Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/685

Rh It U S T. A E A 651 mandibles, the anterior pair with a single joint, the two following pairs being bifurcate. The eye is single, and the labrum and mouth already occupy their permanent posi tions. The hinder body is short, the abdominal segments not being yet developed. In subsequent moults these posterior segments appear, and new limbs sprout forth. In the second stage a fourth pair of extremities is added ; these are the future maxillae ; then follow three new pairs of limbs, the maxillae, and two anterior pairs of natatory feet. The three anterior pairs of appendages still represent rowing-feet. At the next moult the first cyclops-stage is arrived at, when there is a resemblance to the adult in the structure of the an ennae and buccal organs, but the number of body-segments &quot;and appendages is FKI. 53. (1) CLADOCER.V, (2) OSTRACODA, (8-7) COPEPODA. 1, Daphnia pulcx, freshwater, near London. 2, Candona hispida, freshwater, near London. 3, Cydopt quadi-icornis : a, 9, adult with eggs; 6, c, d, three stages of develop ment of nauplius. 4, Cetochilus septerifrionalis, Firth of Forth. 5, Sapphirina oratolanceolata, Dana, ? , Atlantic, off harbour of Rio Janeiro. 6, fiicothoeastaci, ? .with egg-sacs (from gills of common Lobsters, London Market). 7, Nauplius of Copcpod. (After Fritz Muller.) still much less than in the parent. Only the rudi ments of the third and fourth pairs of natatory feet are seen, and the body is made up of an oval cephalo- thorax, the second, third, and fourth thoracic seg ments, and an elongated terminal joint. In the Cyclo- pidre the posterior antennae have lost their secondary branch, and the mandibles have completely thrown off the previously existing character of natatory feet ; whilst in other families these appendages are persistent, although more or less altered. Many of the parasitic Copepoda do not pass beyond this stage of free development. Such forms as Lernanthropus and Chondr acanthus never acquire the third and fourth pairs of limbs, nor does the fifth thoracic somite separate from the abdomen. Others, such as A &quot;Mheres, even fall to a still lower grade, by the subsequent 1 J33 of the two pairs of natatory feet. But all free Copepoda and most of the parasitic Crustacea pass through a longer or shorter series of stages of development, in which the limbs acquire a higher degree of division into joints in continuous sequence, the posterior pairs of feet are de veloped, and the last thoracic segment and the different abdominal segments are successively separated from the common terminal portion (Glaus). Some parasitic Cope- pids, such as Achtheres percarum, certainly quit the egg like the rest in the nauplius-stage ; the oval astomatous body bears two pairs of simple rowing feet, and behind these are two inflations marking the third pair, each having a long seta. Beneath this nauplius-skin a very different larva lies concealed, which in a few hours bursts its clumsy envelope, and makes its appearance in a form which agrees both in the segmentation of its body and the development of its extremities with the first cyclops-stage. The entire series of nauplius-stages which are passed through by the free Copepods are in this case completely overleapt. Although th&amp;gt;? pnraeitic species of Copepoda are all more or less permanently fixed when adult, they pass their youth ful stages as freely locomotive larvae. To this rule there is a singular exception in the genus Caligus. The young animal (described by Burmeister as a peculiar genus, CJialimus) lies at anchor upon a fish by means of a cable springing from its forehead, and having its extremity firmly seated in the skin of the fish. When sexual maturity is attained the cable is cut, and the adult Caligus, which is an admirable swimmer, is not unfrequently captured swim ming freely in the sea (Fritz Muller). The animals belonging to the last division comprise two orders, the Rhizocephala and the Cirripedia. They have long been kept distinct from the Crustacea, and, together with various parasitic forms of Copepoda, whose develop mental history WAS not known, classed as Epizoa and Cirripedia. By later zoologists the Rhizocephala have beefi placed with the Pcecilopoda, but as this division includes many genera which prove to be merely parasitic forms of FIG. 59. A, Ba.la.nw (young), side view with cirri protruded. B, upper surface of same; valvei closed. C, highly magnified view of one of the cirri. (Morse.) Copepoda, it will be more convenient to separate them. All the animals of this last division, which for convenience wo would designate under the general name of Anchora- cephala, are attached when adult ; in the Cirripedia by means of cement ducts which deposit calcareous matter, forming in the adult Bala n idee (figs. 59-60) a broad shelly p IG 60. Early stages of Balanus. A, Xauplius;, eye. B, Larva with ahivalra shell and just before becoming attached (repiesented feet upwards f or cornea; i- son with E, where it is attached) C, After becoming attached, side views. /&amp;gt;, Later stage, viewed from above. E, Side view, later stage and with cirri extended. The dots indicate the actual size. (After Spence Bate.) base, and a simple attachment inthepedunculatedZepaJwtav in the Rhizocephala by ramifying nutritive roots, which sink deep into the interior of the body of the animal upon which they become parasitic (see figs. 82 and 83, p. 665). In all the members of this division the young appear as naupliiform larvfe l which speedily moult their first coat. The body is unsegmented and pyriform, having a median eye, 2 a first 1 See Mr C. Spence Bate s Memoir, Annals and Mag. A T at. Hist. 1851, 2d series, vol. viii. p. 321, plates 6, 7, 8. 1 In Sacculina purpurea and in. some species of Zeptu the rctc eye is wanting.