90) ZOOLOGICAL EXERCISES. 
9. The nervous system.—Carefully remove the muscles 
out of the thorax. On the floor will be seen a large 
oval ganglion. A strong nerve proceeds from this 
into the head, and about six strong nerves come off 
on either side. A much smaller nerve proceeds pos- 
teriorly into the abdomen. The cerebral ganglion is 
not easy to see, but an attempt may be made to slice 
off the crown of the head and upper portion of the 
eyes. | 
10, The dorsal vessel.—Take another fly, open the abdo- 
men from below, and remove the reproductive and 
alimentary systems. ‘The dorsal vessel will be seen 
lying along the middle line. It is white, broad an- 
teriorly, and tapering posteriorly. Dissect it out, 
place it on a slide, and examine. 
11. Integumentary organs.—The proboscis. Pull out the 
proboscis and cut it off close up to the head, above 
the palpi. Spread it out with needles in a drop of 
water, and examine. Notice the labial appendages: 
composed of two broad lobes with a series of trachez- 
like canals with incomplete rings, the false trachee. 
These form a strainer by which the fly separates the 
fluid matter from the substance on which it is feeding. 
These pass backward into the ligula, which is 
enclosed between two chitinous portions, one dorsal, 
the other ventral. The dorsal portion is composed of 
the labrum and the maxille ; the ventral portion is 
the labiwm. Upon the upper portion of the maxillz 
are the single-jointed maxillary palpi. Notice the 
small pores at the extremity of the palpi. The 
antenne are composed of three joints, the first and 
second of which are short, the third long and bearing 
a plumose seta. The third joint is covered by small 
