THE EARTHWORM 23 



pouchy in front. The partitions separating the cavities from 

 one another are covered by large cells which secrete carbonate 

 of lime, the secretion finding its way through the anterior 

 pouch into the oesophagus. 



The crop or proventriculus is a thin-wdlled dilatation of 

 the alimentary canal in somites 15 and 16, and is succeeded 

 by another dilatation with stout muscular walls lying between 

 the septa separating somites 16 — 17 and 18 — 19. This muscular 

 organ, the gizzard, lies therefore in somites 17 and 18, but 

 it may project some way backwards, carrying the septa along 

 with it. The gizzard is succeeded by a wide thin-walled 

 sacculated tube, the intestine, which runs straight backwards 

 to the anus, preserving the same characters throughout its 

 course. The intestine is constricted where it passes through 

 the septa, and is expanded and saccular in the intervening 

 somites. Its external walls are covered with a layer of loosely- 

 packed glandular cells filled with yellow granules, which are 

 not acted upon by alcohol, alkalies, acetic, chromic, or osmic 

 acids. These cells, which give a yellow colour to the 

 intestine, are known as chloragogen cells, or sometimes, but 

 quite erroneously, they are described as " hepatic " cells. 

 They do not communicate with the cavity of the intestine, 

 and their function is unknown. From about the twentieth 

 somite backwards, the dorsal wall of the intestine is deeply 

 infolded so as to form a longitudinal ridge projecting into 

 the cavity of the intestine. This ridge, known as the typhlosole, 

 serves to increase the digestive surface. The space between 

 the limbs of the fold is filled with chloragogen cells, and 

 contains blood-vessels. 



The ccelomic chambers between the septa are filled by a 

 colourless ccelomic fluid, which can easily be squeezed out 

 through the dorsal pores and examined under the microscope. 

 It consists of a plasma coagulable in alcohol and colourless 

 corpuscles of two kinds — the one finely granular and amceboid, 

 the other spherical and loaded with coarse granules. 



In addition to the ccelomic fluid the earthworm possesses 

 red blood, contained in closed vessels. The red colour is 

 due to hemoglobin, which is not, as is the case in Verte- 

 brates, continued in the corpuscles, but is dissolved in the 

 plasma. The blood corpuscles of the earthworm are oval 

 and colourless. A description of the blood-vessels must 



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