HOMOLOGIES OF CRUSTACEA. 35 



chiates, the body of the carapax of the Brachyura may pertain to the 

 second antennary segment, and only small ventral pieces to the 

 mandibular; while the Macroura, in which the shell is somewhat dif- 

 ferent in its connexion with the cephalothorax, and more oblong and 

 narrow in form (as in Cyclops), may have the suture which separates 

 the mandibular and second antennary segments, run across so as 

 nearly to bisect the cephalothorax. Among the Schizopoda, closely 

 related to the Macroura, the second antennary portion of the shell is 

 still smaller. In some Mysidae, there is an appearance of a cephalic 

 suture nearly as in the Pontiaa; and in Lucifer, the second antennary 

 segment is a narrow, neck-like elongation, anterior to the main part 

 of the carapax, from which it is separated by a suture. There are 

 hence variations even in species of the Macroural type (under which 

 the Schizopoda are here included) ; so that, while in some, the suture 

 between the second antennary and mandibular segments nearly bisects 

 the carapax, in others, it separates only a small cephalic segment. 

 Its position in the Brachyura is at the other extreme, the mandibular 

 segment being reduced to the narrow ventral portions of the carapax. 



The carapax in Scyllarus is abruptly inflexed either side, as in the 

 Brachyura ; while in most of the order Macroura, the lateral surface 

 is flat or evenly convex, and no trace of the lateral margin is to be 

 seen except in one or more spines, in a line below the line of the eyes, 

 which appear to mark its position. 



6. Homologies of the Lower Crmtacea. The carapax pertains normally 

 to the anterior segments of the body in many of the lower Crustacea. 

 In the Amphipods and other Choristopoda, it is cut short at the seventh 

 normal segment, and covers only what may be called a head ; this head 

 is very small, and includes the mandibles and three pairs of maxillae, 

 besides the organs of the senses, leaving seven pairs of thoracic feet, 

 each pair to its own well-developed segment. The exact normal rela- 

 tions of the shell of the head is with difficulty determined ; yet the 

 argument that this segment extends across below, just anterior to the 

 mandibles, and only here, probably holds in this group, as in the Deca- 

 poda, so as to show that the shell pertains either to the mandibles or 

 second antennae : farther investigation may possibly bring out a more 

 definite decision. 



In the preceding remarks, we have anticipated, in part, what we 

 .have to say respecting the shell in the Entomostraca. Yet we men- 

 tion the facts here in place, and with some more details. 



