28 



SMITHSONIAN MlSCliLLANEOUS COLLECTIONS 



\OL. 81 



for regarding the region of the carapace formed of the gnathal seg- 

 ments as a part of the head, since there is no evidence that the decapod 

 head ever included more than the protocephalon. 



The generaUzed malacostracan crustacean, Anaspides, also retains 

 the protocephalon as an independent head piece attached within the 

 projecting anterior rim of the mandibular segment. The large mandib- 

 ular segment is likewise free from the following maxillary segment, 

 but the two maxillary segments and the first maxilliped segment are 



Fig. 16. — Four stages in the development of Forficula. (From Heymons, 1895.) 



A, embryo differentiated into a protocephalic head, and a body. B, appendages 

 of gnathal segments (Md, iMx, 2M.r) well developed. C, the gnathocephalic 

 region (Cnc) compact, but still distinct from protocephalic region. D, proto- 

 cephalic and gnathocephalic' regions united in the definitive head (H). 



Ab, abdomen; Ajii, amnion; Ant, antenna; Clio, chorion; Gnc, gnatho- 

 cephalon ; H, definitive head ; Li, first leg ; Lin, labrum ; Md, mandible ; iMx, first 

 maxilla; J>71/.r, second maxilla; Pre, protocephalon; Set, serosa; Th, thorax. 



fused into a composite region bearing the maxillae and the first 

 maxillipeds. 



In most of the other Crustacea, the head either is a unified cephalic 

 structure consisting of the protocephalon and the three gnathal seg- 

 ments, in some forms with one or two of the maxilliped segments 

 added, or it exhibits varying stages in the condensation of the gnathal 

 and maxilliped segments with the protocephalon. A relatively primi- 

 tive condition is shown by Eiihranchipus (Anostraca), in which the 

 protocephalon itself is a distinct and well-developed head capsule (fig. 

 17 A, Pre) carrying the first and second antennae (lAiif, 2Ant), the 



