568 ZOOLOGY SECT. 



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. Ascllns, Phrcatoicus, Antlium 

 Sphacroma, Idotca, Oniscns, 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 Orchcstia, Gammarus (Fig. 461), Hypcria, Caprella, 

 Cyamus (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 Euphausia (Fig. 472), Thysanopoda, Nyctiphancs, and a 

 few other genera. 



ORDER 2. DECAPODA. 



Eucarida in which the first three pairs of thoracic appendages 

 are modified as maxillipedes, with the branchia3 usually in several 

 series 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 antenna; 



