages the pereiopoda, in accordance with the terms used in the History of the British Sessile-eyed Crustacea.'

They invariably consist of seven joints; these are most distinguishable in the Macrura and the lower forms. In the Report on the Sessile-eyed Crustacea, 1855, the author clearly demonstrated the several joints respectively in the Amphipoda. This required no effort on his part to interpret in the Macrura, since in Homarus, Astacus, and Palinurus the general points are very distinguishable; but as we examine higher in the scale of animals, we find that in the Anomura the cox of the several pairs of legs are gradually becoming absorbed and becoming part of the ventral surface of the body; and this in the Brachyura is carried still further, inasmuch as it is difficult to define how much of the structure is due to the legs and how much to the body, and it is not improbable that the appendages have encroached upon and absorbed the generally more important structure.

The coxa or first joint appears to be essential to the existence of the animal, inasmuch as it is the seat of all the more important organs connected with the vital existence. The auditory and olfactory senses are situated in the coxæ of the antennæ, and all the branchial appendages have their origin in the cox of the pereiopoda, while the sexual organs, both male and female, are implanted in the coxæ of the seventh and fifth pairs respectively. The next two joints of the limbs may, and in some of the Stomapoda do, carry appendages attached to them; but none of the joints beyond the ischium are ever so furnished.

The anterior pair is the one most commonly developed in the higher forms into large chelæ or hands. It is the more general in the male than in the female, and I have commonly observed that the female chela generally corresponds more closely with the less-developed chela in males than with

the greater. Sometimes the male appendage is developed so monstrously that they appear inconvenient and burdensome, and are occasionally so long that they are useless in an attempt to reach the mouth. Thus in Homarus the animal feeds itself with the small posterior pair. In Gelassimus no ingenuity on the part of the animal would enable it to reach the mouth with the extremities of the large chelate organ. In the process of feeding they are useful only as holding food while the animal carries it to the mouth with the smaller but more convenient organs. The chela is always formed by the greater or less amount of development that is given to the inferior angle of the distal extremity of the antepenultimate joint. This power of production appears to be dormant in every limb, since we see it occasionally exhibited in all. Thus in Palinurus it is rudimentarily present in the posterior pair of pereiopoda, and in the genus Pagurus it is developed into a small but efficient organ, by which the animal cleanses out and removes obstructive objects that may have found their way into the branchial chamber, and so fulfils the same duties as those performed by the flabella attached to the gnathopoda, and which are wanting in the Anomura.

The fact that the coxa of all the legs attached to the pereion are in some orders absorbed into the sternal plastron, while they are not so in others, offers a ready and safe means by which paleontologists may determine the order to which a fossil Crustacean might belong by the evidence of a single leg. Thus it will be seen invariably that seven distinct and free joints are visible in the Macrura, while only six are free in the Brachyura; whereas in the Anomura there are six free and one partially so. This evidence might be carried still further, inasmuch as in Astacus and Homarus the coxæ are seen to approximate to each other on the opposite sides closely, while in Palinurus they are near anteriorly and broadly separated posteriorly.

The appendages that follow are those that are modified for swimming. When exhibited in the most normal condition, they consist of a long peduncular stalk supporting two oblong leaf-like plates, surrounded by a fringe of small hairs. Sometimes they consist of a series of multiarticulations, as in Amphipoda; sometimes of long cylindrical uniarticular branches, as in Cancer. In some instances, as in Squilla, there is a third branch that springs from the side of the peduncle near the base; this is so membranous in character and ramified in construction, that it is evidently formed for the purpose of assisting in aeration of the blood.

The pleopoda are utilized, according to the habits of the animal, for various purposes, and throughout them all their adaptation to propulsion through the water is not only the most constant but also very generally associated with other offices.

In the Isopoda they appear to be the only organs adapted for respiration that the animal possesses. Yet their rapid motion is the only means which they possess of swimming.

In the Amphipoda, it is this latter use alone for which these organs are adapted, while respiration is fulfilled by other means. But here only the anterior three pairs are adapted for swimming purposes, while the posterior three are utilized for leaping when on land, or forcibly dashing through the water. The Isopoda have only the posterior pair so variated, and the Macrura have two pairs; but in this latter order they are more adapted for producing a retrograde motion, darting backwards as they frequently do to avoid unexpected and sudden danger. In the Macrurous forms they are also available for the purpose of retaining connexion with the ova, and supporting the life of the embryo until it is matured. Throughout most of the Macrurous forms the pleopoda fulfil this double purpose in the female.

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