370 



INVERT EBB A TE MORPHOLOG T. 



The study of the embryology of some of the higher Crustacea has 

 brought out the fact that in these there are indications of a segment desti- 

 tute of appendages but represented by a pair of nerve-ganglia, immedi- 

 ately succeeding the eye-bearing anterior segment. In these cases, then, 

 the head really consists altogether of seven segments. Whether this seg- 

 ment represents the first appendage-bearing segment of the lower forms 



D 



FIG. 163. CRUSTACEAN APPENDAGES. 



A, antennule of Crayfish, Cambarus; B, antennule of Copepod, Oiihona (after 

 GIESBRECHT); (7, antenna of Cambarus; D, antenna of Phyllopod, Eulimnadia 

 (after PACKARD) ; ss, sensory hairs. 



or whether in these also it exists in a degenerate condition has not yet been 

 determined ; for convenience at present the six fully-developed head- 

 segments may be considered homologous throughout the group. 



The appendages vary much in form in different parts of 

 the body and in different forms. Those of the head region 



