KANGE OF THE HERMIT-CRABS 155 



lana. The pleon is soft as in a hermit-crab, but re- 

 flexed. 



That the fifth pair of legs in the crustaceans of this 

 family should be folded in the branchial chambers must 

 seem a very strange arrangement, unless consideration is 

 directed to the advantage which may thus be attained in 

 keeping the branchiae clear from parasites. 



Legion 2. — Fagitriiiea. 



The carapace is elongate, becoming weak or membra- 

 naceous behind the cervical groove, which divides the 

 gastric and hepatic regions from the cardiac and branchial. 

 The second antennas have an acicle. The third maxilli- 

 peds are subpediform, with the third and fourth joints 

 elongate. The chelipeds and two following pairs of legs 

 are well developed, the last two pairs are small, one or 

 both being usually chelate. The sternal plastron is linear. 

 The pleon is spirally twisted or extended ; the tergal ele- 

 ments are as a rule rudimentary. There is generally a 

 single biramous appendage to the second, third, fourth, 

 and fifth segments of the pleon on the left side, the first 

 three of these being well-developed and ovigerous in the 

 female. The sixth segment in both sexes has a pair of 

 appendages. 



Common as the Hermit-Crabs are between tide-marks, 

 they are also found in depths of over two thousand fathoms. 

 The legion contains three families, the Cenobitidae, Pagu- 

 ridae, and Parapaguridae. The first two of these families 

 are phyllobranchiate, that is, have the branchial plumes 

 formed by a series of foliaceous plates, whereas the third 

 family is trichobranchiate, having the branchial plumes 

 made up of long cylindrical filaments. 



Family 1. — Cenobitidce. 



The first antennae have a very elongate peduncle, its 

 first joint deflexed and as long as or longer than the eye- 

 stalks, the second and third joints narrow and cylindrical ; 

 one of the flagella enlarged. In the second antennae the 



