92 MORPHOLOGY OF INVERTEBRATE TYPES 



oesophagus. It occupies the fourteenth and fifteenth segments. 

 The gizzard has about the same size as the crop but is provided 

 with powerful musculature. It occupies the sixteenth, seven- 

 teenth, and eighteenth segments. The gizzard opens into the 

 intestine which extends all the way to the anus. The absorbing 

 surface of the intestine is considerably increased by the presence 

 of a longitudinal dorsal fold or typholosole. The microscopic 

 structure of the intestine presents some interesting peculiarities. 

 The intestinal cavity is lined with striated cuticle produced or 

 secreted by a layer of high columnal epithelial cells. To the 

 outside of this layer comes the vascular layer containing blood 

 vessels. This is followed by the muscular layer. The outermost 

 layer of the intestine is formed by large chloragogue cells which 

 are supposed to possess an excretory function and which are also 

 found floating in the coelomic fluid. 



Excretory system. The excretory system of Lumbricus 

 consists of paired nephridia or segmental organs. One pair of 

 nephridia is found in every segment except the anterior three and 

 the anal segment. Each nephridium is a coiled tube which 

 opens with a ciliated funnel or nephrostome into the body cavity 

 of the preceding segment and with a nephridiopore to the out- 

 side above the b bristle. The nephridium may be divided into 

 three portions. The first, transparent portion following upon the 

 funnel is a long thin tube with fine blood-vessels between its 

 loops. The median or glandular portion is considerably thicker 

 than the preceding one and is opaque because of the secretion of 

 the glandular cells of its wall. The terminal portion is the short- 

 est and widest and plays the role of a collecting bladder. 



Circulatory system. The circulatory system of the earth- 

 worm is characterized by the presence of five longitudinal blood- 

 vessels extending from end to end of the animal. The segmental 

 character of the system is evidenced by the presence of connect- 

 ing blood-vessels or loops. The largest longitudinal vessel is 

 the dorsal vessel which runs in the mid-dorsal line above the 

 intestine. This vessel shows swellings in every segment in the 



