178 



CRUSTACEA EUCARIDA DECArODA 



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 LithoJes maia, 6 (D), 

 there are a series of lateral and marginal plates, while in 

 Acantholithus (E) a number of median plates appear, presum- 

 ably by the fusion of the small spines present in the median 



'V.^N^\ ^VAaaX^ 2^ 



Fig. 122. — Diagrams of abdomen : A, oi Pylopagur^is, sp. : B, of Hapalogasfer cavicauda ; 

 C, of Deniiaturus hispidus ; D, of Lithodes maia, S ; E, of Acantholithus 

 h//stri:c. c, Central plates ; I, lateral plates ; 'in, marginal plates ; T, telson ; 

 1-6, lst-6tli abdominal segments. (After Bouvier.) 



line in Zithodes 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 Ije 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 



