xi PHYLUM ARTHROPODA 567 



ORDER 1. MYSIDACEA. 



Peracarida in which, though the carapace extends over the 

 greater part of the thorax, it does not coalesce dorsal ly with more 

 than the first three segments. The eyes, when present, are 

 supported on movable stalks ; the antennules are biramous, and 

 the antennae have a scale-like exopodite or squame. The first 

 pair of thoracic appendages are specialised as niaxillipedes. The 

 thoracic appendages (except sometimes the first and second pairs) 

 are biramous. The uropods with the telson form a broad fan-like 

 tail-fin. 



This order includes Mysis (Fig. 459), Lopkoyastcr, and other 

 genera. 



ORDER 2. CUMACEA. 



Peracarida in which the carapace coalesces with the first three 

 or four segments of the thorax, is produced on each side to enclose 

 a branchial cavity, and in front is drawn out into a rostrum. The 

 eyes usually coalesce into one, which is not borne on a movable 

 stalk. The antennules are sometimes biramous ; the antenme 

 have no exopoclites. Some of the thoracic appendages are 

 biramous. The telson may coalesce with the last segment of 

 the abdomen. The uropods are styliform. and there is no fan- 

 like tail-fin. 



Includes Cuma (Hodotria), Diastylis, etc. (Fig. 460). 



ORDER 3. TANAIDACEA. 



Paracarida in which the carapace coalesces with the first two 

 thoracic segments and is produced on each side to enclose a 

 branchial cavity. The eyes, when present, are usually supported 

 on short stalks which are not movable. The antennules are some- 

 times biramous : the antenme may possess small exopodites. 

 The first pair of thoracic limbs are modified as maxillipedes. The 

 second and third thoracic limbs sometimes have vestigial exopodites. 

 The uropods are usually narrow. 



This order includes Apseudcs, Tanais, Leptochclia, etc. 



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 ; 



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