532 



ZOOLOGY 



SECT. 



cert m. 



as we have seen, the exopodite springs from the second segment 

 of the axis, and the probabilities are that there is no exact corre- 

 spondence" between the parts of the antennule and those of the 



remaining appendages. 



The eye-stalks, already noticed, 

 arise just above the antemmles and 

 are formed each of a small proximal 

 and a large distal segment. They 

 are sometimes counted as append- 

 ages serially homologous with the 

 antennae, legs, &c. But, as we have 

 seen in the case of Apus, the append- 

 ages of Crustacea are always formed 

 in regular order from before back- 

 wards ; the eye-stalks, on the other 

 hand, always appear later, both in 

 individual development and in the 

 Crustacean series, than the normal 

 anterior appendages. They are 

 therefore more properly to be looked 

 upon as articulated processes of the 

 prostomium, developed in connection 

 with the need for an increased range 

 of vision. Assuming this to be the 

 case, it will be seen that the body of 

 the Crayfish consists of a pro- 

 stomium, nineteen metameres, and 

 a telson. The prostomium bears 

 eye-stalks : the first five metameres 

 are fused with the prostomium to 

 form the head, and bear the an- 

 tennules, antennae, mandibles, first 

 maxillae, and second maxillae : the 

 next eight metameres (6th-13th) 

 constitute the thorax, and bear the 

 three pairs of maxillipeds and the 

 five pairs of legs : the remaining six 

 metameres (14th-19th), together 

 with the telson, constitute the abdo- 

 men, and bear five pairs of pleopods 

 and one of uropods. 



The articulation of the vari- 

 ous podomeres of the appendages 

 is on the same plan as that of the abdominal segments (p. 528). 

 The podomeres are, it must be remembered, rigid tubes : they are 

 connected with one another by flexible articular membranes 

 (Fig. 440, art. m.), but at two points the adjacent ends of the 



FIG. 440. Portion of a leg of Asta- 

 cus, with the exoskeleton partly re- 

 moved, showing articulations and 

 muscles, art. m. articular mem- 

 brane ; en. 2 .5, podomeres of endo- 

 podite ; ext. extensor muscles ; I. 

 flexors ; h. hinge. 



