72 NATURAL HISTORY OF ARTHROPODS. 



host. The males are much smaller than the females, and have not been so ex- 

 tensively altered by parasitism. They usually are found attached to the modified 

 abdominal feet of the females. These forms have not been exhaustively studied, 

 and comparatively little is known of our American forms. The principal char- 

 acters separating the genera are the number and shape of the abdominal branchial 

 feet. 



The last family of Isopoda which we have to notice is the ONISCID^E. The forms 

 belonging here are almost entirely terrestrial, for although some of them may be found 

 on the seashore they are not aquatic in their habits. The antennulae are minute, but 

 the antenna? have from six to nine joints, the differences in number affording im- 

 portant systematic characters. The legs are all fitted for walking, while 

 the abdomen is composed of six distinct segments. In common parlance 

 these forms are known as "sow-bugs," "pill-bugs," and "wood-lice." 

 They live in damp situations, in cellars, under decaying wood, etc. 

 In Armadillo and its allies the body can be rolled up in a compact ball, 

 recalling the similar capacities of the mammalian genera Tatusia, Dasy- 

 pus, and Tolypeutes. The more common forms, belonging to the genera 

 OnisciiS) Porcellio, etc., usually do not have this capacity. These forms 

 are largely if not entirely vegetable feeders, devouring much which 

 FIG. 95. Porcei/io, -\vould become offensive or injurious to man. In olden times, when the 



sow-bug, enlarged. . 



more disgusting a thing was the more erncacious it was considered as 

 a remedial agent, some of these " sow-bugs " played an important part in the Pharma- 

 copoeia, but doubtless they had very little therapeutic value. 



ORDER II.-- AMPHIPODA. 



As a rule the members of this order present a very strong contrast to those of the 

 Isopoda. These have the body flattened vertically, the legs, as the name indicates, 

 divided into two dissimilar series, while some frequently terminate in a more or less 

 perfectly formed pincer. They are not so largely modified by parasitic habits, while 

 their mouthparts are less variable than in the group which AVC have just left. The gills 

 are in the form of sacs attached to the bases of the thoracic limbs. The sessile eyes 

 are compound, but the investing integument is not faceted. The antennulae are 

 sometimes two-branched, and the mandible usually bears a palpus. The two anterior 

 pairs of walking feet are usually fitted for grasping, sometimes bearing a pincer as per- ( 

 feet as those found in the Decapoda. Moulting is effected by a transverse fissure just, 

 behind the head, and a longitudinal one extending from this down the back. The 

 individuals have not that power of amputation of members which has been noticed in 

 connection with certain Decapoda. 



SUB-ORDER I . - - L^MODIPOD A. 



In the older works the forms included under the name Lsemodipoda were regarded 

 as belonging to the Isopoda, the next view placed them as an order intermediate 

 between the Isopoda and the Amphipoda, while more recently the weight of authority 

 is in favor of the position here accorded them. In these aberrant forms the most 

 striking characteristic is the rudimentary condition of the abdomen, which is repre- 

 sented by an inconspicuous tubercle without appendages. The number of thoracic 



