42 THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBAEUS 



most are fastened to plumose hairs upon the pleopods of the second to 

 fifth somites inclusive, and some account of these pleopods may be given here 

 in connection with questions as to how the eggs and larvae are attached to the 

 mother. 



The posterior face of the fourth left pleopod of a female about to lay is 

 represented in figure 44. It will be noted that the endopodite is longer than 

 the exopodite, and both bear a fringe of long plumose setae on their right and 

 left edges. The setae are shorter toward the base of the exopodite and of the 

 endopodite while on the protopodite there are but a few plumose setae in two 

 tufts upon the inner edge. The long setae make of the pleopod a wide fan or 

 flat brush since the setae lie close together and like the wing feathers of a bird 

 form a rather flat firm expanse. The protopodite has a basal part containing 

 several calcified plates in a soft membrane, and a long segment that is well 

 calcified except for a triangular soft area toward its base on the posterior face. 

 The endopodite is made of two segments and the exopodite of one. The endopo- 

 dite and the exopodite are also somewhat annulated in appearance owing to 

 the grouping of the cement glands. The groups are opaque white and from each 

 side tend to run together across the posterior face. Distally they do not meet 

 but proximally they meet and make cross bands. Still farther toward the base 

 the glands cover the entire surface more and more completely. The non-gland- 

 ular areas are clear and not opaque, and in the figure are represented dark. 



The anterior face differs from the posterior (fig. 44) in a greater de- 

 velopment of glands which formed transverse bands more nearly all the way to 

 the tip. As so many of the glands are to the right and left near the setae, they 

 are well placed to smear their secretion over the set*. 



Toward the tips of the pleopods the exoskeleton is so translucent that with 

 Zeiss 2 D, the striation of the muscle fibers, the branched connective tissue 

 cells, granular blood corpuscles, and the polygonal gland cells may be seen. 

 The gland cells are about the diameter of a muscle fiber and larger than a 

 blood corpuscle. 



In most cases the plumose setae spring from over the glands, and the base 

 of a seta is as thick as two gland cells. The setae have a large central cavity 

 and a thick wall which is highly refractive and clear and is of unequal thick- 

 ness so that it projects into the cavity in lumps or waves, and gives the distal 

 part of the axis of the seta a somewhat segmented aspect. At the base each 

 seta is. articulated to the exoskeleton, and its central cavity is constricted by a 

 clear refractive thickening of the wall that leaves very little communication be- 

 tween the cavity of the setae and the cavity of the body. 



The side branches of the seta spring out not only along its sides but also, 

 scatteringly, along its posterior face so that the plumose seta is more like a 

 bottle brush than like a flat feather. While the side branches generally make 



