568 ZOOLOGY skct. 



the rest are usually alike in character. The abdominal appendages 

 are usually biramous ; the rami function as branchiae The body 

 is nearly always dorso-ventrally compressed. This is a large order 

 including many families, e.g. — Asellus, Phrcatoicus, Anthura 

 Sphaeroma, Idotea, Oniscus, Bopyrus (Figs. 462, 464). 



Order 5. — Amphipoda. 



Peracarida with the characters of the preceding order, except 

 that (1) the body is nearly always laterally compressed; (2) the 

 second and third pairs of thoracic appendages are nearly always 

 modified as prehensile organs (gnathopods) ; (3) there are vesicular 

 or lamellar branchiae attached to the bases of more or fewer of the 

 thoracic limbs ; (4) the abdominal appendages are distinguishable 

 into two sets, the three anterior pairs with many -jointed rami, the 

 three posterior (including the uropods) with unjointed styliform 

 rami. 



Includes Orchestia, Gammarus (Fig. 461), Hypcria, Gaprclla, 

 Gyamus (Fig. 463), and many other genera. 



Division 3. — Eucarida. 



Malacostraca in which the carapace coalesces with all the 

 thoracic segments, forming a cephalothorax. The eyes are borne 

 on movable stalks. The heart is short, sac-like, and situated in 

 the thorax. 



Order 1. — Euphausiacea. 



Eucarida in which none of the thoracic limbs take the form of 

 maxillipedes ; with a single series of branchiae (podobranchs) 

 attached to the bases of the thoracic limbs. The larva is a 

 nauplius. 



This is a comparatively small order of pelagic Malacostraca, 

 including Ewphausia (Fig. 472), Thysanopoda, Nydiphanes, and a 

 few other genera. 



Order 2.— Decapoda. 



Eucarida in which the first three pairs of thoracic appendages 

 are modified as maxillipedes, with the branchiae usually in several 

 scries — podobranchs, arthrobranchs, and pleurobranchs. 



Sub -order 1 . — Macrura. 



Decapoda with well-developed, elongated abdomen, which is 

 usually held in the extended position, and terminates in an 

 expanded fan-like tail-fin composed of the telson and the uropods. 

 The eyes are not enclosed in orbits. The antennules and antennae 



