THE CRUSTACEA 



5 



in front of the mouth in all Crustacea (Fig. 2). Perhaps an anterior 

 somite bearing the paired eyes should also be recognised, but some 

 doubt remains as to the appendicular nature of the eye-stalks, and it 

 is possible that the eyes should be 

 referred to a primitively preoral region 

 corresponding to the prostomium of 

 annelid worms. In any case, a 

 prostomial element may be assumed 

 to share in the formation of the head, 

 and to it may be assigned the more 

 or less problematical "frontal sense- 

 organs " found in various larvae and 

 some adult Crustacea. It has been 

 suggested by Bernard that the back- 

 ward shifting of the mouth was 

 accomplished by a bending round of 

 the anterior somites and that the 

 prostomium is represented by the 



loV>riim nv nt-moy lir> incr in frrmt nf 



labrum 01 upper lip just in iront oi 



the mOUth. There 1S, however, little 



definite evidence in. favour of this 

 view. 



In all 

 addition to 



least three postoral somites, corre- 

 sponding to the mandibles, maxillulae, 



and maxillae, are included in the head-region. Even where a larger 

 number of somites are involved there is generally a more or less 

 marked change in the character of the appendages after the third 

 postoral pair, and, since the integumental fold which forms the cara- 

 pace seems to originate at this point, it is usual to take the third 

 postoral (maxillary) somite as the limit of the cephalon throughout 

 the Class. It is quite probable, however, that in the primitive 

 ancestral type of the Crustacea, the head-region included a smaller 

 number of somites. The three anterior pairs of limbs (antennules, 

 antennae, and mandibles), which alone are present in the " nauplius " 

 larva, show peculiarities of structure and development which seem 

 to place them in a different category from the succeeding limbs, and 

 there is some ground for regarding the three corresponding somites 

 as belonging to a " primary head-region." For descriptive purposes, 

 however, it is convenient to treat the two following somites also as 

 cephalic. 



Mention must be made here of a remarkable feature found only 

 in the aberrant group of the Stomatopoda, among the Malacostraca. 

 This is the reappearance of segmentation in the anterior part of the 

 head, where two movably articulated rings carry the eye-stalks and 



Fio. 2. 



Diagram of the Crustacean head- 

 region. (Modified from Goodrich.) The 

 nervous system is shown in black, a', 

 antennule ; a", antenna ; D, deutero- 

 cerebrum, the division of the brain 

 corresponding to the antennules ; K, 

 paired compound eye : e, unpaired 

 " nauplius " eye ; /.;>, frontal papilla 

 or sense-organ ; in, mouth ; ind, man- 

 dible ; maf, maxillula ; mx", maxilla ; P, 

 .protocerebrum, the division of the brain 

 corresponding to the paired eyes ; T, 

 tritocerebrum, the division of the brain 

 corresponding to the antennae. Behind 

 the preoral Somites, at the mouth is seen the transverse com- 



existing Crustacea, in 



missure of the 

 tritocerebrum. 



antennal ganglia or 



