Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/669

belonging to the three divisions of the body differ from each other in a greater or less degree in proportion to the higher or lower grade which we examine. Thus among tho Decapoda (crabs and lobsters) the cephalic, thoracic, and abdominal somites all possess appendages with well marked FIG 5. (1.) Walking-foot of Lobster (Homarw vutgaris), Edw.; (2.) Swimming Jaw-foot of Pterygotus. characters, each series being highly differentiated for the functions to be performed by it. Among the Entomostraca, the appendages of the anterior cephalic somites alone are highly specialized, the others being either mere vegetative repstitions of one another, or else altogether wanting. 13 FIG. 6. Diagram of the segments and appendages of the Common Lobster (Homarus vulgaris). C = cephalon; 13, Th = thorax, showing tho apodenwta; Ab = abdomen. l.Eyes; 2, antennules; 3, antennae,- 4, mandibles; 5, first maxilla:; 6, set-end maxillas; 7, first maxillipedes; 8, second maxillipedes; 9, third pair maxillipeilcs. 10, One of the antepenultimate pair of thoracic legs of female; p. protopodire ; ep, epipodite; g, Rill. 11, One of the last pair of thoracic limbs in male; p, pro- topodite. 14, Third abdominal somite; e.r, exopodite; en, endopodite. / = Jabrum; m = metastoma. 15-20, abdominal segments; T, 21st segment, or telson. First pair. The first cephalic somite supports the eyes, the most constant of all the organs, and probably the only pair of appendages which arc never diverted from their normal use, though sometimes atrophied. In the more highly cephalized forms, the Decapoda- Brachyura (crabs), the eyes are placed on the outer S de of the two pairs of antennae, but the anatomical evidence shows that the most anterior pair of nerves, in this as in all other orders of Crus tacea, is that which is connected with the ophthalmic organs. Second pair. The second somite bears the first pair of antennre (or antennules), called the inner pair in the higher forms, and the upper pair among the lower Crustacea. Ordinarily they are slender, tapering, freely-moving, multi-articulate organs. In the lobster both the endopodite and exopodite are equally developed from a common basal -joint or protopoditc (see 2 in fig. 6). Third pair. The third somite supports the second or posterior pair of antennae, sometimes called the outer pair in the higher, and the inferior pair in the lower forms of Crustacea. In the lobster these are represented by a basal-joint or protopodite, and a long filamentary, multi-segmented endopodite, to the outer base of which a small scale is attached representing the exopodite. In all the higher Crustacea the three most anterior somites are always prcporal, being in front of the buccal orifice, and in most genera, moreover, they are specially set apart as bearing the organs of sense. Generally they are so closely blended together, both in the earlier stages of development and also in the adult forms, as to defy separation, the presence of three somites being only demonstr able by dissection, and by a knowledge of the fact that each pair of appendages, wherever it exists, presupposes a segment or ring to which it belongs. The genus Squilla affords, perhaps, the best evidence of the separate existence of these cephalic rings. In it the first, or ophthalmic somite, is quite distinct from the second and third antennary, which are also separable from one another, although the latter blends with the next somite, and the succeeding ones can only be distinguished by dissection. Usually the antennae or feelers are constant appendages ; but still, the number and dis position of these organs varies extremely. Some of the lowest forms are wholly without antennae, or are furnished with them in a merely rudimentary state. Some species have only a single pair, the normal number, however, as we have already pointea out, is two pairs. In position they are inserted on the superior or inferior surface of the head, according to the development of the somites composing the cephalon. Ordinarily they are slender flexible multi- articulate appendages, but even among the higher forms they are subject to extraordinary modifications ; thus in the Scyllaridae, the external pair are developed into broad flat organs of natation, and probably also for burrowing. In Arcturus, an Isopod, they are the nursery for the young ; in the Entomostraca they are usually natatory organs ; in Ptcrygotits and Limulus they are chelate, serv ing, as in the Copepoda, as clasping organs for the male. In tho last free condition of the Cirripedia and Rhizocephala they serve as the organs for attachment, being converted into cement-ducts in the former, and into root-like organs of nutrition in the latter. The second pair of antennae and sometimes even the first pair become mouth organs in the Merostomata. Fourth to Ninth pairs. Epistomial or Mouth Organs. In the Decapoda the six succeeding pairs may be called mouth-organs or epistomial appendages, being all engaged in duties subservient to nutrition. The fourth somite bears the actual jaws or mandibles proper with their palpi, outside which lie two pairs of maxillae, followed by three pairs of maxillipeds or jaw-feet. In each succes sive somite, these organs become less highly specialized mouth- organs, and betray the fact that they are after all only simple feet modified. Thus in Squilla (a Stomapod) the eighth somite (first thoracic) bears a pair of robust claws, the terminal joint of which is furnished with long and sharp teeth, these forming the principal organs of prehension ; whilst the ninth somite bears a pair of ordi nary feet like the two following pairs (see fig. 71). In most of the Edriophthalmia the mouth-organs extend only to the seventh somite, the eighth and ninth being included with the ambulatory members. In the Decapoda we can detect the more or less rudimentary endopodite and exopodite in the fifth pair of appendages, and in each succeeding pair to the ninth ; the eighth and ninth pairs also bear a third organ called an epipodite, and a gill or branchial organ. Tenth to Fourteenth Pairs. In the higher forms the five somites which follow (and which might be termed the postoral somites) bear the true walking limbs (pereiopoda, Spence Bate), the first pair of which in the Decapoda are usually developed into powerful chelae, and serve as the chief organs of prehension. These podites are usually seven-jointed, and each bears a gill on its basal-joint. They are formed by the endopodite, the exopodite being present only in the larval-limb of the Decapoda : but in the adult Mysis (Stomapoda), eight pairs of limbs (that is to say, the five pairs of pereiopodites or "walking-feet," and the three pairs of maxillipeds or "jaw-feet,") are all furnished with two branches, one the endopodite, the other the exopodite, as in the larval Decapod. Fifteenth to Twentieth pairs. The next six somites bear each a pair of swimming-feet (or phopoditcs). In the Decapoda-Brachyur.i

these remain (like the segments on which they are borne) as er-