312 ZOOLOGY. 



illae. Behind these mouth-parts are eight pairs of short, leaf- 

 like respiratory feet, which do not project beyond the edge of 

 the carapace. These are succeeded by four pairs of large, 

 long swimming feet, and there are two additional pairs of 

 small abdominal feet. There is no metamorphosis, develop- 

 ment being direct, the young hatching in the form of the 

 adult. Of the fossil forms, Hymenocaris was regarded by 

 Salter as the more generalized type. The genera PeUocaris 

 and Discinocaris characterize the Lower Silurian period ; 

 Ceratiocaris the upper ; Dictyocaris the Upper Silurian and 

 Lowest Devonian strata ; Dithyrocaris and Argus the Car- 

 boniferous period. Our northeastern and arctic species is 

 Nebalia bipes (Fabricius), which occurs from Maine to Green- 

 land. 



Order 6. Stomapoda. In the small group represented by 

 Squilla the gills are attached to the base of the hinder ab- 

 dominal feet. Squilla lives in holes below low-water mark. 



Order 7. Decapoda (Shrimps, Lobsters, Crabs). A gen- 

 eral knowedge of the Crustacea representing this, the high- 

 est order of the class, may be obtained by a study of 

 the craw-fish and lobster. All Decapods have twenty seg- 

 ments in the body, a carapace covering the thorax and con- 

 cealing the gills, which are highly specialized and attached 

 to the maxillipedes and to the legs ; usually a pair of stalked 

 eyes, two unequal pairs of antennae, the hinder pair the 

 larger and longer ; a pair of mandibles, often provided with 

 a palpus, two pairs of lobed maxillae, three pairs of maxilli- 

 pedes, while the name of the order is derived from the fact 

 that there are five pairs of well-marked legs, or ten in all. 

 To the abdomen are appended six pairs of swimming feet, 

 called "swimmerets." Another distinctive characteristic of 

 most, in fact all the higher Decapods, is the short, or five 

 or six-sided heart. 



The early phases of embryological development in the De- 

 capods are much as in the Tetradecapods. Most Decapods 

 leave the egg in a larval state called the Zoea. Certain 

 Zoeae having originally been supposed to have been adult 

 Crustaceans, were described under the name Zoea. In this 

 condition there are no thoracic feet, and usually at first no 



