178 CRUSTACEA—-EUCARIDA—DECAPODA ,. CHAP. 
reduction (Fig. 122, B) and disappearance (C) of these original 
plates, their place being taken first by a number of irregularly 
situated small spines and warts, which, however, subsequently fuse 
up to form definite segmental plates. In Lithodes maia, 3 (D), 
there are a series of lateral and marginal plates, while in 
Acantholithus (KE) a number of median plates appear, presum- 
ably by the fusion of the small spines present in the median 
Fia. 122.—Diagrams of abdomen: A, of Pylopagurus, sp.; B, of Hapalogaster cavicauda ; 
C, of Dermaturus hispidus; D, of Lithodes maia, $6 ; E, of Acantholithus 
hystrix. c, Central plates; 7, lateral plates; m, marginal plates; 7, telson ; 
1-6, 1st-6th abdominal segments. (After Bouvier.) 
line in Lithodes maia; finally, a fusion of the marginal and 
lateral plates may take place, so that each abdominal segment 
is covered by a median and two paired lateral plates. 
It is to be noted that the males and females of the various 
species do not follow a parallel course of development, the plates 
in the male being symmetrical, while those of the female are 
often highly asymmetrical (compare Figs. 122, D, and 121), thus 
giving the strongest evidence of a Pagurid ancestry. 
Birgus and the Lithodidae, then, are Pagurids which have 
given up living in shells, and have become adapted to a free 
existence, protecting their soft parts by the development of 
