Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/698

GG4 CRUSTACEA F. Goldenberg has lately described a fossil form belong ing to this division (Lynches ornatus) from the Coal- measures of Saarbruck. 1 V. LOPHYROPODA: (10.) OSTRACODA. In the Ostracoda, of which Cypris, Candona (2 in fig. 58), and Cythere are examples, the body is entirely enclosed in a carapace com posed of two nearly equal parts like a bivalve shell. Two pairs of antennae, one pair long, with hairy filaments, one pair short, stout, and recurved (like feet), and three pairs of short feet, may bo seen protruded from the carapace, which is compact and brittle, yet is capable of protecting the living animal during long periods of drought, buried in the dried-up mud of pools. Cypris frequents stagnant water, living on dead animal matter. Cythere is found in pools along the sea-shore. Like the Phyllopoda, the Ostracoda are of immense geological antiquity, Primitia prima occurring in the Lower Cambrian of St David s. There is abundant evidence, in almost every stratum, of the former existence of these little bivalved Crustacea, often in vast numbers ; their size in early times was much larger, Leperditice as big as large horse beans being found in Silurian strata, but the living forms are all microscopic. M. Ch. Brongniart has just described a fossil Ostracod, Palceocypris Edwardsi, inclosed in silex from the coal of St Etienne, in which all the organs are most perfectly preserved. 2 It closely resembles the modern form. The Ostracoda help with other micro scopic organisms to build up the Chalk. They mnke up the great mass of the Cypris shales of the Wealden, Isle of Wight, and many Tertiary beds are largely composed of their remains. V. LOPHYROPODA: (11.) COPEPODA (a.) LIBERATA. The free Copepods, of which Cyclops, Cantliocamptus, and Cetochilus may servo as examples, have the head and thorax closely enveloped in a carapace with which the front rings of tho thorax aro confluent. The abdominal somites are much diminished in sizo and cylindrical. The single or paired external ovisacs aro attached to two of the posterior somites, which ara usually welded together. The ringle sessile median eyo is situated near the front of the head ; in tho males of Diaptomus and Anomalocera the eye is pedunculated. Tho antennae are very long and powerful natatory organs (in Cetochilus, 4 in fig. 58, and Diaptomus equalling the entire length of the animal s body) ; in the males one or both of the antennae have a swelling near the centro or towards the extremity, followed by a movable joint which acts like a hinge and serves as a clasper to detain tho female. There are five pairs of rowing feet, one pair of which aro usually rudimentary. The species belonging to this family are to be found in both fresh water and the sea. In the muddiest and most stagnant pools and in the clearest springs Cyclops abound (3 in fig. 58). Tho marine species frequent the pools along shore and the open ocean in equal abundance. They assist in producing that luminous appearance in the sea called "phosphorescence," for want of a better name (5 in fig. 58). The fecundity of this order is truly surprising. Cyclops juadricornis is often found with thirty or forty eggs on each side ; and though those species which have but a single ovisac do not carry so many, their number is still very considerable. Jurine isolated female specimens of Cyclops, and found them to lay eight to ten times within three months, each time about forty eggs. At the end of a year one female would have produced 4,442,189,1 20 young ! Cetochilus (4 in fig. 58) is so abundant, both in the Northern Seas and in the South Atlantic, as to serve for food to such an immense animal as the whale. They colour 1 Giebel und Siewert s Zeitschrift, 1870, vol. i. p. 524. 2 Ann. des Soc. Geol. France, t. vii. pt. ri. the sea for many miles in extent, and when the experienced whaler sees this ruddy hue upon the ocean he knows he has arrived at the " pasture of the whales. " They are to be seen in vast quantities off the Isle of May in the .Firth of Forth during the summer months ; many Cetacea are attracted thither, and vast shoals of fish also come to feed upon them. One anomalous type of free Copepods is the Notodelphys acidicola, described by Allman, which is found swimming freely in the branchial sac of the Ascidia communis. In this species the female has the fourth anterior segment of the body peculiarly modified, so as to form on its dorsal surface a marsupium for receiving and retaining the ova until they are hatched, when they escape by an opening between the sac and the upper surface of the body-ring. We have no positive records of Copepoda occurring in a fossil state. V. LOPHYROPODA: (11.) COPEPODA (b.) PARASTTA. The parasitic Copepods are divisible into two groups, the fir.-t comprising the peripatetic genera, in which the male and female both retain their organs of locomotion in the adult state, and can change their habitat whenever needful ; this division would include the fresh- water Argulidce and the marine Caligidce. The second division embraces the fixed parasites, in which the females when adult lose their locomotory appendages and become fixed, deriving their nourishment by a true suctorial mouth, armed with styliform mandibles, from the fishes and other anima s upon which they are parasitic. The larvae when they emerge from the egg are nauplii, like those of other Copepods. The males and females are then alike ; after attachment the female often attains a large size, and is soon litt e more than a maggot-like body, with immense paired ovaries attached to her abdomen. The male is very email and resembles a fat Acarus ; he is usually parasitic on the female, adhering to the vulva. Almost every fish has some form of these Copepod parasites, either on its skin, i 1 - eyes, or its gills. Arguhisfoiiaceusis of a rounded oval shape, the carapace inclosing the thoracic somites in a deep notch behind, and the body terminating with a bilobed telson. The antennae are formed into recurved hook.3 for holding on by, when the animal shifts its position. The second pair of foot- jaws are converted into powerful r.uckers by which it attaches itself to its host. The mouth is tubular, and has j sharp styliform organ within it, affording good evidence of its suctorial habits. There are four pairs of biramcse natatory feet. The animal, when detached, swims with extreme rapidity and elegance, and no fish, however rapid, can escape from its adherence. The female is much larger than the male. She leaves the fish on which she is parasitic when desirous to deposit her eggs, which she fixes to a stone or other inorganic body at the bottom of the water. As many as 400 are deposited at one time by a single female. Argulus catostomi is said to lay 1500 eggs at once. The young are hatched in thirty-five days, and after about three niuults as free Copepod larva} they put on the adult form. It would be impossible to give a detailed account of all the varied forms of Copepoda in an article like the present ; we therefore must refer the reader to the works of Baird, Glaus, 3 and others for fuller information. VI. ANCHORACEPHALA : (12.) BHIZOCEPHALA. These have been referred to under Metamorphosis, so that AYC need not allude to them further here, save only to give illustrations of two genera, Peltojaster (fig. 82) and Sacculina (fig. 83). VI. AXCHORACEPHALA : (13.) CiRRirEDiA. Forty years 5 See Dr C. Glaus, Die Frei Lcbenden Copepoden mit Eesonderer Beracl-sichtigung der Fauna Deutschlands der Nordsee und, des

Mittlemeeres, 37 plates, Leipsic, 1863, 4to.