OF ARTS AND SCIENCES. 91 



pairs. The fork on the convex side of the furcate outer end is much 

 longer, sharper, more curved, and more slender than that on the con- 

 cave side. The number of stylets in tlie ventral fascicles behind the 

 fourth pair varies from 4 to 7, perhaps according to the age of tlie 

 animal. They also vary in length. The fork on the convex side of 

 the stylet (PL I. fig. 3) is very much smaller and sharper than that 

 on the concave side, but not so long. These stylets are therefore 

 readily distinguished from those in the first four pairs of ventral fas- 

 cicles. The dorsal fascicles contain from 1 to o bristles, and from 1 

 to 3 spade-shaped stylets. The latter (PI. I. fig. 4) are not so much 

 curved as the ventral stylets. Their outer ends are flattened and ex- 

 panded into thin triangular blades, one edge of the expanded jDortion 

 being nearly straight, and the other concave, and both being strength- 

 ened by marginal ribs. A third rib sometimes runs midway between 

 the two marginal ones. The bristles are much longer than the sty- 

 lets, pointed, and without a median shoulder. The matrix cells of the 

 stylets and bristles may be seen in sections. They have elliptical, 

 granular nuclei, which are closely applied to the stylets and bristles. 



In the fall, when the Lemna leaves were plenty, the tubes were 

 formed entirely from them. Specimens kept over winter in glass jars 

 in which there was but little Lemna formed their tubes entii-ely from 

 Plumatella eggs, as described by Leidy. An isolated specimen in a 

 small dish with some Qi^dogonium formed its tube entirely from the 

 Qidogonium filaments. The worms were never observed to leave 

 the tube voluntarily, nor were any ever found without a tube. When 

 a single individual becomes divided into two, the two sometimes con- 

 tiime to occupy the same tube, one reversing its direction ; or the 

 newly-formed individual may build a new tube by the side of the old 

 one and attached to it. An individual removed from its tube forms a 

 new one in a very short time, frequently in less than ten minutes. 

 The animal is easily driven from its tube by following it up from the 

 posterior end with needles. 



They stay most of the time at the surface of the water, and, when 

 in a disii, usually at the side. There they lengthen their bodies, head 

 downward, and seize the side of the dish by using the everted pharynx 

 as a sucker. Then, by alternately shortening and lengthening the 

 body, they continually move the tube up and down. The alimentary 

 canal of the worms found at the surface of the water frequently con- 

 tains a great many large air-bubbles, but such bubbles have never 

 been seen in animals taken from the bottom of the jar. The Inib- 

 bles are especially noticeable in specimens whose tubes are formed of 



