554 ZOOLOGY SECT 



ORDER 4. ISOPODA. 



Peracarida in which the dorsal exoskeleton of the head is not 

 produced into a carapace, but in which the first and sometimes 

 also the second segment of the thorax coalesce with the head. 

 The eyes are sessile or borne on short processes which are not 

 movable. The antennules are nearly always uniramous : the 

 antennae sometimes bear a minute exopodite. The thoracic limbs 

 have no exopodites : the first pair are modified as maxillipedes ; 

 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, Phreatoicus, Anthura 

 Sphaeroma, Idotea, Oniscus, Bopyrus (Figs. 469, 471). 



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. 468), Hyperia, Caprella, 

 Cyamus (Fig. 470), 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. 479), Thysanopoda, Nyctiphanes, 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 

 series podobranchs, arthrobranchs, and pleurobranchs. 



