293 ZOOLOGY. 



illse. 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 Peltocaris 

 and Discinocaris characterize the Lower Silurian period ; 

 Ceratiocaris the upper ; Didyocaris the Upper Silurian and 

 Lowest Devonian strata ; Dithyrocaris and Argus the Car- 

 boniferous period. Our northeastern and arctic species is 

 Xebalia iipes (Fabricius), which occurs from Maine to Green- 

 land. 



Order C. Thoracostraca. — In the Stomapods, represented 

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

 <lominal feet. Squilla lives in holes below low-water mark. 



The siil)order Decapodn (Shrimps, Lobster). — A general 

 knowledge of the Crustacea representing this, the high- 

 est group 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- 

 oealing the giUs, which are highly specialized and attached 

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

 eyes, two unequal pairs of antennse, 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 aU. 

 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 EdriopMJi alma. Most Decapods 

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

 shrimps, Lucifer and Peneus, the young is a Nauplius, like 

 a young Entomostracan, having but three pairs of feet, and 

 a single eye. The Zoea has no thoracic feet, and usually at first 



