Common Crayfish. 93 



the Decapods by all three pairs of thoracic limbs. Of the three 

 true jaws the most anteriorly placed, the so-called ^mandible/ 

 carries a tri-articulate palp with an expanded terminal joint, and 

 corresponds as a whole to an ambulatory leg, or to the endopodite 

 of a foot-jaw. The palp, as being of great use in directing 

 floating food towards the mouth, is rarely absent in Crustacea, 

 except in the terrestrial Isopods and Amphipods. The basal portion 

 of the mandibular appendage* appears to correspond to the four 

 basal joints of the ambulatory legs ; and it may be remarked that 

 the denticulation and anchylosis of the second and third segments of 

 the third pair of foot-jaws afford an instructive example of transi- 

 tion towards the modification of the basal segments seen in the 

 mandible. The two pairs of foliaceous maxillae, the posterior one of 

 which remains mesially divided, probably in relation to the ingestion 

 of floating food, and does not form a ' labium^ as in air-breathing 

 Arthropoda, are not seen in this preparation, being closely appressed 

 by the foot-jaws in apposition with them. Of the five pairs of 

 *^ ambulatory' or ' abdominal' legs, whence the order Decapoda take 

 their name, the first has its two terminal joints or tarsus modified, 

 so as to form a large pair of pincers, by having the posterior distal 

 angle of the penultimate joint, or propodite, prolonged so as to be 

 parallel vsith and commensurate with the dactylopodite. Similar 

 but smaller pincers are developed similarly upon the second and 

 third pairs of feet ; the fourth and fifth are not so armed, the 

 penultimate joint not being prolonged beyond its distal articular 

 surface. It may be noted that the chela of the Scorpion, which 

 bears a considerable resemblance to the largest of the three chelae 

 of the Astacus, has its pincer-like portion somewhat differently 

 constituted, the anterior or interior angle of the propodite being 

 produced instead of the posterior or exterior ; and the smaller of 

 the two blades of the prehensile organs lying exteriorly, instead 

 of interiorly as in the Crustacean. The large pincers of the scor- 

 pion are homotypical, as representing the two terminal joints of an 

 appendage with the large pincers of the crayfish, but they are not 



' See figure of edentulous mandible of Matuta Victor in * Kegne Animal,' Crustacea, 

 pi. vii. fig. i. c., where the palp has six joints, the three proximal ones of which are 

 very small, whilst the three distal have the proportions of those which make up the 

 ordinary tri-articulate palp. 



