284 General Biology 



long worm; the tail of one animal may be grafted upon the side of 

 another, producing a double-tailed worm ; or the anterior end of one 

 individual may be united with that of another. In all such experiments 

 the parts must be held together by threads until they become united. 



The Annelida are divided into three classes, as follows: 



(1) Class Archiannelida (Gr. arche, beginning — Lat. annellus, 

 ring). The Polygordius (Fig. 172) is the typical example. This class 

 is without setae or parapodia. 



(2) Class Chaetopoda (Gr. chaite, bristle — pous, foot). Nereis, 

 the common sand-worm, and the earthworm are classic examples. 

 Nereis differs from the earthworm in having a pair of chitinous jaws, 

 a pair of tentacles, and two pairs of eyes on the prostomium, as well as 

 in having a pair of palpi, and four pairs of tentacles on the peristome. 

 The parapodia are used for locomotion, while the lobes of the parapodia 

 are well supplied with blood-vessels and serve as gills. Then, too, there 

 are jointed locomotor-setae on each parapodium, while the muscles which 

 move the parapodium, are attached to two buried bristles, called aciculae, 

 which serve as a sort of internal skeleton. The sense organs of Nereis 

 are also developed more highly than those of Lumbricus, the tentacles 

 serving as organs of touch, while the palpi are thought to act as organs 

 of taste, and the eyes, of sight. 



Nereis (Fig. 173) is the example of the sub-class known as Poly- 

 chaeta (on account of its many foot-like structures), while such worm- 

 like water-animals as Tubifex, Dero, and Nais, usually serve as the ex- 

 ample of the sub-class, Oligochaeta (having few setae). 



(3) Class Hirudinea. (Lat. hirudo, leech.) These are worm-like 

 animals living in fresh water and on land. They are commonly called 

 leeches. They are flattened dorso-ventrally. The external segmentation 

 does not correspond to the internal segmentation. The leeches are dis- 

 tinguished from the earthworm by having definitely thirty-three seg- 

 ments, two suckers (one at each end), and no setae (except in one 

 genus). They are hermaphrodites. 



The most important example is the medicinal leech, known as 

 Hirudo medicinalis (Fig. 174), normally about four inches long, though 

 capable of much contraction and expansion. Not only are these animals 

 used to draw blood from patients, but Lambart advises against drinking 

 water which is not filtered, especially in the tropics, as the small leeches 

 may be swallowed. When swallowed, they attach themselves below the 

 larynx and, instead of releasing themselves when filled with blood as 

 they do on an external surface, they seem to draw a small amount of 

 blood and then migrate to another spot close by and begin the same 

 process, thus causing considerable anaemia (loss of blood). 



This is readily understandable when it is realized that the leech 

 has three chitinous jaws to form the mouth (which lies within the an- 

 terior sucker). These jaws bite into a region, and a secretion from the 



