30 PRACTICAL ZOOLOGY. 



looking for traces of thoracic rings. The number of 

 cephalothoracic segments can be determined only by 

 counting the pairs of appendages. All writers are 

 not agreed as to this number; some regard the eyes 

 as a distinct pair of appendages, comparable to any 

 pair of legs, and representing a distinct ring; but 

 the eyes seem to develop from the ring which bears 

 the antennulae. Again, some regard the metastomata 

 as a distinct pair of appendages. The line of division 

 between the head and thorax is also a matter of 

 dispute. Huxley places it between the second pair 

 of maxillae and the first pair of maxillipeds. Hyatt 

 places the division between the first and second pairs 

 of maxillae, as the space between these is membranous 

 entirely across the sternal region, while back of this 

 line the parts are hard and firmly soldered to- 

 gether. 



The carapace is probably not a consolidation of the 

 dorsal portions of all the cephalothoracic segments, 

 but a backward extension of the consolidated upper 

 parts of the rings of the head ; the dorsal portions of 

 the thoracic rings, being thus covered, are conse- 

 quently no longer developed. In certain lower 

 Crustacea the carapace is unquestionably a backward 

 extension of the head-shield (Hyatt). The groove 

 across the carapace has been generally regarded as 

 indicating the line of division between the head and 

 thorax. If this line be traced its extremities will be 

 found to lie between the antennas and mandibles. 

 Packard states that the carapace, or shield of the 

 head-thorax, may be seen, after close examination, to 

 represent the segments of the antennae and mandibles, 



