MARINE ISOPODA OF NEW ENGLAND, ETC. 361 



VL AKCTUBID^E 



Form elongated ; antennae large and strong ; first four pairs of legs 

 directed forward, ciliated, last three pairs ambulatory; segments of 

 pleon more or less consolidated ; uropods operculiform. 



This well marked family is as yet represented on our coast by a single 

 species of the genus Astacilla Fleming, Leachia or Leacia of Johnston 

 and other authors. The family can be easily recognized by the four 

 anterior pairs of legs, which are directed forward and strongly ciliated 

 on their inner margins with long slender hairs. The form of the body 

 is elongate and may be very much so, as in our species the length of the 

 body in the male is twenty times as great as its diameter at the middle ; 

 in the female eight times. The head is of moderate size and the eyes 

 prominent. The four-jointed antennulae have the basal segment large 

 and swollen. The antennae are large and powerful organs, approaching 

 or even surpassing the body in length, with the first two segments short, 

 the second deeply incised below as in Idotea, the next three segments 

 elongated, and the flagellum varying in the genera, being multiarticu- 

 late in Arcturus, and composed of not more than four segments in Asta- 

 cilla. The mouth parts resemble, in general, those of the Idoteidce. The 

 fourth thoracic segment is more or less elongated. The last three pairs 

 of legs are ambulatory, differing much from the first four pairs. The 

 segments of the pleon are more or less united, and the uropods are mod- 

 ified, as in the preceding family, to form an operculum for the more del- 

 icate anterior pleopods. They are wholly inferior, and consist on each 

 side of a large basal segment, straight on the median line, where it meets 

 its fellow of the opposite side, and bearing, in our genus, two small ter- 

 minal plates at the apex. 



This structure of the pleon and its appendages, together with the 

 structure of the antennulae, antennae, and the parts of the mouth, point 

 to a close relationship between this family and the Idoteidce. With the 

 Antlmridce, however, with which they have often been associated, they 

 seem to have little in common, except, perhaps, the elongate form of 

 body. Even this feature is approached also in the Idoteidce, in Erich- 



sonia, for example. 



Astacilla Fleming. 



Leacia (Leachia) Johnston, Ed. Phil. Jour., vol. xiii, p. 219, 1825 (non Lesueur). 

 Astacilla Fleming, Encyc. Brit., 7th ed., vol. vii, p. 502. 



Johnston, Loud. Mag. Nat. Hist., vol. viii, p. 494, 1835. 



Antennal flagellum short, not more than four-jointed ; fourth thoracic 

 segment elongated, and, in the females, bearing the incubatory pouch 

 on its inferior surface. 



The characters given above seem sufficient to warrant the separation 

 of this genus from Arcturus, notwithstanding the fact that the young 

 of some species, and probably of all, have the fourth thoracic seg- 



