l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 10 



separate lines of evolution within the Chelicerata, and that the arach- 

 nids have not been derived from any Limulus-like progenitor. The 

 xiphosurids, on the other hand, show unmistakable affinities to the 

 trilobites, whose habits of living must have been similar to those of 

 Lifimlus. 



The stomach and digestion. — The greater part of the food tract 

 of the arachnids is formed from the mesenteron, including the stomach 

 proper, the anterior part of the intestine, and the excretory vessels 

 known as "Malpighian tubules" that open with the intestine into a 

 terminal proctodaeal cloaca. The stomach consists of a central canal 

 and of radiating diverticula, which latter may be few and sacklike, 

 or numerous and tubular. The earlier workers on the digestive pro- 

 cesses of Arachnida, including Plateau (1877) and Bertkau (1884), 

 regarded the stomach diverticula as digestive glands, but it was shown 

 by Bernard (1893) and Berlese (1897) that the diverticula constitute 

 the digestive region of the stomach. These writers, furthermore, 

 claimed that digestion with the Arachnida takes place intracelMarly 

 in the epithelium of the diverticula, and their contention has been 

 substantiated by the more detailed studies of later investigators, in- 

 cluding Oetcke (1912), Roesler (1934), Schlottke (1934), Bader 

 (1938), and Frank (1938). Pavlovsky and Zarin (1926) showed 

 that the digestive enzymes in the scorpion, including amylase, lipase, 

 and proteinases, are formed only in the stomach diverticula. 



The epithelium of the stomach diverticula consists of two distinct 

 kinds of functionally active cells ; namely, secretory cells, or "ferment" 

 cells, and digestive cells. Prior to feeding, the secretory cells are filled 

 with globules of secretion products ; the digestive cells are practically 

 devoid of inclusions. After feeding, the secretory cells discharge their 

 contents into the lumina of the diverticula, while the digestive cells 

 soon begin to show loose masses of material in their cytoplasm, 

 which later condense into dark-staining globules. In a starved animal, 

 secretion products are again formed in the secretory cells, but the 

 globules of the digestive cells gradually disappear. From these his- 

 tological phenomena it is deduced that the globules formed in the 

 digestive cells are masses of ingested food material, which is finally 

 digested in the cells and absorbed. Confirmatory evidence is seen in 

 the fact that granules of excretory matter accumulate in the distal 

 parts of these cells, which in most cases are discharged into the 

 stomach lumen by constriction and separation of the ends of the cells. 

 In those Acarina that have no intestinal outlet from the stomach, the 

 excretory granules remain in the epithelial cells. 



Different investigators are not entirely in accord as to the part 



