ASTACUS FLUVIATILIS. 271 



Not only on this ground, but because the fold really repre- 

 sents a true neck, or separation between the head and thorax, 

 it may approximately be termed the cervical fold. The 

 scaphognathite (Fig. 71, O, c, d), an important appendage of 

 the second maxilla, lies in this cervical fold. 



The appendages of the three maxillipedary somites (Fig. 

 71, D, E, I ) are highly interesting, inasmuch as they afford 

 transitional forms between the ambulatory limbs and the 

 gnathites. Each maxillipede is composed of three divisions, 

 articulated with a stout protopodite. The outermost of these 

 divisions is a curved, elongated lamina (d), precisely resem- 

 bling the epipodite of the posterior thoracic limbs in the two 

 hinder maxillipedes {E, F) ; but, in the anterior (-0), not 

 modified so as to serve as a branchia, and rather approaching 

 the scaphogriathite in form. 



The middle division of each maxillipede (c), answering to 

 the exopodite, is long, slender, many-jointed, and palpiform ; 

 while the inner division, or endopodite (a, b), not only corre- 

 sponds with one of the ambulatory limbs, but in the posterior 

 maxillipede (Fig. 71, F) very closely resembles one, and con- 

 tains the same number of joints. In the next maxillipede, 

 however (Fig. 71,-27), the endopodite is proportionally shorter, 

 and in texture and form rather approaches the foliaceous en- 

 dopodite of the anterior maxillipede (Fig. 71, D), in which a 

 flat plate is applied to the posterior surface of the slender 

 exopodite. A perfect transition is thus produced between 

 the corresponding divisions of the second maxillipede and of 

 the second maxilla. 



The intermaxillary apodeme, or that developed from the 

 connecting membrane of the two maxillary somites, is very 

 remarkable for its stoutness and for the great size and ex- 

 panded form of the mesophragmal processes, which unite into 

 a broad plate, whence prolongations are sent forward and 

 outward, in front of the tendon of the great adductor mandi- 

 bulcB muscle on each side. These prolongations appear to be 

 the calcined posterior horizontal apophyses of the mandibulo- 

 maxillary apodeme, which elsewhere remains membranous. 



The second maxilla (Fig. 71, C) much resembles the an- 

 terior maxillipede, but the epipodite (d) and exopodite (c) 

 appear to be combined into a wide oval plate, the scapho- 

 c/nathite, of which mention has already been made. 1 In the 

 first maxilla (Fig. 71, JB) the epipodite and exopodite appear 



1 Until the development of these appendages has been worked out, the de- 

 termination of the homologies of their parts must be regarded as provisional. 

 18 



