No. 2.] STRUCTURE OF BIMASTOS PALUSTRIS. 47C) 



Behind the pharynx the oesophagus gradually increases in 

 diameter, and changes from a flattened to a circular cross-sec- 

 tion, forming a long tubular pouch extending for five or six 

 somites. The papilliform arrangement of the epithelium, seen 

 in the ventral portion of the pharynx, becomes more pro- 

 nounced. When viewed en face, the flat-topped papillae are 

 seen to be closely packed and polygonal in shape; under a low 

 power the interior of this region of the enteric canal has a 

 velvety appearance. As shown in Fig. 7, which is a longitu- 

 dinal section, the epithelial layer alone is concerned in this 

 condition. The increase in surface is enormous. 



Following the region just described is a structure, resem- 

 bling in its general characters, the calciferous glands of Lum- 

 bricus. Externally, this portion of the alimentary canal is of 

 uniform diameter, and there are no pouches such as are con- 

 spicuous in the corresponding region of Lumbricus, and when 

 the walls are ruptured there is no exudation of milky fluid. 

 Longitudinal blood-vessels, seen through the outer wall, pro- 

 duce a striate appearance externally. Figs. 4, 5, 8, 9, and 10, 

 show the structure of this region. 



In somite XI the oesophageal epithelium becomes erected 

 into longitudinal ridges, shown in cross-section in Fig. 8, a. 

 In the lateral regions these rugae increase rapidly in height, as 

 may be seen in Fig. 8, b, which is three sections behind 8, a. 

 At the same time the several ridges begin to fuse, and the 

 epithelium, which has become ciliated, encroaches upon the 

 lumen of the canal, and a few sections further back the latter 

 is reduced to a mere ciliated slit, with its long diameter dorso- 

 ventral. A transverse section in this region shows slender 

 struts radiating in all directions from the ciliated epithelium, 

 and connected at their outer ends with the musculo-peritoneal 

 layer. In other words the epithelial and muscular layers, 

 instead of adjoining one another, or being separated only 

 by a blood sinus, have become arranged into two tubes, 

 a laterally compressed epithelial, and a nearly circular mus- 

 cular one. The two tubes are connected by longitudinal 

 laminae, which cut up the space between into wedge-shaped 

 cavities. 



