654 CRUSTACEA more than twenty-one segments, the introduction of the Trilobita among these may be looked upon as inappropriate. If, however, we admit that the Trilobita had (as there seem good grounds for allowing) true special locomotory appendages other than gill-feet or jaws, as in the Malacos- traca generally, then we submit that they are appropriately classed. The main characteristic of the Malacostraca seems to be, not so much the possession of twenty-one segments, an inheritance really common to the whole class, but the presence of the seven anterior (cephalic) appendages especially set apart for the senses and nutrition, with separate post-cephalic organs of locomotion ; whereas the peculiarity of the Entomostraca seems to be that the seven anterior (cephalic) organs are not specially set apart a 3 organs of sense and nutrition, but are employed in nearly all the class as the chief locomotory organs, the posterior feet being branchial or ovarian lamellae, or altogether want ing. The writer ventures to propose therefore, instead of Estimated thickness of strata in feet. Formations. Evolution and Range in Time of the Orders of the Crustacea. A L. I. C. 6. B. A. T, I. s. M. Aa, Ba. X. A S. 250 Recent . J <
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30,000 Pre-Cambrian ... <*" i -i frtn.iKveMHyXfa* F;a. 64. Diagram showing the probable evolution and actual range in time of the several orders of Crustacea. Explanation. The letters at the top of each column represent the several orders and sub-orders of Crustacea, one family of the Arachnida, the Scorpionidie, being placed beside the Eurypterida to show its range in time and probable derivation from the Crustacea. The vertical lines show the actual range in time, the black dots denote the strata in which remains of each order have been met with. The curved lines, uniting order to order, are intended to show the probable evolution of the class from a common great ancestor : but numerous as are the indications of affinities afforded by the Crustacea, the group is too ancient to be certainly traced back in time to a common parent ; even the lowest Cambrian rocks have yielded evidence of two orders, namely, the Trilobita and Ostracoda, whilst the Middle Cambrian furnishes a Phyllopod Crustacean (Hymenocaris). As the branches of this genealogical tree of the Crustacea are not arranged in a Circular manner, they cannot of course be made to show the affinities which each order presents to every other order or branch, any more than does the dried plant show the natural verticillate arrangement of its leaves and branches when pressed out flat upon the page of a llortus Siccus. L=the LOPKYROPODA. Of this legion, the order Ostracoda is well represented throughout the entire series, from the Cambrian to the Tertiary and also in the seas and fresh-waters of to-day, C=theCiRRipF.DiA. Z=the Lepadidie, or pedunculated Cirripedes, appear first In the Wenlock Limestone (U. Silurian) ; represented by a single form (Turrilepas Wrightianus, H. Woodw.). Numerous forms occur in the Secondary and Tertiary strata, and they are abundantly distributed throughout the warmer seas of the world. 6=the Balanidse, or sessile Cirripedes, are represented by a single form, the Pyrgoma cretacea, II. Woodw., in the Chalk, abundantly in the Tertiary rooks, and in recent deposits, and are distributed in the seas all over the world at the present day. B=the BRANCHIOPODA. Of this division the order Phyllopoda appears in the Cambrian rocks, and is well represented in Silurian and Carboniferous strata ; they are also met with in the Secondary and Tertiary formations, and living in fresh, brackish, and marine waters, widely distributed over the globe. A:=the AMPHIPODA. This order has a single representative in the Upper Silurian, Necrogammarus Salweyi, H. Woodw.; it is represented by Gampsonyx? in the Coal, by Prosoponiscus in the Permian, and by several forms in the Secondary and Tertiary formations, and abundantly in recent freshwater and marine localities. T^the TRILOBITA. This extinct order appears in the Cambrian, attains its maximum development in the Silurian, and terminates in the Carboniferous period. I=the ISOPOBA. This order is represented in the Devonian by a single species, Prcearcturus yiyas, H.Woodw. ,and by remains in the Carboniferous strata. Many species occur in the Secondary and Tertiary strata. It is largely represented to day by land, freshwater, and marine types. S = STOMAPODA. Pygocephalus Huxleyi, II. Woodw., from the Coal-measurcP, probably belongs to this division, True Squillas and J/j/sis-Iike Crustacea occur in the Jurassic rocks (Secondary). Forms of this order are abundant in our modern seas. M = the MACROCRAH division of DECAPODA. A single species, Anthrapa!<emon Groisartii- Salter, appears in the Coal-measures. This order is well represented from the Trias to the present day, and is now one of the prevalent types, occuning both in fresh and i-alt water. Aa=the ANOMOURA, or irregular-tailed DECAPODA. This sub-order embraces forms related both to the Crabs (e.g. Dromia, PorceUana, Dorippe) and to the Lobsters (e.g. Pagurus, Ga/athea, Munida). Their earliest appearance is in the Cretaceous period ; there are numerous living forms, both terrestrial 1111 J marine. Ba=the BRACHTTRA. The oldest known crab is the Palceinachus lonaipes, H. Woodw., from tlie Gt. Oolite. Crabs are well-represented from the Upper Secondary to the present day, when they attain their maximum within the warmer latitudes, being represented by land, freshwater, and marine forms. X=the XIPUOSURA, or king-crabs. These are remarkable for their longevity ; they appear first in the Upper Silurian (Xeolimulus fakatus, II. Woodw.); again in the Coal-measures, next in the Oolite and Tertiaries, and living to-day in the Old and New Worlds. E=EL RYPTERrvj. This extinct order contains some of the largest krown members of the Crustacean class (e.g , Pterygotus anglicus, Devonian). It ranges from the Upper Silurian to the Coal-measures. On morphological grounds there is good reason to conclude that the Eurypterida are the ancestors of the Scorpionida?, to which they present the strongest affinity. A=ARACHNII)A. S = SCORPIONIDA. The scorpions range from the Coal- measures, apparently unaltered, to the present day. the terms Malacostraca 1 and Entomostraca, 2 which convey no idea of any structure or function common to either division to which they are applied, the adoption of the terms Thoracipoda 3 and Gnathopoda. 4 which embody the salient character in each sub-class. 1 From uaait6s, soft, and offrpanov, a shell, a term not specially appropriate or applicable to crabs and lobsters. 3 From tvTo/jios, an insect, and oarpaKov, a shell, a name quite applicable to the Ostracoda, but not to all the sub-class. 3 From Odipat,, the thorax (or middle body), and irouy, iroS6s, a foot, in allusion to the prevalent use in the Malacostraca of the thoracic series of appendages as special organs of locomotion. 4 From 7; 2#os, the jaw, the mouth, and TTOVJ, TroSo j, afoot, in allusion The Trilobita are probably represented to-day by the Isopoda, to which doubtless they are closely related. There is reason to believe the members of the other extinct order, the Eurypterida, to have been the aquatic branclii- ferous ancestors of the terrestrial tracheated air-breathing Scorpionidce ; nevertheless they need not on that account be removed from their present position in the Crustacean to the prevailing character in the Entomostraca, in which the head and mouth-organs are also mainly used in locomotion. We should of course have preferred to use the term Cephalopoda for this sub-class, had
not that designation been already appropriated for the cuttle-fishes,
