292 ZOOLOGY. 
ill. 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 ; 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. Thoracostraca.—In the Stomapods, represented 
by Sguiila, the gills are attached to the base of the hinder ab- 
dominal feet. Sgwilla lives in holes below low-water mark. 
The suborder Decapoda (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- 
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 antenne, the hinder pair the 
larger and longer ; a pair of mandibles, often provided with 
a palpus, two pairs of lobed maxilla; 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- 
eagods are much asin the Hdriophihalma. Most Decapods 
leave the egg in a larval state called the Zoéa. In the 
shrimps, Lucifer and Peneus, the young is a Nauplius, like 
a young Entomostracan, having but three pairs of feet, and 
asingle eye. The Zoéa has no thoracic feet, and usually at first 
