ISOPODS WITH FINGEE AND THUMB 319 



stance tliat the heart occupies the earlier segments of the 

 perseon, whereas in those Isopoda which have -branchial 

 pleopods, the heart is correspondingly situated in and 

 near the pleou. The tribal name refers to the fact that 

 the first pair of trunk-limbs, or gnathopods (corresponding 

 to the second maxillipeds of preceding descriptions), are 

 furnished with a chela, the two terminal joints forming an 

 opposed thumb and finger as in the familiar chelipeds of 

 crabs and lobsters, a character not met with in the rest of 

 the Isopoda. The uropods are slender, the branches often 

 flagelliform. The eggs are carried in a marsupium of thin 

 plates, which either form one pair attached to the fourth 

 free segment alone, or four pairs attached to four segments. 

 The tribe includes only two families, the Apseudidae 

 and Tanaidse, distinguishable by numerous characters both 

 external and internal. 



Family 1. — ApseiididcB. 



The body is depressed, the carapace generally having 

 a well-developed rostrum and carinate sides, with pyriform 

 or spine-like ocular lobes. The segments of the pleon are 

 well defined, narrower than those of the peraeon. 



The first antennae, placed at the front corners of the 

 carapace, are remarkable in this sub-order by having two 

 many-jointed flagella. The second antennae with the 

 bases contiguous are placed between and below the first, 

 and are often furnished with an articulated scale, ciliated 

 all round. The flagellum is many-jointed. 



The mandibles have a three-jointed ' palp." The first 

 maxillae have two incisive lobes and a two-jointed back- 

 ward-directed ' palp,' ending in two or more set£e. The 

 second maxillse are furnished with setee and spines. The 

 maxillipeds have a large laminar epipod, branchial in 

 function. The first gnathopods are strong and chelate, 

 the inner margins of the finger and thumb being usually 

 tuberculate in the male and serrate in the female. The 

 second gnathopods are fossorial, having the last three 

 joints, especially the penultimate or ' hand,' flattened, the 



