CRUSTACEA. 632 CRUSTACEA. and with a disproportionately large abdomen. I'he live anti'rior pairs of thoracic limbs func- tion as niaxillipc'des, the second pair being mucli the largest and juovided with a spiny terminal segment that foUls back upon the penultimate segment, like a knife-blade into its liandle, to form a very efficient clasping organ. A powerful burrowing organ, developed by the union of the si.xth pair of abdominal appendages with the terminal plate or telson, enables the animal to l)ury itself in the ,?andy bottoms near sliore. Fossil forms of this group are rare. The earli- est, Neeroscylla. from the coal-measures of Eng- land, resembles .Squilla. Another allied genus, Sculda, is found in fine preservation in the litlio- graphic slates of the Bavarian Jurassic, and Squilla is found in the same horizon and in suc- ceeding horizons of the Cretaceous and Tertiary. (5) Cu'iiuiceu. — Another small order of prawn- like Crustacea, in which the compound eyes ai-e sessile and sometimes fused together, or are entirely absent. The carapace is further re- duced tluin in the Stomatopoda, for it leaves the five posterior pairs of thoracic segments uncov- ered. Only two pairs of thoracic limbs function as maxillipedes, and six pairs are legs, of which two or tnree anterior pairs are biramous. This group is related to the Schizopoda and Isopoda; no fossil representatives of it are kno^^l. (6) Arfhrostraca. — In this large order the fusion of the segments is still further reduced, only one or two of the anterior being united with the head to form a nidimentary cephalothorax. These fused segments bear maxillipedes, while the remaining free segments bear legs that end in claws or are built for swimming. The two suborders are quite dift'erent in outward appear- ances. The Aiiipkipoda have the body laterally compressed and the legs adapted for swimming and jumping, as in the sand-fleas (Gammarus and Orchestia). The Isopoiln have the body dorsoventrally flattened, the legs adapted for crawling, and the abdominal segments are fused to form a single terminal plate. The amphipods (see Amphipoda) are wholly aquatic, in both marine and fresh water, and are often also found living in the damp flotsam and jetsam of the beach between low and high tides. The isopods, while generally aquatic, afl^ord some fine exam- ples of adaptation to terrestrial conditions in the wood-louse (Oniscus) and pill-bug (Armadillid- ium) , whieh are commonly found under the bark of dead trees and in other like situations. Paleon- tologieally the amphipods are of little impor- tance. The earliest indisputable ancestor is Acanthotelson, of the Carboniferous, and the genus Gammarus itself is found in the Tertiary rocks. The isopods are better known in a fossil state, and the ancestors seem to have been as a rule of larger size than the recent forms. Ar- tbrapleura of the Carboniferous attained a length of about twenty inches. Prjearcturus, also a large form, from the Old Red Sandstone of the English Devonian, is the earliest isopod kno^^'n. Archseoniscus. Cyelosphseroma, and Eo- sphicroma are other genera from the Mesozoic and Tertiary rocks that are closely allied to modern forms. Geological Distribution. The accompanying table indicates in a rough way the larger classi- fication of the Crustacea and the distribution and expansion of the various subdivisions in past geologic times. It shows that certain types, es- pecially the more primitive, played more impor- . tant roles in the early ages than they do at pres- ent, and tluit they have been superseded by the more specialized types. The letters at the "heads of the vertical columns indicate the geological systems as follows: C, Cambrian; 0, Ordovi-
Geological Distribution Of THE Crustacea Paleozoic Mesozoic Cenozoic H ClOlSlD Cb|P T J Cr E M P R m ^_ Entomostraca 1. Order. Phyllopoda
_ _ — Suborder. Cladocera — 4 ? — — — ^
■ ■ ■ — ■ _ _ — ■ ■ ■ — ^ ■ ~ - IM.Order.Copepoda ( Not found in a fossil Stat .) IV.Order.Cirripedia , — _ — -^ _ _ _ ~ — — — - s Suborder, Rhizocephala ( Not fou di of OSS, state) Malacostraca 1 Order, Phyllocarida _ B ll.Order.Schizopoda ■ P> _ B IM.Order.Decapoda _ h - Suborder, Macrura H ■ir -?- r H ■ ■ IV. Order, Stomatopoda ? •"" H V. Order, Cumacea t Not f.u ndi af ssl 5t3 el VI. Order, Arthrostraca Suborder.Amphipoda — 1 ■i Suborder. Isopoda ™ — H ^ HISTORICAL -VIEW OF CRUSTACEA. cian; S. Silurian; D, Devonian: Cb. Carbonifer- ous; P. Permian; T, Triassic: .J, .lurassic; Cr, Cretaceous: E, Eocene; M, Pliocene: PI. Plio- cene; R, Recent. An interrogation-point indi- cates the doubtful presence of a member of the group, and the width of the black line indicates in a very imperfect way the relative amount of exjiansion of the group. The articles on the dif- ferent orders and suborders should be consulted for further infonnation. BiBLiouRAPiiY. The best general introduction to the study of the Crustacea is T. H. Huxley, The Crayfish: An Introduction to the l^tudy of Zoolo(jif, in International Scientific Series (New York, 1884), Other works of a general nature on the morphology and classification are: .T. S. Kingsley, "The Classification of the Arthro- poda," in American yatiiralist. vol. xxviii. (Philadelphia, 1804); K. Groben, The Geneal- ogj- and Classification of the Crustacea," in An- nals and Maga::inc of Natural TJistory. ser, 6, vol. xi. (London. 1803) ; T. R. R. Stebbins, Bis- torn of the Crustacea (New York, 1803) ; R. Rathbun, "Natural History of the Economic Crustaceans," in Bulletin U. .9. Fish Commis- sion (Washington, 1880) ; H. :Milnc-Edwards, Jlistoire naturelle des crustaccs (3 vols., Paris, 1834-40) ; H. Gerstaecker, "Gliederfiissler," in Bronn's Klassen und Ordnungen des Thierreichs,
