CRUSTACEA. 35 



civil the nine thoracic rings hearing the walking-legs. 

 the maxillipeds, and one pair of maxilla-. A dried 

 and (leaned shell from which the teacher can remove 

 the thorax, with the legs, etc., will be found useful at 

 this stage of the lesson. 



Before studying the elements of a ring, which will make 

 the subject of the consolidation of segments clearer, the 



appendages of the cephalothorax, which still remain in 

 good condition on one side of the body, may be compared 

 with those of the third abdominal ring (Fig. 8, E), in 

 order to determine whether or not they are built upon the 

 same plan of structure. 



For this comparison it would certainly be most natural 

 to begin with the eye-stalks and go backward, or else 

 with the last pair of walking-legs and go forward, but, 

 in either case, there would be much difficulty in recog- 

 nizing the similarity of structure which really exists, 

 though it is most successfully disguised by both of these 

 pairs of appendages. Now if the resemblance is less 

 completely masked by any one of the fourteen pairs of 

 appendages, it would be better to begin with that pair 

 wherever it might occur. Just such appendages are 

 found in the third pair of maxillipeds (Fig. 8, />\ J 3). 

 The pupils will recognize the basal section {h 1). with the 

 two parts corresponding tc the two lobes. The inner 

 lobe has taken the form f a ittl: walking-leg (// 3). while 

 the outer lobe has become: slender and tapering (// 2). A 

 fourth and additional section (// 4) is articulated to the 

 basal portion, and extends upward into what is known as 

 the -ill-chamber. This section bears one of the feathery 



gais cr). . 



Examining now the five pain of walking-legs, it is seen 

 that they represent the inner section (h 3), while the outer 

 section is undeveloped. The fourth section (// 4), bearing 

 a -ill {g), is found in all excepting the last pair of walking- 

 legs. 



