CRUSTACEA. 631 CRUSTACEA. of which aiv usually well differentiated to per- form special functions. The compound eves arc stalked in all orders except the fumacea and Arthrostraea. ( 1 ) The order I'lii/llocniida was much more fidly represented in Paleozoic times than it is now by such genera as Coratiocaris. Kchinoearis, Pepliricaris, etc., in wliieh the eephalothorax was covered by a hinged bivalve shell provided with an adductor muscle like that in the bivalve phyllopods and the ostracods. The order enjoyed a long life-period, as it first appeared in the upper Cambrian, and it lasted to the end of the Paleozoic. Tlirough the Jlcsozoic and Tertiary eras the order could not have been very abundant, as no fossil remains referable to it have been found in the rocks of those ages, and the modern genera, three in number, show considerable modifications from their early an- cestors. This group presents synthetic types which may have given rise on the one liand tn the Phyllopoda. and on the other to the higher Crustacea represented by the Schizopoda and Decapoda. (See Puyllocarida.) (2) Schizo- poda. — The animals of this order, the opossum- shrimps (Mysis), resemble in their general form the shrimps and prawns among the decapods. They are, however, more loosely built and the thoracic limbs, of which there are eight pairs, are biramous (whence the name), and all alike. These characters mark the group as of primitive rank. Its relation, as one of lower phylogenetic rank, to the Decapoda. is shown by the fact that certain decapods, such as the prawns, pass in their ontogeny, that is, in their development from egg to adult, through a 'mysis stage,' in which sehizopod characteristics are strongly marked. The fossil genera Arehfeoearis. Cran- gopsis, and Pygocephalus from the Carbonifer- ous, and possibly also Palseocaris and Gamp- sonyx, the latter from the Permian, are the earli- est ancestors of the Schizopoda. These all pre- sent very primitive cliaracters and are separated from the modern descendants by the intei'val of Mesozoie and Tertiary time, from which we have no traces of the group. (-3) Decapoda. — The character upon which the order (shrimps, lobsters, and crabs) is based is the prominence of the five posterior pairs of thoracic limbs developed as walking feet : 'the three anterior pairs lieing adapted as maxilli- pedes or accessory mouth-parts. The first pair of walking feet is usually provided with strong clielie or nipping claws. The ceplialothorax is covered by a single firm carapace, which in many crabs is so shaped as to alTord a most eificient protective covering for the entire animal, includ- ing the legs. The frontal portion of this cara- pace is produced into an anterior rostrum that is often of considerable length, and the lateral portions are extended downward to cover llic gills, which are Iiorne on the bases of the tlioracic limbs. The order is divided into two suborders: the Jlacrur.i, with long-tailed bodies, as the shrimp, prawn (Pahemon). and lobster (Ho- marus) : and the P.rachyura, or crabs, with the abdomen reduced and closely applied to the under surface of the eephalothorax. A number of species allied to the hermit-crab (Pagurus), often distinguislied under the subordinal designa- tion Anomura. are either !Macrura or Tirachyura which, through adaptation to tleir peculiar hab- its of life, in occupying the discarded shells of gastropods, have lost the regularity of their thoracic and abdominal segments. The com- pound eyes of all decapods arc raised on stalks, which in many crabs are developed to extraor- dinary lengtlis. Some of the largest among Crus- tacea are found among the decapods. As a rule the quickswinnning (lecapods (prawns) may be recognized by the lateral compression of the body, while the bottom crawlers have a more or less flattened form. The crabs are the most highly specialized forms of all Crustacea, and they exhil)it many interesting and ])eculiar adap- tations to habit of life. (See Cu.vii; Txm.sxER; Prawn: etc.) The early ancestry of the deca- ])ods is not well known tor those pciiods previous to Mesozoie time. Some doulilful Paleozoic forms that seem to be transitiimal between the schizo- pods and decapods are Paheopahenuni, from the ])evonian of Ohio, and Anthrapabemon, from the coal-measures of Illinois, but during this era the order was greatly sid)ordinated to other crus- taceans like the trilobites and ostracods, and to the merostomates, represented liy Eurypterus. In the Mesozoie of Euro])e the group attained a great development, and this continued through the early Tertiary period. Fossil decapod re- mains are peculiarly sparse in the American de- posits of these periods. The Jurassic litho- graphic slates of Bavaria have aft'orded large niunbers of finely preserved specimens of fossil decapods of many genera and species. Some of these, as Eryon, have modern representatives, as Willemoesia. still living in the abysmal deptlis of the ocean. Other genera, among them the prawn (Pena-us), began in the .Jurassic and Cre- taceous periods and have survived without changes of greater than specific rank throughout the succeeding ages, and are now abundant in the ilediterranean .Sea and the Atlantic and In- ilian oceans. The Maerura declined toward the end of Cretaceous time and gave way to the P.rachyura. which attained great expansion dur- ing the Eocene or early Tertiary period. In fact, the crabs were the dominant types of inverte- brate life in many parts of the Tertiary seas of Europe, and their fossil remains are now found in abundance in the rocks that are relics of those seas. The Eocene deposits of the London Clay of England, those of Bavaria and Hungary, and especially those of Vicenza, in northeastern Italy, have afforded the finest specimens, nearly all of which belong to modern families. (4) Hlomatopoda. — This is a small order com- prising nine marine genera, of which the best known is the mantis-shrimp (Squilla). They have the body loosely built, elongated and flat- tened, with an incomplete carapace that leaves the three posterior thoracic segments uncovered, Explanation of Plate of Cbi'Stacea, Fossu.. 1. Dromiopsis. a roiuKl rral> from the T'lip'T Crotacpoiis of Deiimnrl<. *2, Lepfniitia. au o.stracoil ; Silurian: island of fJotlanit. 3. .Eger. a prawn ; Jiirassic litliofiTapliic lime- stones ; Bavaria. 4. Xaiitliopsis. a c.Vflonietopan crab : Eocene; Ilavaria. .5. Tnrrilepas. a primitive eirripcd. al- lied to the p:ooee-bartiaele : Silurian; Dndle.v. Enjrlnnd. G. Sculda ppiinata. a stoniatopod : Upper .lura ot Bavaria: A. outline of tile fossil ; B. dorsal view of head ; C. ventral view of cntiiv body. KhowinK tlioracic ami abdominal ap- lieiida;;pH. 7. Oratioearis. a phylloi-arid ; Ordoviciaii: Lan- arl^sbiiv, Kii^land ; sliowinp: the bivalve shell, some of the liead api>eiidap:efi. and the .>*ep:mented lonj; abdomen with terminal siiincs. 8. Paheocrciisia. a barnacle, allied to the aeorn-shell. imbedded in a fossil eornl of Lower Tlcvonian age : New York State. '.», Pephriearis. n [di.vHocarid with siiinosi' carapace: Chemliiijr: New York State. 10. Er.von, a. marrnraii ; lithopraiiliio limestones of Bavaria; .. ven- tral aspect : B, dorsal aspect.
