MOEPHOLOGT OF THE PEDIPALPI. 25 



posterior part is a great deal larger than the anterior, and fills the 

 greater part of the ninth free segment. The epithelium lining 

 the stercoral pocket (PI. IV. fig. 15) consists of flat cells containing 

 numerous granules, which stain darkly with hsematoxylin along 

 the outer edge. They are very similar to the cells lining the 

 rest of the intestine. The MalpigJiian tubes arise near the pos- 

 terior end of the stercoral pocket and run forward along the 

 ' sides of its ventral surface. They are somewhat coiled and 

 closely attached to the pocket by connective tissue. In section 

 they are found to possess an indistinct lumen surrounded by 

 large cells with distinct oval granular nuclei. Occasionally 

 darkly staining granules appear in the protoplasm, but for the 

 most part it is apparently structureless (PL IV. fig. 16). The 

 coils of the Malpighian tubes are surrounded and held together 

 by fibrous-looking connective tissue. 



The proctodeum is a short straight tube running back through 

 segments 10-12 to open below the telson. A slight thickening 

 marks its junction with the mesenteron close behind the stercoral 

 pocket. The epithelium lining it is thrown into folds, and con- 

 sists of long cells with apparently a cuticle over their outer 

 surface (PL IV. fig. 17). The distinction between these cells and 

 those lining the stercoral pocket is quite evident, and the tran- 

 sition from one form to the other somewhat abrupt. 



In describing the stercoral pocket as part of the mesenteron, I 

 have been influenced by the character of the epithelium lining it 

 and passing forward into the intestine without any break, while 

 diff'ering so markedly from that lining the proctodaeum, and also 

 by the point of origin of the Malpighian tubes. The condition 

 of things in the embryos of Plirynus, which are described below, 

 admits of little doubt as to the origin of the stercoral pocket 

 from the mesenteron. 



The entosternite, which is so characteristic of Arachnids, de- 

 serves a few words of description in this form (PL III. fig. 4). It lies 

 between the gut and the thoracic ganglion, and is best described 

 as an elongated plate drawn out into a number of processes. 

 The front margin of it lies immediately behind the cerebral 

 ganglion, and a pair of processes run forward one on each side. 

 A large oval foramen perforates the plate near its front end, 

 through which the anterior median diverticulum of the gut 

 passes. Near the level of the posterior end of this foramen a 

 second pair of processes passes onward and dorsalward. A little 



