522 CRUSTACEA. [WAI^ 



WALCOTTIA, Miller & Dyer, 1878, Jour. Gin. 



Soc. Nat. Hist., vol. 1, p. 39. [Ety. 



proper name.] A rugose, flexuous, 



worm-like furrow. Type W. rugosa. 

 cookana, Miller & Dyer, 1878, Cont. to 



Pal., No. 2, p. 11, Hud. Riv. Gr. 

 rugosa, Miller & Dyer, 1878, Jour. Gin. 



Soc. Nat. Hist., vol. 1, p. 39, Hud. 



Riv. Gr. 



FIG. 946. Walcottia rugosa. 



CLASS CRUSTACEA. 



THERE is such an immense diversity among Crustaceous animals, it has been 

 found necessary to make subclasses, orders, and suborders, to give intelligible def- 

 initions to the classification. They are generally covered with a peculiar calcareous 

 secretion or integument, constituting a cutaneous skeleton, inclosing the soft 

 parts of the body. Segments are united by a membrane, giving flexibility to the 

 armor. There being no way to increase the integumentby growth, it is cast offat stated 

 periods, and a new one is secreted to cover the enlarged body. The subclasses are 

 Cirripedia, Entomostraca, Xyphosura, Edriopthalmata, and Podopthalmata. 



The Subclass Cirripedia includes only a single order which bears the same 

 name. The animals, when mature, are attached to submarine objects, and are in- 

 closed in a shell composed of several calcareous plates, from an opening in which 

 articulated cirri are exserted and retracted when the animal is alive in search of 

 prey. The common barnacle, which frequently covers the bottoms of ships so as to 

 impede their progress across the ocean, is a representative of this order. 



The Subclass Entomostraca is divided into several orders, only three of which 

 are Palaeozoic, viz.: Ostracoda, Phyllopoda, and Trilobita. The Ostracoda are 

 minute animals inclosed in a little bivalve shell ; the feet and antennae are pro- 

 truded between the lower edges of the valves. The Cypris, Daphnia, and Po- 

 lyphemus are living examples of this order. The Phyllopoda are so named on ac- 

 count of the broad and leaf-like feet. Some of them are covered with a bivalve shell, 

 and others are without such protection. The Palaeozoic are bivalve shells. The 

 Estheria, which abound in pools and springs, belong to this order. The Trilobita 

 possessed a cephalic shield, a trilobed thorax composed of segments, which were flex- 

 ible and allowed the animal to double itself up, and a tail-piece called the pygidium. 

 The order became extinct in the Palaeozoic era. 



The Subclass Xyphosura has an anterior subcrescentiform carapace, inclosing 

 the cephalothoracic organs, and a posterior abdominal piece, from which a tail 

 spine projects. The upper surface is convex, and the lower concave. There are 

 three orders Amphipeltida, Euripterida, and Xyphosura. Only a fragment of the 

 shell of the Amphipeltida is known. The Euripterida is also an extinct order. A 

 common form of the Xyphosura is the Limulus, or Horseshoe Crab, which is com- 

 mon on the shores of the tropical seas. 



The Subclass Edriopthalmata has the head distinct from the thoracic segments, 

 and therefore has no cephalothorax. The head has a pair of simple compound 

 eyes, not pedunculated. The Palaeozoic orders are Amphipoda and Anisopoda. 



