196 MICROSCOPY FOR BEGINNERS. 



The podal spines are slightly curved, but not forked 

 at the ends. They are arranged in clusters of four 

 spines each, the clusters forming two rows, one on each 

 side of the body. 



The O3sophagus is long and remarkably muscular. It 

 is surrounded and somewhat obscured by a pair of large 

 glands, and has near its posterior extremity two large 

 appendages similar in structure to the oesophagus itself. 

 The blood is yellowish-red. The dorsal vessel, at some 

 distance behind the front end of the body, divides 

 into three branches, which pass forward, and near the 

 anterior border unite by means of a net-work of fine 

 vessels. The worm has four hearts, two on each side of 

 the dorsal vessel, one pair being near the eighth, and 

 one pair near the ninth cluster of podal spines. The 

 dorsal vessel divides in front of the first pair of hearts. 

 The ventral blood-vessel is forked, but with only two 

 branches. 



10. TUBIFEX. 



A common and, in some places, a very abundant little 

 worm, measuring from one-half to one and one-half 

 inches in length. The body is thread-like in its nar- 

 rowness, and is transparent and colorless, although the 

 bright crimson blood gives it a hue so vivid to the naked 

 eye that, where the worms are numerous, it often seems 

 to tinge the mud in which they live. They are seldom 

 found free-swimming, but live a comparatively seden- 

 tary life, with about one-half of the body buried in their 

 burrow, the remaining parts protruding into the water, 



