Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/697

Rh America, by Fischer in Russia and Sweden, and by Huxley, Salter, and Woodward in Britain (see H. Woodward s monograph in Pal. Soc., 4 parts, 1865-1872). The most perfect specimens of the genera Slimonia, Pte- rygotus, and Euryptcrus have been obtained by Dr J. Sli- mon of Lesmahagow, Lanark. The largest known remains, representing specimens from 5 to 6 feet in length, are from the Devonian of Forfarshire, belonging to the great Pterygotus anylicus and to Stylonurus scoticus obtained by Mr James Powrie, F.G.S., Reswallie, Forfar. [n the Upper Silurian we have one English genus, Hemiaspis (6 in fig. 80), and three Russian forms, Exapinu- rus, Pseudoniscus (1 in fig. 80), and Bunodes, which, like the Anomoura, serve to bridge over the interval between the Limuli and Plerygoti, the hind-body being partially deve loped. The best illustration of the Eurypterida is to be found in the zoea of the common shore-crab, Carcinus mcenas (figs. 26 and 27), in which the principal locomotory organs are the maxillipeds, and the abdominal somites are destitute of all appendages. The latest representative of this extinct order has been found in the Lower Carbonr- ferous series of West Lothian, the Eurypterus Scouleri, which however differs greatly from all the other forms. IV. BRANCHIOPODA : (8.) PHYLLOPODA. This order in cludes not only the bivalved Estheria and Nebalia, and the shield-bearing Apus, but two forms of naked gill-footed Crus tacea, Branchipus and Artemia (5 in fig. 57). Of the shield - bearing forms the fresh-water Apus may serve as a good example. The eyes are placed in front on the dorsal surface of the carapace and are nearly confluent. The first pair of feet (maxillipeds) are long and branching ; to these succeed about sixty pairs of branchial feet. The thoracic and ab dominal somites are nearly cylindrical and are composed of about thirty articulations, terminated by two long, many jointed tail-spines. Probably Apus has mors articulations to its appendages and body than any other Crustacean. Schaffer tabulated them, and found they numbered 1,802,604; Latreille puts them down at not less than 2,000,000. Apus affords an excellent illustration of a form in which the mere vegetative repetition of parts is carried to an extreme distance beyond the normal number of body-rings so characteristic of the class. In Nebalia, the marine type (1 in fig. 57), the carapace or head-shield has a well-marked rostrum, and is more compressed laterally than in Apus, covering the head and thorax and even a part of the abdomen. The eyes in Nebalia are placed on pedunclea beneath the carapace ; the number of segments is not excessive as in Apus. In Esthe.ria (fig. 56), the carapace is composed of two valves, subovate in outline, like a bivalve molluscan shell, which it also resembles in being united by the umbones of each valve on the anterior dorsal border, and in each valvo being marked by regular concentric lines of growth. Branchipus (5 in fig. 57) and Artemia are destitute of any carapace, so that the elegant wave-like motion of their many-jointed transparent bodies and branchial feet can be freely observed. The former inhabits our fresh waters, the latter is marine, being peculiarly prolific in the brine-pans at Lymington, where the workmen firmly believe that the &quot; brine-shrimp &quot; aids in some way the rapid deposit of salt, through the constant agitation caused by such myriads of these minute and restless Entomostraca in the water. Chirocephalus or Branchipus is believed to be only a variety of Artemia, resulting from change of conditions be tween a fresh and an extremely saline fluid medium. 1 1 See M. W. J. Schmankewitsch a paper on the transformations of Artemia and Branchipus, Ann. and May. Nat. Hist. vol. xvii. March 1876. 663 In Professor T. Rupert Jones s monograph on fossil Estherice, out of thirty-eight localities recorded for that genus living, three only were met with in brackish water. Mr John Arthur Phillips, F.G.S., states that the Artemia fertilis is exceedingly abundant in the highly-saline waters - of Mono and Owen s Lakes, California. These littb Crustacea congregate in such dense serpentine or annular masses in the water that a breeze sufficient to ruffle the surface of the lake scarcely affects the water filled by the Artem/ ce, which remains perfectly smooth. The only other inhabitant of these salt-lakes is the larva of a dipterous insect, the Epliydra calif ornica, Torrey, which is collected by the Indians, and, when dried in the sun, forms an important article of food. Myriads of gulls and other aquatic birds visit these lakes in summer to fued upon the Artem/ ce and larval Ephydrce. Nebalia, at the present day, seems but the puny and degenerate representative of the once giant pod-shrimps of Silurian times, the caudal somites of one of which measured 8 inches and the tail spines 6 to 7 inches in length, the carapace not being preserved. 3 The ancestors of Nebalia date back to the Menevian group, Hymenocaris major beirg the earliest known. Ceratiocaris papilio is so abundant iu the Upper Silurian of Lesmahagow, Lanark, as to cover entire beds with its remains. Many fossil forms, as Dithyrocaris, Discinocaris, Apt:;- chopsis, and Peltocaris, carried their head-shield flat and expanded like Apus at the present day ; one of these, i Discinocaris from Moffat, Dumfriesshire, had a carapace 7 inches in diameter. These forms occur in all the Palaeo zoic rocks. Estherice are found in the various strata from the Carboniferous Limestone to the Tertiary. Although Branchipus or Chiroccphahis is destitute of any head-shield, its fossil remains have just been transmitted to the writer together with numerous Dipterous and Coleopterous insects from a freshwater deposit, associated with plan f remains, at Gurnet Bay, in the Isle of Wight. IV. BRANCHIOPODA: (9.) CLADOCERA. IntheCladoccra are placed a number of minute animals furnished with branching natatory antennae and five or six pairs of short fe li- aceous feet ; the body (except the head which is distinct and projecting) is entirely inclosed within a carapace, formed of two valves joined together on the back ; the eye is single and very large. The Cladocera are chiefly freshwater, and are distributed over the whole world. Of this order the Daphnia pulex (1 in fig. 58), so abundant in our fresh waters, is a good example. So numerous are they in our ponds in summer as frequently to impart a blood-red hue to the water for many yards in extent. In order to realize the wonderful fecundity of this and allied genera, 4 it is necessary to realize that when a Daphnia is only ten days old, eggs commence to be formed within the carapace, and under favourable conditions of light and temperature, it may have three broods a month or even a greater number, the larger species having as many as forty or fifty eggs at once. No males appear until the autumn, so that all the earlier broods are derived from females, whose parent was fertilized, and died after depositing its eggs, the year before, and these continue to reproduce fertile female offspring throughout the summer. 9 In 1866 Mr Phillips found that the waters of Owen s Lake had a specific gravity of 1 076, and contained 7128 24 grains of solid matter per gallon. This solid matter held in solution consisted of chloride, sulphate, and carbonate of sodium, 6813 28 ; sulphate, phosphate, and silicate of potassium, 298 02 ; and organic matter, 16 9 4 grains per imperial gallon. 3 See Geol. Mar). 1866, vol. iii. pi. 10. fig. 8, p. 203 ; and Geol. Atrj. 1871, vol. viii. pi. 3. fig. 3, p. 104. 4 See Memoir by Professor Leydig, Naturgeschichte der Daphmd-n, 4to. Tubingen, 1860 ; Sir John Lubbock in Phil. Trans. 1657; Baird s British E ntomostraca (Hay Society).